Unsafe evolution: allow await in unsafe context

[jjonescz]

Test plan: #81207
Relevant speclet section: await in unsafe contexts

Microsoft Reviewers: Open in CodeFlow

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[Copilot]

Pull request overview

This PR updates the C# compiler’s await context checks to support the Unsafe Evolution feature: await is permitted in unsafe contexts when the feature is enabled, while await remains disallowed inside a fixed statement (with a new dedicated diagnostic). It also updates compiler tests and the Expression Evaluator tests accordingly, and adds localized resources for the new diagnostic.

Changes:

• Gate ERR_AwaitInUnsafeContext on the Unsafe Evolution feature, and introduce ERR_BadAwaitInFixed for await inside fixed when the feature is enabled.
• Track fixed-statement binding context via a new BinderFlags.InFixedStatement and propagate it through FixedStatementBinder.
• Update/extend tests and resources (resx + xlf) to reflect the new behavior across language versions and debugger EE behavior.

Reviewed changes

Copilot reviewed 27 out of 27 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/ExpressionEvaluator/CSharp/Test/ExpressionCompiler/ExpressionCompilerTests.cs	Expands EE coverage across language versions; adds EE test for await inside fixed under debugger binder flags.
src/Compilers/CSharp/Test/Semantic/Semantics/StackAllocInitializerTests.cs	Pins legacy behavior to C# 14 for await diagnostics in unsafe stackalloc initializers.
src/Compilers/CSharp/Test/Semantic/Semantics/LocalFunctionTests.cs	Updates expectations to reflect that preview/next no longer errors on await in unsafe contexts.
src/Compilers/CSharp/Test/Semantic/Semantics/BindingAsyncTests.cs	Updates/retargets diagnostics checks for legacy vs next behavior around await in unsafe contexts.
src/Compilers/CSharp/Test/Emit3/RefUnsafeInIteratorAndAsyncTests.cs	Updates diagnostics now that await in unsafe can succeed and reach ref-across-await checks.
src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenAwaitUsingTests.cs	Pins legacy await-in-unsafe behavior to C# 14.
src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenAwaitForeachTests.cs	Pins legacy await-in-unsafe behavior to C# 14.
src/Compilers/CSharp/Test/CSharp15/UnsafeEvolutionTests.cs	Adds new coverage for await in unsafe blocks and for the new await-in-fixed restriction/diagnostic.
src/Compilers/CSharp/Portable/xlf/CSharpResources.zh-Hant.xlf	Adds ERR_BadAwaitInFixed localization entry (new).
src/Compilers/CSharp/Portable/xlf/CSharpResources.zh-Hans.xlf	Adds ERR_BadAwaitInFixed localization entry (new).
src/Compilers/CSharp/Portable/xlf/CSharpResources.tr.xlf	Adds ERR_BadAwaitInFixed localization entry (new).
src/Compilers/CSharp/Portable/xlf/CSharpResources.ru.xlf	Adds ERR_BadAwaitInFixed localization entry (new).
src/Compilers/CSharp/Portable/xlf/CSharpResources.pt-BR.xlf	Adds ERR_BadAwaitInFixed localization entry (new).
src/Compilers/CSharp/Portable/xlf/CSharpResources.pl.xlf	Adds ERR_BadAwaitInFixed localization entry (new).
src/Compilers/CSharp/Portable/xlf/CSharpResources.ko.xlf	Adds ERR_BadAwaitInFixed localization entry (new).
src/Compilers/CSharp/Portable/xlf/CSharpResources.ja.xlf	Adds ERR_BadAwaitInFixed localization entry (new).
src/Compilers/CSharp/Portable/xlf/CSharpResources.it.xlf	Adds ERR_BadAwaitInFixed localization entry (new).
src/Compilers/CSharp/Portable/xlf/CSharpResources.fr.xlf	Adds ERR_BadAwaitInFixed localization entry (new).
src/Compilers/CSharp/Portable/xlf/CSharpResources.es.xlf	Adds ERR_BadAwaitInFixed localization entry (new).
src/Compilers/CSharp/Portable/xlf/CSharpResources.de.xlf	Adds ERR_BadAwaitInFixed localization entry (new).
src/Compilers/CSharp/Portable/xlf/CSharpResources.cs.xlf	Adds ERR_BadAwaitInFixed localization entry (new).
src/Compilers/CSharp/Portable/Errors/ErrorFacts.cs	Marks the new error as build-only diagnostic.
src/Compilers/CSharp/Portable/Errors/ErrorCode.cs	Introduces ERR_BadAwaitInFixed = 9396.
src/Compilers/CSharp/Portable/CSharpResources.resx	Adds the user-facing message for ERR_BadAwaitInFixed.
src/Compilers/CSharp/Portable/Binder/FixedStatementBinder.cs	Sets BinderFlags.InFixedStatement for the fixed-statement binding scope.
src/Compilers/CSharp/Portable/Binder/BinderFlags.cs	Introduces InFixedStatement and clears it at executable code boundaries.
src/Compilers/CSharp/Portable/Binder/Binder_Await.cs	Updates await-context checking for Unsafe Evolution + new fixed-statement restriction.

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[AlekseyTs]

Done with review pass (commit 2)

[Copilot]

Pull request overview

Copilot reviewed 29 out of 29 changed files in this pull request and generated no new comments.

Comments suppressed due to low confidence (1)

src/Compilers/CSharp/Portable/Binder/Binder_Await.cs:228

• ReportBadAwaitContext reports ERR_BadAwaitInFixed for any await in a fixed statement body, even when BinderFlags.AllowAwaitInUnsafeContext is set. This looks inconsistent with the intended debugger behavior (and the new ExpressionCompiler test remarks) where the debugger should allow awaiting in unsafe contexts, including within fixed bodies.

            if (this.Flags.Includes(BinderFlags.InFixedStatementBody))
            {
                Error(diagnostics, ErrorCode.ERR_BadAwaitInFixed, awaitNodeOrToken);
                return true;
            }

[AlekseyTs]

Cannot await in the body or the initializer of a 'fixed' statement

Not blocking, but I would go with: "Cannot await in context of a 'fixed' statement"

[AlekseyTs]

LGTM (commit 6)

[AlekseyTs]

LGTM (commit 6)

[Copilot]

Pull request overview

Copilot reviewed 29 out of 29 changed files in this pull request and generated no new comments.

[jjonescz]

@333fred for a second review, thanks

[Copilot]

Pull request overview

Copilot reviewed 30 out of 30 changed files in this pull request and generated no new comments.

Suppressed comments (3)

src/Compilers/CSharp/Test/Semantic/Semantics/UnsafeTests.cs:900

• Same as above: this comment mentions "ref and unsafe in async and iterator methods", but the test is asserting a preview-gated diagnostic for await in an unsafe context. Please update the comment to match the scenario being tested.

            // https://github.com/dotnet/roslyn/issues/73280 - these should ideally be langversion errors for 'ref and unsafe in async and iterator methods'

src/Compilers/CSharp/Portable/CSharpResources.resx:3836

• The new diagnostic text is slightly ungrammatical compared to adjacent messages (e.g., "Cannot await in the body of a lock statement"). Consider adding "the" so the message reads naturally.

  <data name="ERR_BadAwaitInFixed" xml:space="preserve">
    <value>Cannot await in context of a 'fixed' statement</value>
  </data>

src/Compilers/CSharp/Test/Semantic/Semantics/UnsafeTests.cs:873

• This comment references issue #73280 and "ref and unsafe in async and iterator methods", but the asserted diagnostic here is for the preview-gated "updated memory safety rules" feature (triggered by await in an unsafe context). The comment should be updated to reflect what this test is actually asserting.

This issue also appears on line 900 of the same file.

            // https://github.com/dotnet/roslyn/issues/73280 - these should ideally be langversion errors for 'ref and unsafe in async and iterator methods'

[jjonescz]

@333fred for a second review, thanks

[Copilot]

Pull request overview

Copilot reviewed 30 out of 30 changed files in this pull request and generated 1 comment.