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Clarify struct-to-enum breaking changes and R2R cast folding #137100

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Merged
AaronRobinsonMSFT merged 6 commits into from
Nov 7, 2026
Merged

Clarify struct-to-enum breaking changes and R2R cast folding #137100

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f9fbc49
Initial plan
Copilot Nov 6, 2026
b92f076
Apply remaining changes
Copilot Nov 6, 2026
b2ebaa4
Merge branch 'main' into copilot/clarify-breaking-change-rules
jkotas Nov 7, 2026
087be5e
Update breaking change rules to include struct-enum change
AaronRobinsonMSFT Nov 7, 2026
b33bc96
Align breaking-change rule wording
Copilot Nov 7, 2026
bc4cf45
Apply suggestion from @AaronRobinsonMSFT
AaronRobinsonMSFT Nov 7, 2026
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2 changes: 2 additions & 0 deletions docs/coding-guidelines/breaking-change-rules.md
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Expand Up @@ -169,6 +169,8 @@ Breaking Change Rules

* Changing a `struct` type to a `ref struct` type and vice versa

* Changing a `struct` type to an `enum` type and vice versa

* Changing the underlying type of an enum

This is a compile-time and behavioral breaking change as well as a binary breaking change which can make attribute arguments unparsable.
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3 changes: 3 additions & 0 deletions src/coreclr/tools/aot/ILCompiler.ReadyToRun.Tests/TestCases/CastFolding/CastFolding.cs
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Expand Up @@ -86,6 +86,9 @@ public static class CastFolding
[MethodImpl(MethodImplOptions.NoInlining)]
public static bool StructIsOtherStruct(SourceStruct s) => (object)s is TargetStruct;

[MethodImpl(MethodImplOptions.NoInlining)]
public static bool StructIsEnum(SourceStruct s) => (object)s is Enum;

[MethodImpl(MethodImplOptions.NoInlining)]
public static bool StructIsInterface(ControlStruct s) => (object)s is INotImplemented;

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1 change: 1 addition & 0 deletions src/coreclr/tools/aot/ILCompiler.ReadyToRun.Tests/TestCases/R2RTestSuites.cs
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Expand Up @@ -649,6 +649,7 @@ void Validate(ReadyToRunReader reader)
Assert.False(importedTypes.Contains("byte"), diagnostic);
Assert.False(importedTypes.Contains("SourceStruct"), diagnostic);
Assert.False(importedTypes.Contains("TargetStruct"), diagnostic);
Assert.False(importedTypes.Contains("System.Enum"), diagnostic);

// Casts to interfaces fold only when every type in the hierarchy is in the version bubble,
// which includes System.Object and System.ValueType only with the framework in the bubble.
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4 changes: 2 additions & 2 deletions src/coreclr/tools/aot/ILCompiler.ReadyToRun/JitInterface/CorInfoImpl.ReadyToRun.cs
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Expand Up @@ -3972,8 +3972,8 @@ private bool IsNegativeCastResultVersionResilient(TypeDesc fromType, TypeDesc to
if (fromType.HasTypeEquivalence || toType.HasTypeEquivalence)
return false;

// A value type can gain interfaces (or become an enum), but its other cast targets are fixed.
if (fromType.IsValueType && !toType.IsInterface && !toType.IsWellKnownType(WellKnownType.Enum))
// A value type can gain interfaces, but its other cast targets are fixed.
if (fromType.IsValueType && !toType.IsInterface)
return true;

// Otherwise every type that decides the cast must version with this code. Array and variant casts
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