Speclet: https://github.com/dotnet/csharplang/blob/main/proposals/csharp-13.0/ref-unsafe-in-iterators-async.md
Feature branch: https://github.com/dotnet/roslyn/tree/features/RefInAsync
Label: https://github.com/dotnet/roslyn/labels/New%20Feature%20-%20Ref%2FUnsafe%20in%20Iterators%2FAsync

• Review feature specification at LDM
• Breaking change around unsafe context within iterators is documented
• language version
  • ref / ref struct local in
    • async method
    • iterator method
  • unsafe block is allowed in C# 13 in
    • async method (this is a preserved legacy behavior)
    • iterator method
  • unsafe modifier is allowed on an iterator in C# 13, still an error (a language version error) in previous versions
    • regular method
    • property get
    • local function
    • operator
  • effect on unsafe context within iterator's body (the breaking change)
    • regular method
    • property get
    • local function
    • operator
• use ref / ref struct local
  • within / across await
  • within / across yield return; in
    • regular iterators
    • async iterators
  • across yield break; in
    • regular iterators
    • async iterators
  • within / across async foreach block
  • yield in nested block
  • within async lambda / local function
  • within iterator local function
  • within property / indexer get iterator
  • error in async / iterator even in unsafe context
  • await in elements of stackalloc, the result span should be usable afterwards
  • async Task<int[]> M() => ... ReadOnlySpan<int> x = await M(); is expected to work
  • Buffer2<int> b = [await a, await b]; // inline array type (ref struct)
  • using (refStruct = ...) { await ...; }
  • foreach (var _ in refStruct) { await ...; }
  • Interpolation handlers with awaits in the middle handler is a ref struct
    • no awaits in async method should work
    • Has await should be blocked (turns out it works but doesn't use the interpolation handler)
• unsafe context
  • unannotated / unsafe containing type
  • unannotated / unsafe iterator method declaration
  • unannotated / unsafe iterator block
  • unannotated / unsafe local function
  • unannotated lambda
  • unannotated / unsafe property
  • unannotated / unsafe property / indexer get iterator
• within unsafe context
  • error for await (confirm we have a legacy test)
  • error for yield return;
  • no diagnostic for yield break;
• within lock
  • next warning waive warning for yield return; for legacy lock
  • error for yield return; for Lock type
  • no diagnostic for yield break;
• taking address of local / parameter
  • error in iterator method
  • error in iterator method for an indirect case
  • warning in async method (C#12 warning wave)
• taking address of a field is allowed in async/iterator

Related:

• Consider relaxing restrictions of async methods in blocks that do not contain await. (VS 17.11, .NET 9) csharplang#1331