Fix #1603: recognize foreach over enumerators that cannot be disposable - #3916
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The foreach pattern was only matched against using instructions, but the compiler emits no using/try-finally at all when the enumerator's static type can never require disposal: a struct or a sealed class that does not implement IDisposable (SerializationInfoEnumerator in the issue's example). Such loops stayed while loops. Recognize the bare 'enumerator = x.GetEnumerator(); while (enumerator.MoveNext())' shape during statement building and reuse the existing foreach transformation core for it. The transformation is restricted to exactly the cases where recompilation would produce the same IL: the enumerator type rules above (ref structs are excluded because of pattern-based disposal), a single-store enumerator variable unused outside the loop, and synchronous enumeration only, since async enumerators are always IAsyncDisposable. Assisted-by: Claude:claude-fable-5:Claude Code
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Fixes #1603.
The compiler emits no using/try-finally at all when the enumerator's static type can never require disposal: a struct or a sealed class that does not implement
IDisposable(SerializationInfoEnumeratorin the issue's example). Since the decompiler's foreach pattern was only matched against using instructions, such loops stayedwhile (enumerator.MoveNext())loops.This PR recognizes the bare
enumerator = x.GetEnumerator(); while (enumerator.MoveNext()) { ... }shape during statement building and reuses the existing foreach transformation core for it (TransformToForeachis split into theUsingInstructionwrapper and a core taking the containers/enumerator variable explicitly).The transformation is restricted to exactly the cases where recompilation produces the same IL:
IDisposable(a non-sealed class would be enumerated with afinally { (enumerator as IDisposable)?.Dispose(); }block; ref structs are excluded because of pattern-based disposal);IAsyncDisposable.Tests: the two commented-out TODO cases in
Pretty/Loops.cs(ForEachOnCustomStructEnumeratorand its generic variant) are revived, plus a new sealed-class enumerator case andIssue1603using the actualSerializationInfo/SerializationEntrytypes from the issue. All were verified to decompile to while loops before the fix. Full decompiler suite on Linux: 2544 total, 0 failed, 38 skipped. Note the legacy-csc and mcs configurations of the Loops test do not run on Linux, so the Windows CI matrix is the first place they get exercised.🤖 This PR was prepared by an AI agent (Claude Code) operated by @siegfriedpammer.