Revert "Simplify TrimmerSingleWarn intermediate assembly update using MSBuild item Update"

[sbomer]

Reverts #125630

The Update in a target doesn't work (see dotnet/msbuild#2835 for the MSBuild behavior).

Fixes the dependency flow issue here: dotnet/sdk#53847 (comment).

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[Copilot]

Pull request overview

Reverts the prior simplification that used MSBuild item Update in _PrepareTrimConfiguration, restoring a set-based approach to ensure TrimmerSingleWarn=false metadata is correctly applied to the ResolvedFileToPublish item(s) corresponding to @(IntermediateAssembly).

Changes:

• Replace the ResolvedFileToPublish Update="@(IntermediateAssembly)" metadata update with explicit include/remove set operations to compute the intersection between @(ResolvedFileToPublish) and @(IntermediateAssembly).
• Re-add the updated intersection items back into @(ResolvedFileToPublish) so downstream consumers observe the metadata.

Show a summary per file

File	Description
src/tools/illink/src/ILLink.Tasks/build/Microsoft.NET.ILLink.targets	Restores reliable MSBuild item manipulation to apply TrimmerSingleWarn metadata to the intermediate assembly’s publish items without using Update in a target.

Copilot's findings

• Files reviewed: 1/1 changed files
• Comments generated: 0

[github-actions]

🤖 Copilot Code Review — PR #127656

Note

This review was generated by GitHub Copilot.

Holistic Assessment

Motivation: This PR reverts PR #125630 which simplified the TrimmerSingleWarn intermediate assembly metadata update logic. The revert is well-justified — MSBuild's Update syntax inside targets has a known bug (dotnet/msbuild#2835) where it updates all items in the list instead of only matching ones, causing a real dependency flow failure.

Approach: A clean git revert is the correct approach for undoing a broken simplification. The restored intersection-based pattern is the established workaround for this MSBuild limitation.

Summary: ✅ LGTM. This is a clean revert that restores known-working logic. The set-intersection pattern is correct and well-commented. One optional suggestion for preventing future re-simplification attempts.

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Detailed Findings

✅ Clean Revert — Verified correct

The revert is clean and focused. Only the expected file is modified, and the restored code exactly matches the pre-simplification state. No unrelated changes or scope creep.

✅ Restored MSBuild Logic — Verified correct

The set-intersection pattern is mathematically sound:

1. __SingleWarnIntermediateAssembly = ResolvedFileToPublish \\ IntermediateAssembly (set difference)
2. _SingleWarnIntermediateAssembly = ResolvedFileToPublish \\ __SingleWarnIntermediateAssembly = ResolvedFileToPublish ∩ IntermediateAssembly
3. Metadata is set on the intersection (only when TrimmerSingleWarn is not already set)
4. Items are replaced in ResolvedFileToPublish via Remove+Include

This correctly limits TrimmerSingleWarn=false to only items that exist in both ResolvedFileToPublish and IntermediateAssembly, working around the MSBuild Update bug.

💡 Suggestion — Add preventive comment (follow-up, not blocking)

Future maintainers may attempt to "simplify" this code again. Consider adding a comment explaining why the simpler Update syntax cannot be used:

<!-- NOTE: Cannot use <ResolvedFileToPublish Update="@(IntermediateAssembly)"> because
     Update inside targets has a bug (dotnet/msbuild#2835) that updates ALL items
     instead of only matching ones. Use set operations instead. -->

This was flagged independently by multiple review models (Claude Sonnet 4.5, GPT-5.3-Codex).

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Review contributed by: Claude Opus 4.6 (primary), Claude Sonnet 4.5, GPT-5.3-Codex

Generated by Code Review for issue #127656 · ◷

[sbomer]

/backport to release/11.0-preview4

[github-actions]

Started backporting to release/11.0-preview4 (link to workflow run)