Fix #3825: reconstruct async iterators with [EnumeratorCancellation] and await in finally

[sailro]

Fixes #3825.

An async IAsyncEnumerable<T> iterator that has both an [EnumeratorCancellation]
cancellation-token parameter and an await inside a finally was not
reconstructed: the decompiler emitted the raw compiler-generated state machine
(catch (object), goto case, <>1__state, ...), which does not compile.

Root cause

For such an iterator, the hoisted-local cleanup (stfld <>u__N(this, null)) is
emitted before the combined CancellationTokenSource disposal in both the
set-result block and the catch block. CheckSetResultReturnBlock and
ValidateCatchBlock only consumed that cleanup after the disposal, so the
pos + 2 == count test missed the dispose pattern, the symbolic analysis failed,
and AsyncAwaitDecompiler left the raw state machine.

Fix

Allow the hoisted-local cleanup to appear before the combined-tokens disposal as
well (one extra MatchHoistedLocalCleanup call in each of the two methods).

Test

Pretty/AsyncStreams.AwaitInFinallyWithCancellation combines
[EnumeratorCancellation] with an await in a finally; it now round-trips.

Real-world occurrence: Renci.SshNet SftpClient.ListDirectoryAsync (net462).

[Copilot]

Pull request overview

This PR fixes a decompilation failure for async IAsyncEnumerable<T> methods that combine an [EnumeratorCancellation] cancellation token parameter with an await inside a finally, where ILSpy previously fell back to emitting the raw compiler-generated state machine (which does not compile).

Changes:

• Adjusts async iterator reconstruction to accept hoisted-local cleanup (stfld <>u__N(..., null)) appearing before combined CancellationTokenSource disposal in both the set-result/return path and the catch handler path.
• Adds a Pretty test case covering [EnumeratorCancellation] + await in finally to ensure round-trip decompilation.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
ICSharpCode.Decompiler/IL/ControlFlow/AsyncAwaitDecompiler.cs	Allows MatchHoistedLocalCleanup to be consumed before the combined-tokens disposal pattern in the set-result block and catch block, fixing reconstruction for the reported iterator shape.
ICSharpCode.Decompiler.Tests/TestCases/Pretty/AsyncStreams.cs	Adds a regression test that exercises an async iterator with [EnumeratorCancellation] and an await in finally.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.