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📚 A comprehensive study of how the Compose compiler determines type stability for recomposition optimization.

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# Compose Compiler Stability Inference System

A comprehensive study of how the Compose compiler determines type stability for recomposition optimization. All details that [Optimize App Performance By Mastering Stability in Jetpack Compose](https://medium.com/proandroiddev/optimize-app-performance-by-mastering-stability-in-jetpack-compose-69f40a8c785d) and [compose-performance](https://github.com/skydoves/compose-performance) couldn't take in.

## 💝 Sponsors

## 📗 Jetpack Compose Mechanisms Book

[Jetpack Compose Mechanisms](https://howcomposeworks.com/) takes you from "how to use Compose" into "how Compose actually works," tracing the AOSP source line by line through the compiler, runtime, and UI layers beneath every Composable, with practical, production-ready examples from the author's own Compose tooling and libraries. It then ties all three layers together into deep, real-world performance tuning, from stability inference to the skip decision. Fully updated for Kotlin 2.4.0 and Compose Compiler 2.4.0.

## Table of Contents

- [Compose Compiler Stability Inference System](#compose-compiler-stability-inference-system)
  - [📘 Manifest Android Interview](#-manifest-android-interview)
  - [🕊️ Dove Letter](#️-dove-letter)
  - [Table of Contents](#table-of-contents)
  - [Chapter 1: Foundations](#chapter-1-foundations)
    - [1.1 Introduction](#11-introduction)
    - [1.2 Core Concepts](#12-core-concepts)
      - [Stability Definition](#stability-definition)
      - [Recomposition Mechanics](#recomposition-mechanics)
    - [1.3 The Role of Stability](#13-the-role-of-stability)
      - [Performance Impact](#performance-impact)
  - [Chapter 2: Stability Type System](#chapter-2-stability-type-system)
    - [2.1 Type Hierarchy](#21-type-hierarchy)
    - [2.2 Compile-Time Stability](#22-compile-time-stability)
      - [Stability.Certain](#stabilitycertain)
    - [2.3 Runtime Stability](#23-runtime-stability)
      - [Stability.Runtime](#stabilityruntime)
    - [2.4 Uncertain Stability](#24-uncertain-stability)
      - [Stability.Unknown](#stabilityunknown)
    - [2.5 Parametric Stability](#25-parametric-stability)
      - [Stability.Parameter](#stabilityparameter)
    - [2.6 Combined Stability](#26-combined-stability)
      - [Stability.Combined](#stabilitycombined)
    - [2.7 Stability Decision Tree](#27-stability-decision-tree)
      - [Complete Decision Tree](#complete-decision-tree)
      - [Decision Tree for Generic Types](#decision-tree-for-generic-types)
      - [Expression Stability Decision Tree](#expression-stability-decision-tree)
      - [Key Decision Points Explained](#key-decision-points-explained)
  - [Chapter 3: The Inference Algorithm](#chapter-3-the-inference-algorithm)
    - [3.1 Algorithm Overview](#31-algorithm-overview)
    - [3.2 Type-Level Analysis](#32-type-level-analysis)
      - [Phase 1: Fast Path Type Checks](#phase-1-fast-path-type-checks)
      - [Phase 2: Type Parameter Handling](#phase-2-type-parameter-handling)
      - [Phase 3: Nullable Type Unwrapping](#phase-3-nullable-type-unwrapping)
      - [Phase 4: Inline Class Handling](#phase-4-inline-class-handling)
    - [3.3 Class-Level Analysis](#33-class-level-analysis)
      - [Phase 5: Cycle Detection](#phase-5-cycle-detection)
      - [Phase 6: Annotation and Marker Checks](#phase-6-annotation-and-marker-checks)
      - [Phase 7: Known Constructs](#phase-7-known-constructs)
      - [Phase 8: External Configuration](#phase-8-external-configuration)
      - [Phase 9: External Module Handling](#phase-9-external-module-handling)
      - [Phase 10: Java Type Handling](#phase-10-java-type-handling)
      - [Phase 11: General Interface Handling](#phase-11-general-interface-handling)
      - [Phase 12: Field-by-Field Analysis](#phase-12-field-by-field-analysis)
    - [3.4 Expression-Level Analysis](#34-expression-level-analysis)
      - [Constant Expressions](#constant-expressions)
      - [Function Call Expressions](#function-call-expressions)
      - [Variable Reference Expressions](#variable-reference-expressions)
  - [Chapter 4: Implementation Mechanisms](#chapter-4-implementation-mechanisms)
    - [4.1 Bitmask Encoding](#41-bitmask-encoding)
      - [Encoding Scheme](#encoding-scheme)
      - [Special Bit: Known Stable](#special-bit-known-stable)
      - [Bitmask Application](#bitmask-application)
    - [4.2 Runtime Field Generation](#42-runtime-field-generation)
      - [JVM Platform](#jvm-platform)
      - [Non-JVM Platforms](#non-jvm-platforms)
    - [4.3 Annotation Processing](#43-annotation-processing)
      - [@StabilityInferred Annotation](#stabilityinferred-annotation)
      - [Annotation Generation](#annotation-generation)
    - [4.4 Normalization Process](#44-normalization-process)
  - [Chapter 5: Case Studies](#chapter-5-case-studies)
    - [5.1 Primitive and Built-in Types](#51-primitive-and-built-in-types)
      - [Integer Types](#integer-types)
      - [String Type](#string-type)
      - [Function Types](#function-types)
    - [5.2 User-Defined Classes](#52-user-defined-classes)
      - [Simple Data Class](#simple-data-class)
      - [Class with Mutable Property](#class-with-mutable-property)
      - [Class with Mixed Properties](#class-with-mixed-properties)
    - [5.3 Generic Types](#53-generic-types)
      - [Simple Generic Container](#simple-generic-container)
      - [Multiple Type Parameters](#multiple-type-parameters)
      - [Nested Generic Types](#nested-generic-types)
    - [5.4 External Dependencies](#54-external-dependencies)
      - [External Class with Annotation](#external-class-with-annotation)
      - [External Class Without Annotation](#external-class-without-annotation)
    - [5.5 Interface and Abstract Types](#55-interface-and-abstract-types)
      - [Interface Parameter](#interface-parameter)
      - [Abstract Class](#abstract-class)
      - [Interface with @Stable](#interface-with-stable)
    - [5.6 Inheritance Hierarchies](#56-inheritance-hierarchies)
      - [Stable Inheritance](#stable-inheritance)
      - [Unstable Inheritance](#unstable-inheritance)
  - [Chapter 6: Configuration and Tooling](#chapter-6-configuration-and-tooling)
    - [6.1 Stability Annotations](#61-stability-annotations)
      - [@Stable Annotation](#stable-annotation)
      - [@Immutable Annotation](#immutable-annotation)
      - [Compiler-Level Differences: @Stable vs @Immutable](#compiler-level-differences-stable-vs-immutable)
      - [@StableMarker Meta-Annotation](#stablemarker-meta-annotation)
    - [6.2 Configuration Files](#62-configuration-files)
      - [File Format](#file-format)
      - [Pattern Syntax](#pattern-syntax)
      - [Gradle Configuration](#gradle-configuration)
    - [6.3 Compiler Reports](#63-compiler-reports)
      - [Enabling Reports](#enabling-reports)
      - [Generated Files](#generated-files)
    - [6.4 Common Issues and Solutions](#64-common-issues-and-solutions)
      - [Issue 1: Accidental var Usage](#issue-1-accidental-var-usage)
      - [Issue 2: Mutable Collections](#issue-2-mutable-collections)
      - [Issue 3: Interface Parameters](#issue-3-interface-parameters)
      - [Issue 4: External Library Types](#issue-4-external-library-types)
      - [Issue 5: Inheritance from Unstable Base](#issue-5-inheritance-from-unstable-base)
  - [Chapter 7: Advanced Topics](#chapter-7-advanced-topics)
    - [7.1 Type Substitution](#71-type-substitution)
      - [Substitution Map Construction](#substitution-map-construction)
      - [Substitution Application](#substitution-application)
      - [Nested Substitution](#nested-substitution)
    - [7.2 Cycle Detection](#72-cycle-detection)
      - [Detection Mechanism](#detection-mechanism)
      - [Example: Self-Referential Type](#example-self-referential-type)
      - [Limitation](#limitation)
    - [7.3 Special Cases](#73-special-cases)
      - [Protobuf Types](#protobuf-types)
      - [Delegated Properties](#delegated-properties)
      - [Inline Classes with Markers](#inline-classes-with-markers)
  - [Chapter 8: Compiler Analysis System](#chapter-8-compiler-analysis-system)
    - [8.1 Analysis Infrastructure](#81-analysis-infrastructure)
      - [WritableSlices: Data Flow Storage](#writableslices-data-flow-storage)
      - [BindingContext and BindingTrace](#bindingcontext-and-bindingtrace)
    - [8.2 Composable Call Validation](#82-composable-call-validation)
      - [Context Checking Algorithm](#context-checking-algorithm)
      - [Inline Lambda Restrictions](#inline-lambda-restrictions)
      - [Type Compatibility Checking](#type-compatibility-checking)
    - [8.3 Declaration Validation](#83-declaration-validation)
      - [Composable Function Rules](#composable-function-rules)
      - [Property Restrictions](#property-restrictions)
      - [Override Consistency](#override-consistency)
    - [8.4 Applier Target System](#84-applier-target-system)
      - [Scheme Structure](#scheme-structure)
      - [Target Inference Algorithm](#target-inference-algorithm)
      - [Cross-Target Validation](#cross-target-validation)
    - [8.5 Type Resolution and Inference](#85-type-resolution-and-inference)
      - [Automatic Composable Inference](#automatic-composable-inference)
      - [Lambda Type Adaptation](#lambda-type-adaptation)
    - [8.6 Analysis Pipeline](#86-analysis-pipeline)
      - [Compilation Phases](#compilation-phases)
      - [Data Flow Through Phases](#data-flow-through-phases)
    - [8.7 Practical Examples](#87-practical-examples)
      - [Example: Composable Context Validation](#example-composable-context-validation)
      - [Example: Inline Lambda Analysis](#example-inline-lambda-analysis)
      - [Example: Stability and Skipping](#example-stability-and-skipping)
  - [Conclusion](#conclusion)
  - [Find this repository useful? :heart:](#find-this-repository-useful-heart)
- [License](#license)

## Chapter 1: Foundations

### 1.1 Introduction

The Compose compiler implements a stability inference system to enable recomposition optimization. This system analyzes types at compile time to determine whether their values can be safely compared for equality during recomposition.

The inference process involves analyzing type declarations, examining field properties, and tracking stability through generic type parameters. The results inform the runtime whether to skip recomposition when parameter values remain unchanged.

### 1.2 Core Concepts

#### Stability Definition

A type is considered stable when it satisfies three conditions:

1. **Immutability**: The observable state of an instance does not change after construction
2. **Equality semantics**: Two instances with equal observable state are equal via `equals()`
3. **Change notification**: If the type contains observable mutable state, all state changes trigger composition invalidation

These properties allow the runtime to make optimization decisions based on value comparison.

#### Recomposition Mechanics

When a composable function receives parameters, the runtime determines whether to execute the function body:

```kotlin
@Composable
fun UserProfile(user: User) {
    // Function body
}
```

The decision process:
1. Compare the new `user` value with the previous value
2. If equal and the type is stable, skip recomposition
3. If different or unstable, execute the function body

Without stability information, the runtime must conservatively recompose on every invocation, regardless of whether parameters changed.

### 1.3 The Role of Stability

#### Performance Impact

Stability inference affects recomposition in three ways:

**Smart Skipping**: Composable functions with stable parameters can be skipped when parameter values remain unchanged. This reduces the number of function executions during recomposition.

**Comparison Propagation**: The compiler passes stability information to child composable calls, enabling nested optimizations throughout the composition tree.

**Comparison Strategy**: The runtime selects between structural equality (`equals()`) for stable types and referential equality (`===`) for unstable types, affecting change detection behavior.

Consider this example:

```kotlin
// Unstable parameter type - interface with unknown stability
@Composable
fun ExpensiveList(items: List<String>) {
    // List is an interface - has Unknown stability
    // Falls back to instance comparison
}

// Stable parameter type - using immutable collection
@Composable
fun ExpensiveList(items: ImmutableList<String>) {
    // ImmutableList is in KnownStableConstructs
    // Can skip recomposition when unchanged
}

// Alternative: Using listOf() result
@Composable
fun ExpensiveList(items: List<String>) {
    // If items comes from listOf(), the expression is stable
    // But the List type itself is still an interface with Unknown stability
}
```

The key insight: `List` and `MutableList` are both interfaces with `Unknown` stability. To achieve stable parameters, use:
1. `ImmutableList` from kotlinx.collections.immutable (in KnownStableConstructs)
2. Add `kotlin.collections.List` to your stability configuration file
3. Use `@Stable` annotation on your data classes containing List

## Chapter 2: Stability Type System

### 2.1 Type Hierarchy

The compiler represents stability through a sealed class hierarchy defined in `Stability.kt`:

```kotlin
sealed class Stability {
    class Certain(val stable: Boolean) : Stability()
    class Runtime(val declaration: IrClass) : Stability()
    class Unknown(val declaration: IrClass) : Stability()
    class Parameter(val parameter: IrTypeParameter) : Stability()
    class Combined(val elements: List<Stability>) : Stability()
}
```

Each subtype represents a different category of stability information available to the compiler.

### 2.2 Compile-Time Stability

#### Stability.Certain

This type represents stability that can be determined completely at compile time.

**Structure:**
```kotlin
class Certain(val stable: Boolean) : Stability()
```

The `stable` field indicates whether the type is definitely stable (`true`) or definitely unstable (`false`).

**Examples:**

```kotlin
// Certain(stable = true)
class Point(val x: Int, val y: Int)

// Certain(stable = false)
class Counter(var count: Int)
```

**Usage Conditions:**
- Primitive types (`Int`, `Long`, `Boolean`, etc.)
- `String` and `Unit`
- Function types (`FunctionN`, `KFunctionN`)
- Classes with only stable `val` properties
- Classes with any `var` property (immediately unstable)
- Classes marked with `@Stable` or `@Immutable` annotations

**Implementation:** See `Stability.kt` for the `knownStable()` extension function.

### 2.3 Runtime Stability

#### Stability.Runtime

This type indicates that stability must be checked at runtime by reading a generated `$stable` field.

**Structure:**
```kotlin
class Runtime(val declaration: IrClass) : Stability()
```

The `declaration` references the class whose stability requires runtime determination.

**Generated Code Example:**

```kotlin
// Source code
class Box<T>(val value: T)

// Compiler-generated code
@StabilityInferred(parameters = 0b1)
class Box<T>(val value: T) {
    companion object {
        @JvmField
        val $stable: Int = /* computed based on type parameters */
    }
}
```

**When Applied:**
- Classes from external modules (separately compiled)
- Generic classes where type parameters affect stability
- Classes with `@StabilityInferred` annotation

**Runtime Behavior:**

At instantiation sites, the runtime computes the `$stable` field value:

```kotlin
Box<Int>         // $stable = STABLE (0b000)
Box<MutableList> // $stable = UNSTABLE (0b100)
```

**Implementation:** See `Stability.kt` (the `Runtime` handling in `StabilityInferencer.stabilityOf`) and `ClassStabilityTransformer.kt` (the generated `$stable` field).

### 2.4 Uncertain Stability

#### Stability.Unknown

This type represents cases where the compiler cannot determine stability.

**Structure:**
```kotlin
class Unknown(val declaration: IrClass) : Stability()
```

**Examples:**

```kotlin
interface Repository {
    fun getData(): String
}

class Screen(val source: Repository)
// Repository has Unknown stability
```

**Usage Conditions:**
- Interface types (unknown implementations)
- Abstract classes without concrete analysis
- Types in incremental compilation scenarios

**Runtime Behavior:**

When encountering Unknown stability, the runtime falls back to instance comparison (`===`) for change detection. This conservative approach ensures correctness but prevents skipping optimizations.

**Implementation:** See `Stability.kt` (the `Stability.Unknown` branch in `StabilityInferencer.stabilityOf`).

### 2.5 Parametric Stability

#### Stability.Parameter

This type represents stability that depends on a generic type parameter.

**Structure:**
```kotlin
class Parameter(val parameter: IrTypeParameter) : Stability()
```

**Example:**

```kotlin
class Wrapper<T>(val value: T)
//              ^^^^^^^^^^^^
// Stability depends on T

// Instantiation examples:
Wrapper<Int>       // Stable (Int is stable)
Wrapper<Counter>   // Unstable (Counter from 2.2 is unstable)
```

**Resolution Process:**

When analyzing `Wrapper<Int>`:
1. Identify `value: T` has `Stability.Parameter(T)`
2. Substitute `T` with `Int`
3. Evaluate `stabilityOf(Int)` = `Stable`
4. Result: `Wrapper<Int>` is stable

**Implementation:** See `Stability.kt` for type parameter handling (the `isTypeParameter()` branch and `applyTypeParameterMask`).

### 2.6 Combined Stability

#### Stability.Combined

This type aggregates multiple stability factors from different sources.

**Structure:**
```kotlin
class Combined(val elements: List<Stability>) : Stability()
```

**Examples:**

```kotlin
class Complex<T, U>(
    val primitive: Int,     // Certain(stable = true)
    val param1: T,          // Parameter(T)
    val param2: U           // Parameter(U)
)
// Combined([Certain(true), Parameter(T), Parameter(U)])
```

**Combination Rules:**

The compiler combines stabilities using the `plus` operator (defined in `Stability.kt`):

```kotlin
operator fun plus(other: Stability): Stability = when {
    other is Certain -> if (other.stable) this else other
    this is Certain -> if (stable) other else this
    else -> Combined(listOf(this, other))
}
```

Worked examples:

```kotlin
Stable     + Stable     = Stable
Stable     + Unstable   = Unstable
Unstable   + Stable     = Unstable
Stable     + Parameter  = Parameter                     // a stable Certain is absorbed
Parameter  + Parameter  = Combined([Parameter, Parameter])
Runtime    + Parameter  = Combined([Runtime, Parameter])
```

**Key insight:** Adding a stable `Certain` returns the *other* operand unchanged, so only genuinely uncertain factors (`Parameter`, `Runtime`, `Unknown`) accumulate into a `Combined`. A `Combined` is only produced when neither operand is `Certain`.

**Key Property:** Unstable stability dominates all combinations. A single unstable component makes the entire result unstable.

### 2.7 Stability Decision Tree

The Compose compiler follows a systematic decision tree when determining stability. This tree represents the actual logic flow implemented in the compiler.

#### Complete Decision Tree

```
┌─────────────────────────────────┐
│     Start: Analyze Type/Class   │
└────────────┬────────────────────┘
             │
             ▼
┌─────────────────────────────────┐
│  Is it a primitive type?        │───Yes──→ [STABLE]
│  (Int, Boolean, Float, etc.)    │
└────────────┬────────────────────┘
             │ No
             ▼
┌─────────────────────────────────┐
│  Is it String or Unit?          │───Yes──→ [STABLE]
└────────────┬────────────────────┘
             │ No
             ▼
┌─────────────────────────────────┐
│  Is it a function type?         │───Yes──→ [STABLE]
│  (Function<*>, KFunction<*>)    │
└────────────┬────────────────────┘
             │ No
             ▼
┌─────────────────────────────────┐
│  Has @Stable or @Immutable?     │───Yes──→ [STABLE]
└────────────┬────────────────────┘
             │ No
             ▼
┌─────────────────────────────────┐
│  Has @StableMarker descendant?  │───Yes──→ [STABLE]
└────────────┬────────────────────┘
             │ No
             ▼
┌─────────────────────────────────┐
│  Is it an Enum class/entry?     │───Yes──→ [STABLE]
└────────────┬────────────────────┘
             │ No
             ▼
┌─────────────────────────────────┐
│  Is it an object (singleton)?   │───Yes──→ [STABLE]
└────────────┬────────────────────┘
             │ No
             ▼
┌─────────────────────────────────┐
│  Is it a Protobuf type?         │───Yes──→ [STABLE]
│  (GeneratedMessage/Lite)        │
└────────────┬────────────────────┘
             │ No
             ▼
┌─────────────────────────────────┐
│  Is it in KnownStableConstructs?│───Yes──→ [STABLE/RUNTIME]
│  (Pair, Triple, etc.)           │          (check type params)
└────────────┬────────────────────┘
             │ No
             ▼
┌─────────────────────────────────┐
│  Matches external config?       │───Yes──→ [STABLE/RUNTIME]
│  (stability-config.conf)        │          (check type params)
└────────────┬────────────────────┘
             │ No
             ▼
┌─────────────────────────────────┐
│  Is it an interface?            │───Yes──→ [UNKNOWN]
└────────────┬────────────────────┘
             │ No
             ▼
┌─────────────────────────────────┐
│  Is it from external Java?      │───Yes──→ [UNSTABLE]
└────────────┬────────────────────┘
             │ No
             ▼
┌─────────────────────────────────┐
│  Public/internal & declared in  │───Yes──→ [RUNTIME]
│  a different file? (JVM, for     │          (read $stable,
│  incremental compilation)       │           apply type-param mask)
└────────────┬────────────────────┘
             │ No
             ▼
┌─────────────────────────────────┐
│  External stub with a stability │───Yes──→ [RUNTIME] / [UNSTABLE]
│  bitmask? (@StabilityInferred)  │          (no bitmask → UNSTABLE)
└────────────┬────────────────────┘
             │ No
             ▼
┌─────────────────────────────────┐
│  Seed stability:                │
│  final class → start STABLE     │
│  non-final   → start UNKNOWN     │
└────────────┬────────────────────┘
             │
             ▼
┌─────────────────────────────────┐
│  Analyze class members:         │
│  - Check all properties         │
│  - Check backing fields         │
│  - Check superclass             │
│    (ignored if Unknown)         │
└────────────┬────────────────────┘
             │
             ▼
┌─────────────────────────────────┐
│  Any non-delegated var          │───Yes──→ [UNSTABLE]
│  (mutable) property?            │
└────────────┬────────────────────┘
             │ No
             ▼
┌─────────────────────────────────┐
│  All members stable?            │───Yes──→ [STABLE]
└────────────┬────────────────────┘
             │ No
             ▼
        [UNSTABLE/COMBINED/UNKNOWN]
```

#### Decision Tree for Generic Types

When analyzing generic types, the compiler follows an additional decision path:

```
┌─────────────────────────────────┐
│   Generic Type: Class<T1, T2>   │
└────────────┬────────────────────┘
             │
             ▼
┌─────────────────────────────────┐
│  Base class stable?             │───No───→ [UNSTABLE]
└────────────┬────────────────────┘
             │ Yes
             ▼
┌─────────────────────────────────┐
│  Has stability bitmask?         │───No───→ Analyze each
│  (from KnownStableConstructs    │         type parameter
│   or external config)           │         individually
└────────────┬────────────────────┘
             │ Yes
             ▼
┌─────────────────────────────────┐
│  For each type parameter Ti:    │
│  Is bit i set in bitmask?       │───No───→ Ti doesn't affect
└────────────┬────────────────────┘         stability
             │ Yes
             ▼
┌─────────────────────────────────┐
│  Check stability of actual      │
│  type argument for Ti           │───→ Combine all results
└─────────────────────────────────┘
```

#### Expression Stability Decision Tree

For expressions (used in default parameters and composable bodies):

```
┌─────────────────────────────────┐
│    Expression to analyze        │
└────────────┬────────────────────┘
             │
             ▼
┌─────────────────────────────────┐
│  Is expression type stable?     │───Yes──→ [STABLE]
└────────────┬────────────────────┘
             │ No
             ▼
┌─────────────────────────────────┐
│  Is it a constant (IrConst)?    │───Yes──→ [STABLE]
└────────────┬────────────────────┘
             │ No
             ▼
┌─────────────────────────────────┐
│  Is it a stable function call?  │───Yes──→ Check function
│  (listOf, mapOf, etc.)          │          type parameters
└────────────┬────────────────────┘
             │ No
             ▼
┌─────────────────────────────────┐
│  Is it a val reference?         │───Yes──→ Check initializer
│  (non-mutable variable)         │          stability
└────────────┬────────────────────┘
             │ No
             ▼
┌─────────────────────────────────┐
│  Is it a composite with all     │───Yes──→ [STABLE]
│  stable sub-expressions?        │
└────────────┬────────────────────┘
             │ No
             ▼
         [UNSTABLE]
```

#### Key Decision Points Explained

**1. Early Exit Conditions:**
- Primitives, String, Unit, and function types are immediately stable
- Stability annotations override all other checks
- Enums (classes and entries) are always stable (finite, immutable values)
- Objects (singletons) are always stable

**2. Interface Handling:**
- Interfaces return `Unknown` stability because implementations can vary
- Exception: Interfaces with `@Stable` marker are trusted

**3. External Types:**
- Java types default to unstable (mutable by default in Java)
- Protobuf types are special-cased as stable (immutable messages)
- External Kotlin modules use `@StabilityInferred` bitmasks

**4. Member Analysis:**
- The seed stability is `Stable` for `final` classes and `Unknown(declaration)` for non-final (`open`/`abstract`) classes
- Any non-delegated `var` property makes the entire class unstable
- Delegated properties are analyzed based on their delegate type
- Superclass stability is combined into the result, but an `Unknown` superclass result is ignored (so an open superclass doesn't poison the subclass)

**5. Generic Type Resolution:**
- Bitmask encodes which type parameters affect stability
- Maximum 32 type parameters supported (32-bit bitmask)
- Type arguments are substituted and analyzed recursively

This decision tree is implemented across several key functions in the compiler, with the main entry point being `StabilityInferencer.stabilityOf()`.

## Chapter 3: The Inference Algorithm

### 3.1 Algorithm Overview

The stability inference algorithm operates following the decision tree shown above, with multiple optimization paths for early termination. The implementation spans several key components working together.

**Algorithm Structure:**

The algorithm short-circuits when it can determine stability definitively, avoiding unnecessary analysis.

### 3.2 Type-Level Analysis

#### Phase 1: Fast Path Type Checks

The compiler first checks for common stable types:

```kotlin
when {
    type is IrErrorType -> Stability.Unstable
    type is IrDynamicType -> Stability.Unstable

type.isUnit() ||
    type.isPrimitiveType() ||
    type.isFunctionOrKFunction() ||
    type.isSyntheticComposableFunction() ||
    type.isString() -> Stability.Stable
}
```

**Primitive Types:**
- Numeric: `Byte`, `Short`, `Int`, `Long`, `Float`, `Double`
- Boolean: `Boolean`
- Character: `Char`

**Function Types:**
- Standard functions: `Function0`, `Function1`, ..., `FunctionN`
- Kotlin functions: `KFunction0`, `KFunction1`, ..., `KFunctionN`
- Composable functions: `ComposableFunction0`, etc.

#### Phase 2: Type Parameter Handling

For generic type parameters:

```kotlin
type.isTypeParameter() -> {
    val classifier = type.classifierOrFail
    val arg = substitutions[classifier]
    val symbol = SymbolForAnalysis(classifier, emptyList(), analysisEntryFile)
    if (arg != null && symbol !in currentlyAnalyzing) {
        stabilityOf(arg, substitutions, currentlyAnalyzing + symbol, analysisEntryFile)
    } else {
        Stability.Parameter(classifier.owner as IrTypeParameter)
    }
}
```

**Substitution Example:**

```kotlin
class Container<T>(val item: T)

// Analyzing Container<Int>
// T is IrTypeParameter
// substitutions map: {T: Int}
// Result: stabilityOf(Int) = Stable
```

#### Phase 3: Nullable Type Unwrapping

Nullable types defer to their non-null counterpart:

```kotlin
type.isNullable() ->
    stabilityOf(type.makeNotNull(), substitutions, currentlyAnalyzing)
```

**Examples:**
- `Int?` → analyze `Int` → Stable
- `User?` → analyze `User` → depends on User structure

#### Phase 4: Inline Class Handling

Inline classes (value classes) have special handling:

```kotlin
type.isInlineClassType() -> {
    val inlineClassDeclaration = type.getClass()

if (inlineClassDeclaration.hasStableMarker()) {
        Stability.Stable
    } else {
        stabilityOf(
            type = getInlineClassUnderlyingType(inlineClassDeclaration),
            substitutions = substitutions,
            currentlyAnalyzing = currentlyAnalyzing
        )
    }
}
```

**Examples:**

```kotlin
@JvmInline
value class UserId(val value: Int)
// Checks: stabilityOf(Int) = Stable

@JvmInline
value class Token(val value: String)
// Checks: stabilityOf(String) = Stable

@JvmInline
@Stable
value class SpecialId(val list: MutableList<Int>)
// @Stable marker overrides underlying type analysis
// Result: Stable (by annotation)
```

### 3.3 Class-Level Analysis

#### Phase 5: Cycle Detection

To prevent infinite recursion with recursive types:

```kotlin
if (currentlyAnalyzing.contains(fullSymbol))
    return Stability.Unstable
```

**Example:**

```kotlin
class Node(val value: Int, val next: Node?)

// Analysis trace:
// 1. stabilityOf(Node) → add to currentlyAnalyzing
// 2. Check field: value: Int → Stable
// 3. Check field: next: Node? → unwrap nullable
// 4. Check field: next: Node → CYCLE DETECTED
// 5. Return Unstable
```

This conservative approach ensures termination while potentially marking some stable recursive types as unstable.

#### Phase 6: Annotation and Marker Checks

Quick checks for annotated or special types:

```kotlin
if (declaration.hasStableMarkedDescendant()) return Stability.Stable
if (declaration.isEnumClass || declaration.isEnumEntry) return Stability.Stable
if (declaration.isObject) return Stability.Stable
if (declaration.defaultType.isPrimitiveType()) return Stability.Stable
if (declaration.isProtobufType()) return Stability.Stable
```

**Stable Markers:**
- `@Stable` annotation
- `@Immutable` annotation
- Annotations marked with `@StableMarker`

**Enum Handling:**

All enum classes and enum entries are considered stable because:
1. Enum instances are singletons (referential equality works)
2. Enum state is immutable after initialization
3. Enum equality is based on identity

**Object Handling:**

`object` declarations (singletons) are always stable. There is exactly one instance, so identity comparison is always valid regardless of the object's members.

**Protobuf Detection:**

```kotlin
private fun IrClass.isProtobufType(): Boolean {
    if (!isFinalClass) return false
    val directParentClassName = superTypes
        .lastOrNull { !it.isInterface() }
        ?.classOrNull?.owner?.fqNameWhenAvailable?.toString()
    return directParentClassName == "com.google.protobuf.GeneratedMessageLite" ||
           directParentClassName == "com.google.protobuf.GeneratedMessage"
}
```

Generated protobuf classes are marked stable despite potentially containing mutable implementation details.

#### Phase 7: Known Constructs

The compiler maintains a registry of known stable types:

```kotlin
val stableTypes = mapOf(
    Pair::class.qualifiedName!! to 0b11,
    Triple::class.qualifiedName!! to 0b111,
    Comparator::class.qualifiedName!! to 0b1,
    Result::class.qualifiedName!! to 0b1,
    ClosedRange::class.qualifiedName!! to 0b1,
    ClosedFloatingPointRange::class.qualifiedName!! to 0b1,
    // Guava
    "com.google.common.collect.ImmutableList" to 0b1,
    "com.google.common.collect.ImmutableEnumMap" to 0b11,
    "com.google.common.collect.ImmutableMap" to 0b11,
    "com.google.common.collect.ImmutableEnumSet" to 0b1,
    "com.google.common.collect.ImmutableSet" to 0b1,
    // Kotlinx immutable
    "kotlinx.collections.immutable.ImmutableCollection" to 0b1,
    "kotlinx.collections.immutable.ImmutableList" to 0b1,
    "kotlinx.collections.immutable.ImmutableSet" to 0b1,
    "kotlinx.collections.immutable.ImmutableMap" to 0b11,
    "kotlinx.collections.immutable.PersistentCollection" to 0b1,
    "kotlinx.collections.immutable.PersistentList" to 0b1,
    "kotlinx.collections.immutable.PersistentSet" to 0b1,
    "kotlinx.collections.immutable.PersistentMap" to 0b11,
    // Dagger
    "dagger.Lazy" to 0b1,
    // Coroutines
    EmptyCoroutineContext::class.qualifiedName!! to 0,
    // Java types
    BigInteger::class.qualifiedName!! to 0,
    BigDecimal::class.qualifiedName!! to 0,
    Locale::class.qualifiedName!! to 0,
)
```

The integer value represents a bitmask indicating which type parameters affect stability (covered in Chapter 4). A mask of `0` means the type is stable regardless of its type arguments (e.g. `BigInteger`, `Locale`).

#### Phase 8: External Configuration

Users can provide configuration files declaring types as stable:

```kotlin
if (declaration.isExternalStableType()) {
    val baseStability = Stability.Stable
    return baseStability.applyTypeParameterMask(
        mask = externalTypeMatcherCollection
            .maskForName(declaration.fqNameWhenAvailable) ?: 0,
        typeParameters = typeParameters,
        substitutions,
        analyzing,
        analysisEntryFile,
    )
}
```

The same `applyTypeParameterMask` helper used for `KnownStableConstructs` (Phase 7) combines the base stability with the stability of the type arguments selected by the configured bitmask. Configuration file format is covered in Chapter 6.

#### Phase 9: Runtime Stability for Separately-Compiled Classes

Stability inference must be stable across **incremental compilation**, which is separated by file. If the compiler inferred concrete stability for a class declared in another file, a later edit to that file could silently invalidate the result without recompiling the dependents. To avoid this, classes that are part of the public/internal API and are declared in a **different file** than the one that started the analysis are forced to use *runtime* stability — i.e. the value of their generated `$stable` field is read at runtime instead of being inferred at compile time.

```kotlin
// `analysisEntryFile` is the file containing the element that started this
// stabilityOf() call tree; `fileContainingDeclaration` is where `declaration` lives.
val forcedToUseRuntimeStability = isTargetJvm &&
    (declaration.visibility.isPublicAPI ||
        declaration.visibility == DescriptorVisibilities.INTERNAL) &&
    (fileContainingDeclaration == null || fileContainingDeclaration != analysisEntryFile)

if (forcedToUseRuntimeStability) {
    val baseStability = Stability.Runtime(declaration)
    return baseStability.applyTypeParameterMask(
        mask = null, // null = consider every type parameter
        typeParameters = typeParameters,
        substitutions,
        analyzing,
        analysisEntryFile,
    )
}

// Classes that come from a separately-compiled module arrive as external stubs.
// Their stability is encoded in the @StabilityInferred bitmask.
if (declaration.origin == IrDeclarationOrigin.IR_EXTERNAL_DECLARATION_STUB) {
    val mask = declaration.stabilityParamBitmask() ?: return Stability.Unstable
    val baseStability = Stability.Runtime(declaration)
    return baseStability.applyTypeParameterMask(
        mask,
        typeParameters = typeParameters,
        substitutions,
        analyzing,
        analysisEntryFile,
    )
}
```

**Key points:**

1. The decision is driven by the **file** the declaration lives in (via `analysisEntryFile`), not by a "current module" check. This is what makes the result safe under incremental compilation.
2. `Stability.Runtime(declaration)` means "emit a read of `declaration.$stable` at runtime"; it is combined with the stability of the relevant type arguments via `applyTypeParameterMask`.
3. When `forcedToUseRuntimeStability`, `mask` is `null`, so **all** type parameters are taken into account. For genuine external stubs the precise `@StabilityInferred` bitmask is used instead.
4. An external stub with no `@StabilityInferred` bitmask (e.g. a third-party class compiled without the Compose compiler) is treated as `Unstable`.

> Note: the order of checks in the source is `Java stub → interface → external-stub-without-bitmask → forcedToUseRuntimeStability → external stub`. Phases 10 and 11 below (Java and interface handling) actually execute *before* this runtime-stability logic.

#### Phase 10: Java Type Handling

```kotlin
if (declaration.origin == IrDeclarationOrigin.IR_EXTERNAL_JAVA_DECLARATION_STUB) {
    return Stability.Unstable
}
```

Java types default to unstable because:
1. Java allows unrestricted mutability
2. No equivalent of Kotlin's `val` guarantee
3. No stability annotations in Java standard library

#### Phase 11: General Interface Handling

```kotlin
if (declaration.isInterface) {
    return Stability.Unknown(declaration)
}
```

Without concrete implementation details, interfaces have unknown stability.

#### Phase 12: Field-by-Field Analysis

For concrete classes in the current module:

```kotlin
var stability = if (declaration.modality == Modality.FINAL) {
    Stability.Stable
} else {
    Stability.Unknown(declaration)
}

for (member in declaration.declarations) {
    when (member) {
        is IrProperty -> {
            member.backingField?.let {
                if (member.isVar && !member.isDelegated) return Stability.Unstable
                stability += stabilityOf(it.type, substitutions, analyzing, analysisEntryFile)
            }
        }

is IrField -> {
            stability += stabilityOf(member.type, substitutions, analyzing, analysisEntryFile)
        }
    }
}

declaration.superClass?.let {
    val superClassStability = stabilityOf(it, substitutions, analyzing, analysisEntryFile)
    if (superClassStability !is Stability.Unknown) {
        stability += superClassStability
    }
}

return stability
```

**Key Points:**

1. The seed is `Stable` only for `final` classes; non-final (`open`/`abstract`) classes seed as `Unknown(declaration)`, since an unknown subclass could add unstable state
2. Any non-delegated `var` property immediately returns `Unstable`
3. Combine stability of all backing-field (`val`) property types
4. Include superclass stability — but only when it is **not** `Unknown` (an `Unknown` superclass result is dropped rather than propagated)
5. Use the `+` operator for combination (see 2.6)

### 3.4 Expression-Level Analysis

Beyond type stability, the compiler analyzes expression stability:

```kotlin
fun stabilityOf(expr: IrExpression): Stability {
    val stability = stabilityOf(expr.type)
    if (stability.knownStable()) return stability

return when (expr) {
        is IrConst -> Stability.Stable
        is IrCall -> stabilityOf(expr, stability)
        is IrGetValue -> /* analyze variable */
        is IrLocalDelegatedPropertyReference -> Stability.Stable
        is IrComposite -> /* analyze all statements */
        else -> stability
    }
}
```

#### Constant Expressions

Literal constants are always stable:

```kotlin
val x = 42           // IrConst(42) → Stable
val s = "text"       // IrConst("text") → Stable
val b = true         // IrConst(true) → Stable
```

#### Function Call Expressions

The compiler checks known stable functions:

```kotlin
private fun stabilityOf(expr: IrCall, baseStability: Stability): Stability {
    val function = expr.symbol.owner
    val fqName = function.kotlinFqName

return when (val mask = KnownStableConstructs.stableFunctions[fqName.asString()]) {
        null -> baseStability
        0 -> Stability.Stable
        else -> Stability.Combined(/* check type arguments */)
    }
}
```

**Known Stable Functions:**

```kotlin
val stableFunctions = mapOf(
    "kotlin.collections.emptyList" to 0,
    "kotlin.collections.listOf" to 0b1,
    "kotlin.collections.listOfNotNull" to 0b1,
    "kotlin.collections.mapOf" to 0b11,
    "kotlin.collections.emptyMap" to 0,
    "kotlin.collections.setOf" to 0b1,
    "kotlin.collections.emptySet" to 0,
    "kotlin.to" to 0b11,
    // Kotlinx immutable
    "kotlinx.collections.immutable.immutableListOf" to 0b1,
    "kotlinx.collections.immutable.immutableSetOf" to 0b1,
    "kotlinx.collections.immutable.immutableMapOf" to 0b11,
    "kotlinx.collections.immutable.persistentListOf" to 0b1,
    "kotlinx.collections.immutable.persistentSetOf" to 0b1,
    "kotlinx.collections.immutable.persistentMapOf" to 0b11,
)
```

#### Variable Reference Expressions

For `val` variables, the compiler checks initializer stability:

```kotlin
val x = 42              // Stable initializer
val y = x               // IrGetValue(x) → Stable

var z = 42              // Mutable variable
val w = z               // IrGetValue(z) → use type stability
```

## Chapter 4: Implementation Mechanisms

### 4.1 Bitmask Encoding

Generic types use bitmasks to encode type parameter dependencies.

#### Encoding Scheme

Each type parameter is represented by a single bit:
- Bit N = 1: Type parameter N affects stability
- Bit N = 0: Type parameter N does not affect stability

**Maximum Limit:** 32 type parameters (Int size constraint)

**Examples:**

```kotlin
// Pair<A, B>
@StabilityInferred(parameters = 0b11)
// Binary: 00000000000000000000000000000011
// Bit 0: A affects stability
// Bit 1: B affects stability

// Triple<A, B, C>
@StabilityInferred(parameters = 0b111)
// Binary: 00000000000000000000000000000111
// Bit 0: A affects stability
// Bit 1: B affects stability
// Bit 2: C affects stability

// Result<T>
@StabilityInferred(parameters = 0b1)
// Binary: 00000000000000000000000000000001
// Bit 0: T affects stability
```

#### Special Bit: Known Stable

If bit position `typeParams.size` is set, the class is known stable:

```kotlin
@StabilityInferred(parameters = 0b101)
class Container<T, U>
// Binary: 00000000000000000000000000000101
// Bit 0: T affects stability
// Bit 1: U does not affect stability
// Bit 2: Known stable bit (1 shl 2 where typeParams.size = 2)
```

This indicates the class is stable regardless of type parameter instantiation.

#### Bitmask Application

This is implemented by the `Stability.applyTypeParameterMask` extension function. Note that `mask` is nullable: a `null` mask means "consider every type parameter" (used for the incremental-compilation `Runtime` path), while a concrete mask selects parameters bit-by-bit.

```kotlin
private fun Stability.applyTypeParameterMask(
    mask: Int?,
    typeParameters: List<IrTypeParameter>,
    substitutions: Map<IrTypeParameterSymbol, IrTypeArgument>,
    currentlyAnalyzing: Set<SymbolForAnalysis>,
    analysisEntryFile: IrFile?,
): Stability {
    return when {
        mask == 0 || typeParameters.isEmpty() -> this
        else -> this + Stability.Combined(
            typeParameters.mapIndexedNotNull { index, irTypeParameter ->
                if (index >= 32) return@mapIndexedNotNull null
                if (mask == null || mask and (0b1 shl index) != 0) {
                    val sub = substitutions[irTypeParameter.symbol]
                    if (sub != null)
                        stabilityOf(sub, substitutions, currentlyAnalyzing, analysisEntryFile)
                    else
                        Stability.Parameter(irTypeParameter)
                } else null
            }
        )
    }
}
```

**Process:**
1. For each type parameter at index I (capped at 32)
2. If `mask` is `null`, or bit I is set in `mask`, include that parameter
3. When included, add the stability of the substituted type argument (or `Parameter` if not yet substituted)
4. Otherwise, ignore that parameter

### 4.2 Runtime Field Generation

#### JVM Platform

For JVM targets, the compiler generates a static field:

```kotlin
// Source
class Box<T>(val value: T)

// Generated
@StabilityInferred(parameters = 0b1)
class Box<T>(val value: T) {
    companion object {
        @JvmField
        public static final int $stable = 0
    }
}
```

The field name is always `$stable`, and it is initialized with a computed stability value.

**Stability Values:**

```kotlin
enum class StabilityBits(val bits: Int) {
    UNSTABLE(0b100),
    STABLE(0b000);

fun bitsForSlot(slot: Int): Int = bits shl (1 + slot * 3)
}
```

`bitsForSlot` positions the stability bits for a given parameter slot; the `$stable` field stored on a class uses slot `0` (i.e. `bits shl 1`).

#### Non-JVM Platforms

For Native and JS targets, the compiler generates, at the package (top) level:
1. A private backing field with a mangled, FQN-derived name (`<fqName>$stableprop_field` style)
2. A property wrapping that field
3. A separate getter **function** with a mangled name, registered as metadata-visible

```kotlin
// Generated for Native/JS (names are derived from the class FQN)
private val `com_example_Box$stableprop_field`: Int = /* computed */

// Registered via metadataDeclarationRegistrar.registerFunctionAsMetadataVisible(...)
@Deprecated(
    level = DeprecationLevel.HIDDEN,
    message = "Synthetic declaration generated by the Compose compiler. Please do not use."
)
// @HiddenFromObjC is added only on Native targets
fun `com_example_Box$stableprop_getter`(): Int =
    `com_example_Box$stableprop_field`
```

**Rationale:**

A separate getter function is used (instead of a plain field getter) because `registerFunctionAsMetadataVisible` does not work for a field getter and there is no API to register properties as metadata-visible. Making the getter metadata-visible is what enables cross-module stability reads. On Native, the getter is additionally annotated with `@HiddenFromObjC`. Dependencies compiled with an older plugin that lack this getter are treated as `Unstable`, and a configuration warning is emitted advising an upgrade (to avoid extra recompositions on non-JVM targets).

### 4.3 Annotation Processing

#### @StabilityInferred Annotation

```kotlin
private fun IrAnnotationContainer.stabilityParamBitmask(): Int? =
    (annotations.findAnnotation(ComposeFqNames.StabilityInferred)?.arguments[0] as? IrConst)
        ?.value as? Int
```

The annotation carries a single integer parameter representing the bitmask.

#### Annotation Generation

```kotlin
val annotation = IrAnnotationImpl(
    UNDEFINED_OFFSET,
    UNDEFINED_OFFSET,
    StabilityInferredClass.defaultType,
    StabilityInferredClass.constructors.first(),
    typeArgumentsCount = 0,
    constructorTypeArgumentsCount = 0,
    origin = null
).also {
    it.arguments[0] = irConst(parameterMask)
}

if (useK2 && cls.hasFirDeclaration()) {
    context.metadataDeclarationRegistrar.addMetadataVisibleAnnotationsToElement(
        cls,
        annotation,
    )
} else {
    cls.annotations += annotation
    classStabilityInferredCollection?.addClass(cls, parameterMask)
}
```

The annotation is created as an `IrAnnotationImpl` (earlier compiler versions used `IrConstructorCallImpl`). Under K2 it is attached as a metadata-visible annotation; otherwise it is added directly to the class and also recorded in the `classStabilityInferredCollection`.

### 4.4 Normalization Process

Before using stability for code generation, the compiler normalizes it:

```kotlin
fun Stability.normalize(): Stability {
    when (this) {
        is Stability.Certain,
        is Stability.Parameter,
        is Stability.Runtime,
        is Stability.Unknown,
        -> return this

is Stability.Combined -> { /* normalize */ }
    }

val parameters = mutableSetOf<IrTypeParameterSymbol>()
    val parts = mutableListOf<Stability>()
    val stack = mutableListOf<Stability>(this)

while (stack.isNotEmpty()) {
        when (val stability: Stability = stack.removeAt(stack.size - 1)) {
            is Stability.Combined -> {
                stack.addAll(stability.elements)
            }

is Stability.Certain -> {
                if (!stability.stable)
                    return Stability.Unstable
            }

is Stability.Parameter -> {
                if (stability.parameter.symbol !in parameters) {
                    parameters.add(stability.parameter.symbol)
                    parts.add(stability)
                }
            }

is Stability.Runtime -> parts.add(stability)
            is Stability.Unknown -> { /* ignore */ }
        }
    }

return Stability.Combined(parts)
}
```

**Normalization Operations:**

1. **Flatten Combined**: Recursively expand nested `Combined` instances
2. **Remove Unknown**: `Unknown` elements are discarded (treated as uncertain)
3. **Deduplicate Parameters**: Keep only unique type parameters
4. **Short-circuit on Unstable**: Return immediately if any `Certain(false)` found
5. **Collect Runtime and Parameter**: Preserve these for runtime checks

**Result Types:**
- `Stability.Unstable` if any component is unstable
- `Stability.Combined([...])` with deduplicated elements otherwise

## Chapter 5: Case Studies

### 5.1 Primitive and Built-in Types

#### Integer Types

```kotlin
val x: Int = 42
// Analysis: type.isPrimitiveType() → true
// Result: Stability.Certain(stable = true)
```

All primitive numeric types follow the same pattern.

#### String Type

```kotlin
val s: String = "text"
// Analysis: type.isString() → true
// Result: Stability.Certain(stable = true)
```

`String` receives special treatment due to its immutability guarantees.

#### Function Types

```kotlin
val f: (Int) -> String = { it.toString() }
// Analysis: type.isFunctionOrKFunction() → true
// Result: Stability.Certain(stable = true)
```

Function types are stable because:
1. Function references are immutable
2. Capturing lambdas capture immutable values (or create new closures)
3. Function equality is well-defined

### 5.2 User-Defined Classes

#### Simple Data Class

```kotlin
data class User(
    val id: Int,
    val name: String
)

// Analysis:
// 1. Not in fast path
// 2. No annotations
// 3. Not in known constructs
// 4. Field analysis:
//    - id: Int → Stable
//    - name: String → Stable
// 5. Combine: Stable + Stable = Stable
// Result: Stability.Certain(stable = true)
```

#### Class with Mutable Property

```kotlin
class Counter(
    var count: Int
)

// Analysis:
// 1. Field analysis:
//    - count is var → immediate return
// Result: Stability.Certain(stable = false)
```

#### Class with Mixed Properties

```kotlin
class Mixed(
    val stable: String,
    var unstable: Int
)

// Analysis:
// 1. Field analysis:
//    - stable: String → Stable
//    - unstable is var → immediate return
// Result: Stability.Certain(stable = false)
```

The presence of any `var` property makes the entire class unstable.

### 5.3 Generic Types

#### Simple Generic Container

```kotlin
class Box<T>(val value: T)

// Analysis:
// 1. Field analysis:
//    - value: T → Stability.Parameter(T)
// 2. Generate annotation: @StabilityInferred(parameters = 0b1)
// Result: Stability.Combined([Stability.Parameter(T)])

// Instantiation:
val intBox: Box<Int>
// Substitute T → Int
// stabilityOf(Int) = Stable
// Result: Stable

val counterBox: Box<Counter>
// Substitute T → Counter
// stabilityOf(Counter) = Unstable (from 5.2)
// Result: Unstable
```

#### Multiple Type Parameters

```kotlin
class Pair<A, B>(val first: A, val second: B)

// Analysis:
// 1. Field analysis:
//    - first: A → Parameter(A)
//    - second: B → Parameter(B)
// 2. Generate annotation: @StabilityInferred(parameters = 0b11)
// Result: Combined([Parameter(A), Parameter(B)])

// Instantiation:
val pair: Pair<Int, String>
// Substitute A → Int, B → String
// stabilityOf(Int) = Stable
// stabilityOf(String) = Stable
// Result: Stable
```

#### Nested Generic Types

```kotlin
class Container<T>(val items: List<T>)

// Analysis:
// 1. Field analysis:
//    - items: List<T>
//      - List is known stable construct (mask = 0b1)
//      - Check type arg T → Parameter(T)
// 2. Result: Combined([Parameter(T)])

// Instantiation:
val container: Container<String>
// items: List<String>
// List stability depends on String
// stabilityOf(String) = Stable
// Result: Stable
```

### 5.4 External Dependencies

#### External Class with Annotation

```kotlin
// Library module (compiled separately)
@StabilityInferred(parameters = 0b1)
class LibraryBox<T>(val value: T) {
    companion object {
        @JvmField
        val $stable: Int = 0
    }
}

// Your module
fun useLibraryBox(box: LibraryBox<Int>) {
    // Analysis:
    // 1. box: LibraryBox<Int>
    // 2. LibraryBox is external
    // 3. Has @StabilityInferred(0b1)
    // 4. Create Stability.Runtime(LibraryBox)
    // 5. Check bit 0 (T parameter)
    // 6. Add stabilityOf(Int) = Stable
    // Result: Combined([Runtime(LibraryBox), Stable])
    // At runtime: check LibraryBox.$stable field
}
```

#### External Class Without Annotation

```kotlin
// Third-party library (no Compose compiler)
class ThirdPartyType(val data: String)

// Your module
fun useThirdParty(obj: ThirdPartyType) {
    // Analysis:
    // 1. ThirdPartyType is external
    // 2. No @StabilityInferred annotation
    // 3. stabilityParamBitmask() returns null
    // Result: Stability.Unstable
}
```

### 5.5 Interface and Abstract Types

#### Interface Parameter

```kotlin
interface Repository {
    fun getData(): String
}

class Screen(val repo: Repository)

// Analysis of Repository:
// 1. Repository is interface
// 2. Unknown implementations
// Result: Stability.Unknown(Repository)

// Analysis of Screen:
// 1. Field repo: Repository → Unknown
// Result: Combined([Unknown(Repository)])
// Runtime: use instance comparison for repo
```

#### Abstract Class

```kotlin
abstract class BaseViewModel {
    abstract val state: String
}

class Screen(val viewModel: BaseViewModel)

// Analysis:
// 1. BaseViewModel is abstract (not an interface), so it reaches member analysis
// 2. modality != FINAL -> seed stability is Unknown(BaseViewModel)
// 3. `abstract val state` has no backing field, so no member contributes
// Result: Stability.Unknown(BaseViewModel)
```

A non-final class seeds as `Unknown`. If it had concrete `val` properties with backing fields, those would still be combined in — but the `Unknown` seed keeps the overall result uncertain unless something resolves it.

#### Interface with @Stable

```kotlin
@Stable
interface StableRepository {
    fun getData(): String
}

class Screen(val repo: StableRepository)

// Analysis of StableRepository:
// 1. Check annotations
// 2. Has @Stable marker
// Result: Stability.Certain(stable = true)

// Analysis of Screen:
// 1. Field repo: StableRepository → Stable
// Result: Stable
```

### 5.6 Inheritance Hierarchies

#### Stable Inheritance

```kotlin
open class Base(val id: Int)
class Derived(val name: String) : Base(0)

// Analysis of Derived:
// 1. Field analysis:
//    - name: String → Stable
// 2. Check superclass: Base
//    - Field id: Int → Stable
// 3. Combine: Stable + Stable = Stable
// Result: Stability.Certain(stable = true)
```

#### Unstable Inheritance

```kotlin
open class Base(var state: Int)
class Derived(val data: String) : Base(0)

// Analysis of Base:
// 1. Field state is var
// Result: Unstable

// Analysis of Derived:
// 1. Field data: String → Stable
// 2. Check superclass: Base → Unstable
// 3. Combine: Stable + Unstable = Unstable
// Result: Stability.Certain(stable = false)
```

The unstable superclass makes all derived classes unstable.

## Chapter 6: Configuration and Tooling

### 6.1 Stability Annotations

#### @Stable Annotation

Declares that a type's public API is stable:

```kotlin
@Stable
class MutableCounter(private var count: Int) {
    fun increment() {
        count++
        // Must trigger recomposition
    }

override fun equals(other: Any?): Boolean {
        return other is MutableCounter && count == other.count
    }
}
```

**Contract:**
1. Public API appears immutable (private var is internal)
2. `equals()` implements structural equality
3. State changes trigger composition invalidation

**Warning:** Incorrect usage violates runtime assumptions.

#### @Immutable Annotation

Stronger guarantee than `@Stable`:

```kotlin
@Immutable
class ImmutableData(val value: String)
```

**Contract:**
1. All observable state is truly immutable
2. No mutable fields (even private)
3. `equals()` implements structural equality

#### Compiler-Level Differences: @Stable vs @Immutable

While both annotations mark types as stable for recomposition skipping, there is one significant compiler-level difference.

**Stability Inference Treatment**

Both annotations are processed identically through `hasStableMarker()`:

```kotlin
fun IrAnnotationContainer.hasStableMarker(): Boolean =
    annotations.any { it.isStableMarker() }
```

Both result in:
- Same stability inference (types marked as `Stability.Certain(stable = true)`)
- Same `@StabilityInferred` annotation generation
- Same `$stable` runtime field generation
- Same recomposition skipping behavior

**Static Expression Optimization (Key Difference)**

`@Immutable` has special treatment for static expression detection.

```kotlin
private fun IrConstructorCall.isStatic(): Boolean {
    // special case for inline classes
    if (type.isInlineClassType()) {
        return stabilityInferencer.stabilityOf(type.unboxInlineClass()).knownStable() &&
                arguments[0]?.isStatic() == true
    }

// @Immutable constructors with static args are static
    if (symbol.owner.parentAsClass.hasAnnotationSafe(ComposeFqNames.Immutable)) {
        return areAllArgumentsStatic()
    }

// @Stable constructors are NOT considered static
    return false
}
```

**Practical Impact:**

```kotlin
@Immutable
data class ImmutablePoint(val x: Int, val y: Int)

@Stable
data class StablePoint(val x: Int, val y: Int)

@Composable
fun Example() {
    // Static expression - enables additional optimizations
    val immutable = ImmutablePoint(10, 20)

// NOT a static expression - standard optimizations only
    val stable = StablePoint(10, 20)

// Lambda with immutable capture can be more aggressively memoized
    val lambda1 = { immutable.x }  // Better optimization

// Lambda with stable capture has standard memoization
    val lambda2 = { stable.x }      // Standard optimization
}
```

**Optimization Benefits of Static Expressions:**

1. **Lambda Memoization**: Static captures don't prevent lambda singleton optimization
2. **Default Parameters**: Static defaults can be computed at compile time
3. **Remember Optimization**: Compiler may skip unnecessary remember calls for static values

**Summary Table:**

| Aspect | @Stable | @Immutable |
|--------|---------|------------|
| Stability inference | Stable | Stable |
| Recomposition skipping | Enabled | Enabled |
| Constructor staticness | No | Yes (with static args) |
| Lambda capture optimization | Standard | Enhanced |
| Semantic contract | Allows private mutability | Truly immutable |

The compiler treats `@Immutable` as a stronger guarantee that enables additional compile-time optimizations, particularly for static expression evaluation and lambda memoization.

#### @StableMarker Meta-Annotation

Create custom stability markers:

```kotlin
@StableMarker
annotation class MyStable

@MyStable
class CustomType(val data: String)
// Treated as @Stable
```

### 6.2 Configuration Files

#### File Format

Create a [Stability configuration file](https://developer.android.com/develop/ui/compose/performance/stability/fix#configuration-file), `stability_config.conf`:

```
// Single class
com.example.ExternalType

// Package wildcard
com.example.models.**

// Single-segment wildcard
com.example.*.data

// Generic parameter inclusion
com.example.Container<*>

// Generic parameter exclusion
com.example.Wrapper<*,_>

// Mixed generic parameters
com.example.Complex<*,_,*>
```

#### Pattern Syntax

**Wildcard Rules:**
- `*`: Matches single package segment
- `**`: Matches multiple package segments
- `<*>`: Generic parameter affects stability
- `<_>`: Generic parameter ignored for stability

**Generic Parameter Encoding:**

```
Container<*,_,*>
          │ │ │
          │ │ └─ Bit 2 set (third param matters)
          │ └─── Bit 1 clear (second param ignored)
          └───── Bit 0 set (first param matters)
// Bitmask: 0b101 = 5
```

#### Gradle Configuration

To enable this feature, pass the path of the configuration file to the composeCompiler options block of the [Compose compiler Gradle plugin](https://developer.android.com/develop/ui/compose/compiler) configuration.

```kotlin
composeCompiler {
  stabilityConfigurationFile = rootProject.layout.projectDirectory.file("stability_config.conf")
}
```

### 6.3 Compiler Reports

#### Enabling Reports

```kotlin
composeCompiler {
    reportsDestination = layout.buildDirectory.dir("compose_reports")
    metricsDestination = layout.buildDirectory.dir("compose_metrics")
}
```

#### Generated Files

**`<module>-classes.txt`**: Class stability analysis

```
stable class User {
  stable val id: Int
  stable val name: String
}

unstable class Counter {
  unstable var count: Int
}

runtime stable class Box {
  stable val value: T
}
```

**`<module>-composables.txt`**: Composable function analysis

```
restartable skippable scheme("[androidx.compose.ui.UiComposable]") fun UserProfile(
  stable user: User
)

restartable scheme("[androidx.compose.ui.UiComposable]") fun Counter(
  unstable counter: Counter
)
```

**`<module>-module.json`**: Metrics summary

```json
{
  "skippableComposables": 45,
  "restartableComposables": 50,
  "readonlyComposables": 5,
  "totalComposables": 100,
  "restartGroups": 50,
  "totalGroups": 75
}
```

### 6.4 Common Issues and Solutions

#### Issue 1: Accidental var Usage

**Problem:**

```kotlin
data class UserState(var loading: Boolean)
// Unstable due to var
```

**Solution:**

```kotlin
data class UserState(val loading: Boolean)
// Stable
```

#### Issue 2: Mutable Collections

**Problem:**

```kotlin
class ViewModel(val items: MutableList<String>)
// MutableList is unstable
```

**Solution:**

```kotlin
import kotlinx.collections.immutable.ImmutableList

class ViewModel(val items: ImmutableList<String>)
// ImmutableList is stable (in KnownStableConstructs)
```

**Alternative (requires configuration):**

```kotlin
class ViewModel(val items: List<String>)
// List is an interface with Unknown stability by default
// To make it stable, add to stability-config.txt:
// kotlin.collections.List
```

#### Issue 3: Interface Parameters

**Problem:**

```kotlin
interface DataSource { }

@Composable
fun Screen(source: DataSource) {
    // DataSource has Unknown stability
    // Falls back to instance comparison
}
```

**Solution A: Add @Stable**

```kotlin
@Stable
interface DataSource { }
```

**Solution B: Use concrete type**

```kotlin
@Composable
fun Screen(source: ConcreteDataSource) {
    // Concrete class can have known stability
}
```

#### Issue 4: External Library Types

**Problem:**

```kotlin
// Third-party library without Compose support
class LibraryClass(val data: String)

@Composable
fun Display(obj: LibraryClass) {
    // LibraryClass marked unstable
}
```

**Solution:**

Add to `stability-config.txt`:

```
com.thirdparty.LibraryClass
```

#### Issue 5: Inheritance from Unstable Base

**Problem:**

```kotlin
open class MutableBase(var state: Int)

class DerivedData(val name: String) : MutableBase(0)
// Unstable due to base class
```

**Solution:**

Restructure to avoid mutable inheritance:

```kotlin
open class Base(val id: Int)

class DerivedData(val name: String, val state: Int) : Base(0)
// Stable if all fields are stable
```

## Chapter 7: Advanced Topics

### 7.1 Type Substitution

#### Substitution Map Construction

```kotlin
private fun IrSimpleType.substitutionMap(): Map<IrTypeParameterSymbol, IrTypeArgument> {
    val cls = classOrNull ?: return emptyMap()
    val params = cls.owner.typeParameters.map { it.symbol }
    val args = arguments
    return params.zip(args).filter { (param, arg) ->
        param != (arg as? IrSimpleType)?.classifier
    }.toMap()
}
```

**Example:**

```kotlin
class Container<T, U>(val first: T, val second: U)

// Analyzing Container<Int, String>
// typeParameters = [T, U]
// arguments = [Int, String]
// substitutionMap = {T: Int, U: String}
```

#### Substitution Application

When analyzing `Container<Int, String>`:

```kotlin
// Field: first: T
stabilityOf(T, substitutions = {T: Int, U: String})
// Lookup T in substitutions → Int
// Result: stabilityOf(Int) = Stable

// Field: second: U
stabilityOf(U, substitutions = {T: Int, U: String})
// Lookup U in substitutions → String
// Result: stabilityOf(String) = Stable
```

#### Nested Substitution

```kotlin
class Outer<T>(val inner: Inner<T>)
class Inner<U>(val value: U)

// Analyzing Outer<Int>
// 1. Field inner: Inner<T>
// 2. Inner has type parameter U
// 3. U is substituted with T
// 4. T is substituted with Int
// 5. Result: stabilityOf(Int) = Stable
```

### 7.2 Cycle Detection

#### Detection Mechanism

```kotlin
data class SymbolForAnalysis(
    val symbol: IrClassifierSymbol,
    val typeParameters: List<IrTypeArgument?>,
    // The file containing the element that initiated this stabilityOf request tree.
    // Two identical symbols analyzed from different entry files are distinct keys,
    // which is what keeps the per-file caching/runtime-stability behavior correct.
    val analysisEntryFile: IrFile?,
)

// In stabilityOf(declaration: IrClass)
val fullSymbol = SymbolForAnalysis(symbol, typeArguments, analysisEntryFile)

if (currentlyAnalyzing.contains(fullSymbol))
    return Stability.Unstable
```

The `currentlyAnalyzing` set tracks the analysis stack to detect cycles. The `analysisEntryFile` is part of the key because the same type can resolve to different stability depending on which file initiated the analysis (see Phase 9).

#### Example: Self-Referential Type

```kotlin
class Node(val value: Int, val next: Node?)

// Analysis trace:
// stabilityOf(Node, currentlyAnalyzing = {})
//   analyzing = {Node}
//   field: value: Int → Stable
//   field: next: Node?
//     unwrap nullable
//     stabilityOf(Node, currentlyAnalyzing = {Node})
//       CYCLE DETECTED: Node in currentlyAnalyzing
//       return Unstable
```

#### Limitation

Some recursive types that could be stable are marked unstable:

```kotlin
class TreeNode(val value: Int, val left: TreeNode?, val right: TreeNode?)
// Could be stable (immutable structure)
// Marked unstable due to cycle detection
```

This conservative approach ensures algorithm termination.

### 7.3 Special Cases

#### Protobuf Types

**Detection:**

**Rationale:**

Generated protobuf classes use internal mutability for builder patterns but present an immutable API. The compiler treats them as stable based on their parent class.

#### Delegated Properties

```kotlin
if (member.isVar && !member.isDelegated)
    return Stability.Unstable
```

Delegated `var` properties are not automatically unstable. The stability depends on the delegate implementation.

**Example:**

```kotlin
class WithDelegate {
    var value: String by mutableStateOf("")
    // Delegated to MutableState
    // Compose tracks state changes
    // Can be stable with proper delegate
}
```

#### Inline Classes with Markers

```kotlin
if (inlineClassDeclaration.hasStableMarker()) {
    Stability.Stable
}
```

Inline classes can override underlying type stability with annotations:

```kotlin
@JvmInline
@Stable
value class Wrapper(val list: MutableList<Int>)
// Underlying type (MutableList) is unstable
// But @Stable annotation overrides
// Result: Stable (developer responsibility)
```

## Chapter 8: Compiler Analysis System

### 8.1 Analysis Infrastructure

The Compose compiler stores per-element metadata during compilation and reads it back in later phases. There are two distinct mechanisms: **IR attributes** for the IR (backend/lowering) phase, and **WritableSlices** on the K1 frontend.

> Note: most of Chapter 8 describes the **K1 (descriptor/PSI-based) frontend**, which is now the *legacy* path. K2/FIR is the default frontend and lives under the `k2/` package. The concepts below are still useful, but the exact APIs shown are mostly K1.

#### IR Attributes: IR-phase Data Flow

`lower/ComposePluginAttributes.kt`

In the IR phase the compiler no longer uses a `BindingContext`. Instead, metadata is attached directly to IR nodes via delegated **IR attribute / flag** properties (`irAttribute(...)` / `irFlag(...)`):

```kotlin
// lower/ComposePluginAttributes.kt (illustrative)
var IrExpression.isStaticExpression: Boolean by irFlag(/* ... */)
var IrExpression.isStaticFunctionExpression: Boolean by irFlag(/* ... */)
var IrElement.isComposableSingleton: Boolean by irFlag(/* ... */)
var IrElement.isComposableSingletonClass: Boolean by irFlag(/* ... */)
var IrElement.durableFunctionKey: KeyInfo? by irAttribute(/* ... */)
var IrElement.hasTransformedLambda: Boolean by irFlag(/* ... */)
```

These attributes carry critical metadata:
- **isStaticExpression**: Marks expressions that can be evaluated at compile time
- **durableFunctionKey**: Stores unique keys for functions to enable hot reload
- **isComposableSingleton / isComposableSingletonClass**: Marks hoisted composable lambda singletons

They are written and read as plain properties on the IR node, e.g. `expr.isStaticExpression = true` during analysis and `if (expr.isStaticExpression) { ... }` during lowering — no trace/context lookup is involved.

#### Frontend WritableSlices (K1)

`k1/FrontendWritableSlices.kt`

On the K1 frontend, analysis results are recorded in a `BindingTrace` via `WritableSlice` keys:

```kotlin
object FrontendWritableSlices {
    val INFERRED_COMPOSABLE_DESCRIPTOR = WritableSlice<FunctionDescriptor, Boolean>()
    val LAMBDA_CAPABLE_OF_COMPOSER_CAPTURE = WritableSlice<FunctionDescriptor, Boolean>()
    val INFERRED_COMPOSABLE_LITERAL = WritableSlice<KtLambdaExpression, Boolean>()
    val COMPOSE_LAZY_SCHEME = WritableSlice<Any, LazyScheme>()
}
```

These slices are stored in the `BindingContext`/`BindingTrace` that persists throughout K1 frontend analysis:

```kotlin
// During analysis
trace.record(FrontendWritableSlices.INFERRED_COMPOSABLE_DESCRIPTOR, descriptor, true)

// Reading back
val inferred = trace.bindingContext[
    FrontendWritableSlices.INFERRED_COMPOSABLE_DESCRIPTOR, descriptor
]
```

### 8.2 Composable Call Validation

`k1/ComposableCallChecker.kt`

The composable call checker ensures composable functions are only called from valid contexts.

#### Context Checking Algorithm

The checker walks up the PSI tree from each composable call site:

```kotlin
override fun check(resolvedCall: ResolvedCall<*>, reportOn: PsiElement, context: CallCheckerContext) {
    // Walk up PSI tree to find composable context
    var node: PsiElement? = reportOn
    while (node != null) {
        when (node) {
            is KtLambdaExpression -> {
                val descriptor = bindingContext[BindingContext.FUNCTION, node.functionLiteral]
                if (descriptor?.isComposableCallable() == true) {
                    return // Valid: inside composable lambda
                }
            }

is KtFunction -> {
                val descriptor = bindingContext[BindingContext.FUNCTION, node]
                if (descriptor?.isComposableCallable() == true) {
                    return // Valid: inside composable function
                }
            }

is KtPropertyAccessor -> {
                val descriptor = bindingContext[BindingContext.PROPERTY_ACCESSOR, node]
                if (descriptor?.isComposableCallable() == true) {
                    return // Valid: inside composable property
                }
            }

is KtTryExpression -> {
                // Check if composable call is inside try/catch
                if (node.tryBlock.isAncestor(reportOn)) {
                    context.trace.report(ILLEGAL_TRY_CATCH_AROUND_COMPOSABLE.on(reportOn))
                    return
                }
            }

is KtClass, is KtFile -> {
                // Reached non-composable boundary
                context.trace.report(COMPOSABLE_INVOCATION.on(reportOn))
                return
            }
        }
        node = node.parent
    }
}
```

#### Inline Lambda Restrictions

Special handling for inline functions with composable lambda parameters:

```kotlin
private fun checkInlineLambdaCall(
    resolvedCall: ResolvedCall<*>,
    reportOn: PsiElement,
    context: CallCheckerContext
) {
    val descriptor = resolvedCall.resultingDescriptor

for ((parameter, argument) in resolvedCall.valueArguments) {
        if (!parameter.type.isComposableType()) continue

if (parameter.hasDisallowComposableCalls()) {
            // This lambda cannot contain composable calls
            val lambda = argument.getLambdaExpression()
            lambda?.forEachDescendantOfType<KtCallExpression> { call ->
                if (call.isComposableCall()) {
                    context.trace.report(
                        CAPTURED_COMPOSABLE_INVOCATION.on(call, parameter, descriptor)
                    )
                }
            }
        }
    }
}
```

**Example:**

```kotlin
inline fun runWithoutComposables(
    @DisallowComposableCalls block: () -> Unit
) = block()

@Composable
fun MyComposable() {
    runWithoutComposables {
        Text("Error")  // ERROR: Composable calls not allowed
    }
}
```

#### Type Compatibility Checking

Validates that composable and non-composable function types match expected signatures:

```kotlin
override fun checkType(
    expression: KtExpression,
    expressionType: KotlinType,
    expectedType: KotlinType,
    context: CallCheckerContext
) {
    val isComposableExpression = expressionType.isComposableType()
    val isComposableExpected = expectedType.isComposableType()

when {
        isComposableExpression && !isComposableExpected -> {
            // Trying to pass composable lambda where non-composable expected
            if (!isInlineConversion(expression)) {
                context.trace.report(
                    TYPE_MISMATCH.on(expression, expectedType, expressionType)
                )
            }
        }

!isComposableExpression && isComposableExpected -> {
            // Trying to pass non-composable where composable expected
            context.trace.report(
                TYPE_MISMATCH.on(expression, expectedType, expressionType)
            )
        }
    }
}
```

### 8.3 Declaration Validation

Validates composable function and property declarations follow Compose rules.

#### Composable Function Rules

Key validation rules enforced:

```kotlin
class ComposableDeclarationChecker : DeclarationChecker {
    override fun check(declaration: KtDeclaration, descriptor: DeclarationDescriptor, context: DeclarationCheckerContext) {
        when (descriptor) {
            is FunctionDescriptor -> checkFunction(descriptor, declaration, context)
            is PropertyDescriptor -> checkProperty(descriptor, declaration, context)
        }
    }

private fun checkFunction(descriptor: FunctionDescriptor, declaration: KtDeclaration, context: DeclarationCheckerContext) {
        // Rule 1: Composable functions cannot be suspend
        if (descriptor.isComposableCallable() && descriptor.isSuspend) {
            context.trace.report(COMPOSABLE_SUSPEND_FUN.on(declaration))
        }

// Rule 2: Main function cannot be composable
        if (descriptor.isComposableCallable() && descriptor.name.asString() == "main") {
            context.trace.report(COMPOSABLE_FUN_MAIN.on(declaration))
        }

// Rule 3: Composability must be consistent in overrides
        descriptor.overriddenDescriptors.forEach { overridden ->
            if (descriptor.isComposableCallable() != overridden.isComposableCallable()) {
                context.trace.report(
                    CONFLICTING_OVERLOADS.on(declaration, listOf(descriptor, overridden))
                )
            }
        }
    }
}
```

#### Property Restrictions

Composable properties have specific limitations:

```kotlin
private fun checkProperty(descriptor: PropertyDescriptor, declaration: KtDeclaration, context: DeclarationCheckerContext) {
    if (!descriptor.isComposableCallable()) return

// Rule 1: Composable properties cannot have backing fields
    if (descriptor.hasBackingField()) {
        context.trace.report(COMPOSABLE_PROPERTY_BACKING_FIELD.on(declaration))
    }

// Rule 2: Composable properties cannot be var (have setters)
    if (descriptor.isVar) {
        context.trace.report(COMPOSABLE_VAR.on(declaration))
    }

// Rule 3: Composable delegates restricted to getValue only
    if (declaration is KtProperty && declaration.hasDelegate()) {
        if (descriptor.isVar) {
            // setValue on composable delegate not allowed
            context.trace.report(COMPOSE_INVALID_DELEGATE.on(declaration))
        }
    }
}
```

#### Override Consistency

Ensures override hierarchies maintain consistent composability:

```kotlin
private fun checkOverrideConsistency(
    descriptor: CallableDescriptor,
    context: DeclarationCheckerContext
) {
    val isComposable = descriptor.isComposableCallable()

for (overridden in descriptor.overriddenDescriptors) {
        val overriddenComposable = overridden.isComposableCallable()

if (isComposable != overriddenComposable) {
            context.trace.report(
                CONFLICTING_OVERLOADS.on(
                    descriptor.source.getPsi()!!,
                    listOf(descriptor, overridden)
                )
            )
        }

// Check applier compatibility (scheme-based): the overridden declaration's
        // scheme must be able to override this declaration's scheme.
        if (!overridden.toScheme().canOverride(descriptor.toScheme())) {
            context.trace.report(
                COMPOSE_APPLIER_DECLARATION_MISMATCH.on(
                    descriptor.source.getPsi()!!
                )
            )
        }
    }
}
```

### 8.4 Applier Target System

The applier target system ensures composable functions target compatible UI frameworks.

#### Scheme Structure

Applier schemes encode which UI framework a composable targets:

```kotlin
sealed class Item

// Concrete applier, e.g. "androidx.compose.ui.UiComposable"
class Token(val value: String) : Item()

// Generic/unknown applier, identified by an index that unification can bind
class Open(val index: Int, override val isUnspecified: Boolean = false) : Item()

class Scheme(
    val target: Item,
    val parameters: List<Scheme> = emptyList(),
    val result: Scheme? = null,
    val anyParameters: Boolean = false,
) {
    // Whether this scheme can legally override `other`
    // (alpha-renames open targets, then compares structurally).
    fun canOverride(other: Scheme): Boolean = /* ... */
}
```

Note that `Token` and `Open` are top-level subclasses of `Item` (not nested as `Item.Token`/`Item.Open`), and `Scheme` exposes a `canOverride` check used for override validation rather than `isOpen()`/`isConcrete()` helpers.

**Example Schemes:**

```kotlin
// UI Composable targeting Android UI
Scheme(Token("androidx.compose.ui.UiComposable"))

// Generic composable (works with any applier)
Scheme(Open(-1))

// Composable with lambda expecting UI target
Scheme(
    target = Token("androidx.compose.ui.UiComposable"),
    parameters = listOf(
        Scheme(Token("androidx.compose.ui.UiComposable"))
    )
)
```

#### Target Inference Algorithm

The `ApplierInferencer` class performs unification to resolve applier targets. It is **generic and adapter-driven** — it is not tied to descriptors or a `ModuleDescriptor`. Instead, callers supply adapters that teach it how to read schemes from their own node/type representation (so the same engine works for both K1 and K2):

```kotlin
class ApplierInferencer<Type, Node>(
    private val typeAdapter: TypeAdapter<Type>,
    private val nodeAdapter: NodeAdapter<Type, Node>,
    private val lazySchemeStorage: LazySchemeStorage<Node>,
    private val errorReporter: ErrorReporter<Node>,
) {
    // Infers/visits a node, unifying the schemes of a call with its callees.
}
```

Conceptually, inference proceeds by:

1. Reading the declared scheme from `@ComposableTarget`/`@ComposableInferredTarget` annotations (or a fully-open scheme when none is present).
2. Walking the call graph via the adapters and **unifying** the scheme of each call site with the scheme of its callee.
3. Resolving open targets through a `Bindings` table: an `Open(index)` target gets *bound* to a concrete `Token` when it meets one, and two different concrete `Token`s that meet are a conflict (reported via `errorReporter`, not by throwing).
4. Merging results with `Scheme.mergeWith` / `bindings.unify` to produce the final, possibly-still-open scheme.

There are no `Constraint`/`Substitution`/`IncompatibleApplierException` types; unification state lives in `Bindings` and `LazyScheme`.

#### Cross-Target Validation

Validates that composable calls use compatible appliers:

```kotlin
private fun checkApplierCompatibility(
    caller: CallableDescriptor,
    callee: CallableDescriptor,
    context: CallCheckerContext
) {
    val callerScheme = inferredScheme(caller)
    val calleeScheme = inferredScheme(callee)

if (!areCompatible(callerScheme, calleeScheme)) {
        context.trace.report(
            COMPOSE_APPLIER_CALL_MISMATCH.on(
                context.element,
                calleeScheme.target.toString(),
                callerScheme.target.toString()
            )
        )
    }
}
```

**Example Error:**

```kotlin
@Composable
@ComposableTarget("androidx.compose.ui.UiComposable")
fun UiButton(text: String) { /* ... */ }

@Composable
@ComposableTarget("com.example.CustomApplier")
fun CustomWidget() { /* ... */ }

@Composable
fun Screen() {
    UiButton("Click") {
        CustomWidget()  // COMPOSE_APPLIER_CALL_MISMATCH
    }
}
```

> Note: under K1 applier-target mismatches are reported as **errors**, but under K2 (the default frontend) `COMPOSE_APPLIER_CALL_MISMATCH`, `COMPOSE_APPLIER_PARAMETER_MISMATCH`, and `COMPOSE_APPLIER_DECLARATION_MISMATCH` are reported as **warnings**.

### 8.5 Type Resolution and Inference

Automatically infers `@Composable` annotation on lambda expressions based on expected type.

> Note: `ComposeTypeResolutionInterceptorExtension` shown below is the **K1-only** mechanism (package `...kotlin.k1`). Under K2 (the default), composable-lambda inference is performed in FIR, not via a `TypeResolutionInterceptor`.

#### Automatic Composable Inference

```kotlin
class ComposeTypeResolutionInterceptorExtension : TypeResolutionInterceptorExtension {
    override fun interceptFunctionLiteralDescriptor(
        expression: KtLambdaExpression,
        context: TypeResolutionContext,
        descriptor: FunctionDescriptor
    ): FunctionDescriptor {
        val expectedType = context.expectedType ?: return descriptor

if (!expectedType.isComposableType()) return descriptor
        if (descriptor.isComposableCallable()) return descriptor

// Infer @Composable on lambda
        val composableDescriptor = descriptor.copy(
            annotations = descriptor.annotations + ComposableAnnotation
        )

// Record inference
        context.trace.record(
            FrontendWritableSlices.INFERRED_COMPOSABLE_DESCRIPTOR,
            composableDescriptor,
            true
        )

context.trace.record(
            FrontendWritableSlices.INFERRED_COMPOSABLE_LITERAL,
            expression.functionLiteral,
            true
        )

return composableDescriptor
    }
}
```

#### Lambda Type Adaptation

The system automatically adapts lambda types to match composable expectations:

```kotlin
// Expected: @Composable () -> Unit
val content: @Composable () -> Unit = {
    // Lambda automatically becomes @Composable
    Text("Hello")  // Composable call allowed
}

// Without inference, this would be an error
Column(
    content = {  // Automatically @Composable
        Text("Item 1")
        Text("Item 2")
    }
)
```

### 8.6 Analysis Pipeline

#### Compilation Phases

The analysis system operates through distinct compilation phases:

```
┌─────────────────────────┐
│    1. PARSING           │
│  PSI Tree Construction  │
└───────────┬─────────────┘
            │
┌───────────▼─────────────┐
│  2. RESOLUTION (K1/K2)  │
│  Type & Symbol Binding  │
└───────────┬─────────────┘
            │
┌───────────▼─────────────┐
│   3. DIAGNOSTIC PHASE   │
│  Checkers & Validators  │
│  ┌──────────────────┐   │
│  │ Annotation Check │   │
│  │ Declaration Check│   │
│  │ Call Check       │   │
│  │ Target Check     │   │
│  └──────────────────┘   │
└───────────┬─────────────┘
            │
┌───────────▼─────────────┐
│   4. IR GENERATION      │
│  Convert PSI to IR Tree │
└───────────┬─────────────┘
            │
┌───────────▼─────────────┐
│   5. IR ANALYSIS        │
│  Stability Inference    │
│  Static Detection       │
└───────────┬─────────────┘
            │
┌───────────▼─────────────┐
│   6. IR LOWERING        │
│  Transform Composables  │
└─────────────────────────┘
```

#### Data Flow Through Phases

**Frontend Analysis → WritableSlices:**

```kotlin
// During type resolution
ComposeTypeResolutionInterceptor {
    record(INFERRED_COMPOSABLE_DESCRIPTOR, descriptor, true)
}

// During call checking
ComposableCallChecker {
    record(LAMBDA_CAPABLE_OF_COMPOSER_CAPTURE, lambda, true)
}

// During target checking
ComposableTargetChecker {
    record(COMPOSE_LAZY_SCHEME, function, scheme)
}
```

**IR Analysis → IR Attributes:**

```kotlin
// During static-expression analysis
expression.isStaticExpression = isStatic

// During key generation
DurableFunctionKeyTransformer {
    function.durableFunctionKey = keyInfo
}
```

**IR Lowering → Read Attributes:**

```kotlin
// During composable transformation
ComposableFunctionBodyTransformer {
    val isStatic = expr.isStaticExpression
    val key = function.durableFunctionKey

if (isStatic) {
        // Generate optimized code
    } else {
        // Generate standard code
    }
}
```

### 8.7 Practical Examples

#### Example: Composable Context Validation

```kotlin
// Source code
class MainActivity {
    fun onCreate() {
        Text("Hello")  // ERROR: Not in composable context
    }

@Composable
    fun Content() {
        Text("Hello")  // OK: In composable function

runBlocking {
            Text("Error")  // ERROR: In non-composable lambda
        }

LaunchedEffect(Unit) {
            Text("Error")  // ERROR: LaunchedEffect block is suspend, not composable
        }
    }
}
```

**Analysis Flow:**

1. `ComposableCallChecker` sees `Text("Hello")` call
2. Walks up PSI tree from call site
3. In `onCreate`: Reaches `KtFunction` without `@Composable` → Reports `COMPOSABLE_INVOCATION`
4. In `Content`: Finds `@Composable` function → Valid
5. In `runBlocking`: Lambda not composable → Reports `COMPOSABLE_INVOCATION`
6. In `LaunchedEffect`: Suspend lambda context → Reports `COMPOSABLE_INVOCATION`

#### Example: Inline Lambda Analysis

```kotlin
inline fun <T> withoutComposables(
    @DisallowComposableCalls noinline block: () -> T
): T = block()

@Composable
fun Screen() {
    withoutComposables {
        val data = loadData()  // OK
        Text(data)  // ERROR: CAPTURED_COMPOSABLE_INVOCATION
    }
}
```

**Analysis Flow:**

1. `ComposableCallChecker.checkInlineLambdaCall()` examines `withoutComposables` call
2. Finds parameter `block` has `@DisallowComposableCalls`
3. Traverses lambda body AST
4. Finds composable call to `Text()`
5. Reports `CAPTURED_COMPOSABLE_INVOCATION` with parameter name and function

#### Example: Stability and Skipping

```kotlin
// Source
data class StableUser(val name: String, val age: Int)
data class UnstableUser(var name: String, var age: Int)

@Composable
fun StableUserCard(user: StableUser) {
    Text(user.name)
}

@Composable
fun UnstableUserCard(user: UnstableUser) {
    Text(user.name)
}
```

**Analysis and Generation:**

1. **Stability Analysis:**
   - `StableUser`: All `val` properties of stable types → `Certain(stable = true)`
   - `UnstableUser`: Has `var` properties → `Certain(stable = false)`

2. **IR Attribute Recording:**
   ```kotlin
   stableUserParam.isStaticExpression = false
   unstableUserParam.isStaticExpression = false
   ```

3. **Code Generation Difference:**

**StableUserCard (can skip):**
   ```kotlin
   fun StableUserCard(user: StableUser, $composer: Composer, $changed: Int) {
       $composer.startRestartGroup(key)
       if ($changed and 0x1 == 0 && $composer.skipping) {
           $composer.skipToGroupEnd()  // Skip if user unchanged
       } else {
           Text(user.name, $composer, 0)
       }
       $composer.endRestartGroup()?.updateScope {
           StableUserCard(user, it, $changed or 0x1)
       }
   }
   ```

**UnstableUserCard (always executes):**
   ```kotlin
   fun UnstableUserCard(user: UnstableUser, $composer: Composer, $changed: Int) {
       $composer.startRestartGroup(key)
       // No skipping logic - always executes
       Text(user.name, $composer, 0)
       $composer.endRestartGroup()?.updateScope {
           UnstableUserCard(user, it, 0x1)
       }
   }
   ```

The analysis system's decisions directly impact runtime performance through intelligent code generation based on stability inference and validation results.

## Conclusion

The Compose compiler's stability inference system operates through a multi-phase analysis process that examines types, classes, and expressions. The system balances compile-time analysis with runtime checks, enabling optimization while maintaining correctness guarantees.

Key principles:
1. Stability enables recomposition skipping through value comparison
2. The algorithm proceeds from fast paths to detailed analysis
3. Bitmasks encode generic type parameter dependencies
4. External modules require runtime stability fields
5. Configuration files extend stability to external types
6. Conservative handling ensures correctness over optimization

Understanding this system allows developers to structure code for optimal Compose performance while maintaining type safety and correctness. If you want to get more information about the Compose performance, check out [compose-performance repository](https://github.com/skydoves/compose-performance).

## 📘 Manifest Android Interview

[Manifest Android Interview](https://www.android.skydoves.me/) is a comprehensive guide designed to enhance your Android development expertise through 108 interview questions with detailed answers, 162 additional practical questions, and 50+ "Pro Tips for Mastery" sections. The interview questions primarily focus on Android development—including the Framework, UI, Jetpack Libraries, and Business Logic—as well as Jetpack Compose, covering Fundamentals, Runtime, and UI.

## 🕊️ Dove Letter

If you're eager to dive deeper into Kotlin and Android, explore [Dove Letter](https://github.com/doveletter), a private subscription repository where you can learn, discuss, and share knowledge. To get more details about this unique opportunity, check out the [Learn Kotlin and Android With Dove Letter](https://medium.com/@skydoves/learn-kotlin-and-android-with-dove-letter-26265da11903) article.

## Find this repository useful? :heart:
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# License
```xml
Designed and developed by 2025 skydoves (Jaewoong Eum)

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
```

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