[Copilot]

The breaking-change rules did not explicitly prohibit converting between a struct and an enum, though R2R relies on that distinction remaining stable across version bubbles.

• Rules: Marked conversions between struct and enum as disallowed.
• R2R: Removed the enum exception from negative-cast resilience:

  if (fromType.IsValueType && !toType.IsInterface)

• Coverage: Added an R2R cast-folding regression case for a struct cast to System.Enum.

• Fixes Clarify in breaking-change rules and R2R that converting a struct to an enum is a breaking change #137055

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[AaronRobinsonMSFT]

@copilot Please examine the failures on this PR.

[Copilot]

@copilot Please examine the failures on this PR.

I examined the failed checks. Eight jobs across runtime build 1668155 and runtime-dev-innerloop build 1668156 fail with CS1929 at src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Legacy/Dbi/DacDbiImpl.cs (lines 6789, 6843, 7084): ReadOnlySpan<byte> has no AsSpan method. Those lines are already present in the PR’s base commit, and this file is outside the PR diff, so this does not appear to be caused by this change. The failures need follow-up in the CDAC area; I don’t see a change warranted in this PR. The Linux and Windows x64 R2R test jobs passed, and the Helix monitor reported zero failed work items.

[jkotas]

the failures on this PR.

Build break is fixed in main.

[Copilot]

🟢 Approval recommended

No active review issues are identified for the ReadyToRun cast-folding compatibility update.

No open issues 🎉

What changed in this PR

This PR aligns ReadyToRun cast folding with the clarified compatibility rule that structs and enums cannot be converted into one another across versions, addressing #137055.

Changes:

• Explicitly disallows changing a struct to an enum or vice versa.
• Removes the System.Enum exception from negative-cast version-resilience checks.
• Adds a struct-to-System.Enum regression case and checks that folding avoids importing System.Enum.

File	Description
src/​coreclr/​tools/​aot/​ILCompiler.ReadyToRun/​JitInterface/​CorInfoImpl.ReadyToRun.cs	Simplifies value-type negative-cast resilience logic and updates its comment.
src/​coreclr/​tools/​aot/​ILCompiler.ReadyToRun.Tests/​TestCases/​R2RTestSuites.cs	Asserts that System.Enum is absent from imported types.
src/​coreclr/​tools/​aot/​ILCompiler.ReadyToRun.Tests/​TestCases/​CastFolding/​CastFolding.cs	Adds the struct-to-enum cast-folding test case.
docs/​coding-guidelines/​breaking-change-rules.md	Explicitly prohibits conversions between structs and enums.

_{Review effort: Balanced}

[jkotas]

/ba-g build analysis stuck