commit f2df0ad6d3ef30ed033a794e7150a5600be50a95 (async2-instrumentation)
Author: Copilot <198982749+Copilot@users.noreply.github.com>
Date:   Sat Nov 8 00:06:25 2025 +0000

    Don't emit DebuggerStepThroughAttribute for runtime async methods (#81035)

    ## ✅ Completed: Fix DebuggerStepThroughAttribute for Runtime Async
    Methods

    - [x] Understand the current state by exploring the codebase and
    identifying where DebuggerStepThroughAttribute is synthesized
    - [x] Modify `SourceMethodSymbol.cs` to NOT emit
    DebuggerStepThroughAttribute when runtime async is enabled
    - [x] Check for other attributes that might need similar conditioning
    (none found - only DebuggerStepThroughAttribute is synthesized)
    - [x] Create tests to validate the fix using
    CreateRuntimeAsyncCompilation helper
    - [x] Build and test the changes
    - [x] Run broader test suite to ensure no regressions
    - [x] Run code review and security scanning (CodeQL: no issues detected)
    - [x] Address review feedback - use CreateRuntimeAsyncCompilation helper
    - [x] Address review feedback from Julien and Aleksey

    ## Summary of Changes

    ### Production Code
    **File**:
    `src/Compilers/CSharp/Portable/Symbols/Source/SourceMethodSymbol.cs`
    - Added check for `compilation.IsRuntimeAsyncEnabledIn(target)` before
    emitting `DebuggerStepThroughAttribute`
    - The attribute was being incorrectly added to all async methods, even
    when runtime async is enabled
    - When runtime async is enabled, no state machine is generated, so the
    attribute is not needed

    ### Test Code
    **File**:
    `src/Compilers/CSharp/Test/Emit3/Attributes/AttributeTests_Synthesized.cs`
    - Added `AsyncStateMachineAttribute_RuntimeAsync` test with Debug and
    Release variants for regular async methods
    - Added `AsyncStateMachineAttribute_RuntimeAsync_Lambda` test with Debug
    and Release variants for async lambdas
    - Added `AsyncStateMachineAttribute_RuntimeAsync_LocalFunction` test
    with Debug and Release variants for async local functions
    - Uses `CreateRuntimeAsyncCompilation` helper for proper runtime async
    compilation setup
    - Made symbolValidator static as requested in review
    - Inlined attributes variable for conciseness
    - Removed unnecessary MetadataImportOptions.All
    - Uses GetTypeMembers() to enumerate nested types instead of checking
    specific names
    - Validates that when runtime async is enabled:
    - No `DebuggerStepThroughAttribute` is emitted on async methods,
    lambdas, or local functions
      - No state machine types are generated

    ## Test Results
    ✅ All 6 runtime async attribute tests pass (regular methods, lambdas,
    local functions - each with Debug and Release)
    ✅ All broader AsyncStateMachineAttribute tests pass
    ✅ CodeQL security scan: No issues detected

    ## Key Findings
    - **Only DebuggerStepThroughAttribute needed fixing**: Other attributes
    like `AsyncStateMachineAttribute` were already correctly handled (only
    added when state machine exists)
    - **Lambdas and local functions supported**: Runtime async works with
    lambdas and local functions, no state machines generated
    - **Minimal change**: Only 7 lines of production code changed (added
    conditional check)
    - **No breaking changes**: All existing tests continue to pass

    Relates to test plan https://github.com/dotnet/roslyn/issues/75960

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