Fix CS4007: ReadOnlySpan across await in TrailingHeadersTest

[Copilot]

Assert.Equal<byte>(Array.Empty<byte>(), await ...) resolves the ReadOnlySpan<byte> overload, causing CS4007 since the span can't be preserved across the await boundary — breaking functional test builds for System.Net.Http.WinHttpHandler.

Description

• TrailingHeadersTest.cs: In Http2GetAsync_TrailingHeaders_NoData_EmptyResponseObserved, extract the awaited call into a local variable before Assert.Equal:

// Before (broken):
Assert.Equal<byte>(Array.Empty<byte>(), await response.Content.ReadAsByteArrayAsync());

// After (fixed):
byte[] responseBody = await response.Content.ReadAsByteArrayAsync();
Assert.Equal(Array.Empty<byte>(), responseBody);

Original prompt

Problem

Building the functional tests for System.Net.Http.WinHttpHandler fails with the following error:

src\libraries\System.Net.Http.WinHttpHandler\tests\FunctionalTests\TrailingHeadersTest.cs(220,36): error CS4007: Instance of type 'System.ReadOnlySpan<byte>' cannot be preserved across 'await' or 'yield' boundary.

Root Cause

In src/libraries/System.Net.Http.WinHttpHandler/tests/FunctionalTests/TrailingHeadersTest.cs, line 220, inside the Http2GetAsync_TrailingHeaders_NoData_EmptyResponseObserved async test method:

Assert.Equal<byte>(Array.Empty<byte>(), await response.Content.ReadAsByteArrayAsync());

Assert.Equal<byte>(ReadOnlySpan<byte>, ReadOnlySpan<byte>) overload is being resolved here, which means Array.Empty<byte>() is implicitly converted to a ReadOnlySpan<byte>. Since there is an await expression in the same statement (as the second argument), the compiler cannot preserve the ReadOnlySpan<byte> across the await boundary — resulting in CS4007.

Fix

In the Http2GetAsync_TrailingHeaders_NoData_EmptyResponseObserved method, extract the await call to a local variable before the Assert.Equal call, so that no ReadOnlySpan<byte> lives across an await:

// Before (broken):
Assert.Equal<byte>(Array.Empty<byte>(), await response.Content.ReadAsByteArrayAsync());

// After (fixed):
byte[] responseBody = await response.Content.ReadAsByteArrayAsync();
Assert.Equal(Array.Empty<byte>(), responseBody);

The file to change is:
src/libraries/System.Net.Http.WinHttpHandler/tests/FunctionalTests/TrailingHeadersTest.cs

The change is in the Http2GetAsync_TrailingHeaders_NoData_EmptyResponseObserved method, around line 220.

The following is the prior conversation context from the user's chat exploration (may be truncated):

User: in testing we started hitting following error, fix it: src\libraries\System.Net.Http.WinHttpHandler\tests\FunctionalTests\TrailingHeadersTest.cs(220,36): error CS4007: (NETCORE_ENGINEERING_TELEMETRY=Build) Instance of type 'System.ReadOnlySpan' cannot be preserved across 'await' or 'yield' boundary.

Assistant: The issue is clear. On line 220, Assert.Equal<byte> is being called inside an async method where the first argument is Array.Empty<byte>() — which returns a ReadOnlySpan<byte> — and the second argument is await response.Content.ReadAsByteArrayAsync(). The ReadOnlySpan<byte> cannot be preserved across the await boundary.

The fix is to await the ReadAsByteArrayAsync() call first, store it in a local variable, and then pass it to Assert.Equal.

Assistant: We're waiting for your response to the tool call dialog to continue — please accept or reject the proposed pull request action so I can proceed.

This pull request was created from Copilot chat.

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Fixes #125952

[MichalStrehovsky]

Cc @agocke, I think this is from the xunit upgrade, not completely sure why we didn't see this in that PR's CI run, but I now see:

src\libraries\System.Net.Http.WinHttpHandler\tests\FunctionalTests\TrailingHeadersTest.cs(220,36): error CS4007: (NETCORE_ENGINEERING_TELEMETRY=Build) Instance of type 'System.ReadOnlySpan<byte>' cannot be preserved across 'await' or 'yield' boundary.

in various CI legs.

[Copilot]

Pull request overview

Fixes a functional test build failure in System.Net.Http.WinHttpHandler caused by overload resolution selecting an Assert.Equal<byte>(ReadOnlySpan<byte>, ReadOnlySpan<byte>) overload in the presence of an await, which triggers CS4007.

Changes:

• Store await response.Content.ReadAsByteArrayAsync() into a local byte[] before asserting.
• Update the assertion to compare the empty byte array against the local variable without an await in the same statement.

[MichalStrehovsky]

/azp run runtime-nativeaot-outerloop

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[MichalStrehovsky]

(Triggering a native AOT outerloop, since we very like won't run the relevant legs here either, but naot outerloops for sure hit this.)

[dotnet-policy-service]

Tagging subscribers to this area: @karelz, @dotnet/ncl
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[MichalStrehovsky]

/ba-g one is #125953, another has a pr out by me, another is a timeout, all unrelated

[agocke]

Huh. Little weird but Lgtm