Fix #3827: keep the while-loop for a ref local used after the loop (avoid an uninitialized hoisted ref decl)

[sailro]

Fixes #3827.

A ref local used after a while-loop has its declaration hoisted in front of the
loop. PatternStatementTransform then rewrote the while into a for, moving the
sole ref-assignment into the for-initializer — leaving ref T x; (no initializer)
in front of a headless for (; cond; x = ref …), which does not compile
(CS8174: a by-reference variable must have an initializer).

Fix

Per @siegfriedpammer's review, instead of emitting a headless for-loop,
PatternStatementTransform.TransformFor now leaves the loop as a while when the
iteration variable is a by-ref-like local used after the loop. The ref-assignment
stays on the hoisted declaration and the output matches the original source — both
simpler and free of the not-so-nice headless-for form.

Before (does not recompile — CS8174):

ref Node reference;
for (reference = ref _buckets[hash & 3]; reference != null; reference = ref reference.next)
{
    ...
}

After:

ref Node reference = ref _buckets[hash & 3];
while (reference != null)
{
    ...
    reference = ref reference.next;
}

Test

Pretty/RefLocalsAndReturns.RefLocalUsedAfterForLoop round-trips the hoisted-decl +
while-loop form.

Real-world occurrence: Enums.NET (net461) hash-bucket walk. Same symptom as the
long-closed #1630 (stale-bot closed, no fix); the if/else ref-assignment variant
from #1520 is not addressed here.

[Copilot]

Pull request overview

This PR fixes a C# decompilation/codegen edge case where a ref local whose declaration is hoisted ahead of a for loop could lose its initializer (ending up as ref T x;), producing invalid C# (CS8174). The change teaches DeclareVariables to detect when the only initialization is the first for-initializer assignment and to move that initializer into the hoisted declaration while removing the for initializer.

Changes:

• Add detection for for (v = …; …) as the sole initialization when the declaration must be hoisted before the for statement.
• Emit ref T v = ref …; before the loop and remove the corresponding for initializer to preserve valid C# for ref locals.
• Add a pretty test case covering the “ref local used after loop” scenario (Issue #3827) to ensure round-tripping.

Reviewed changes

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

File	Description
ICSharpCode.Decompiler/CSharp/Transforms/DeclareVariables.cs	Detects and rewrites the “hoisted ref local + for-initializer assignment” pattern to keep required initializers on ref local declarations.
ICSharpCode.Decompiler.Tests/TestCases/Pretty/RefLocalsAndReturns.cs	Adds a regression test case that exercises the fixed pattern and validates the emitted form.

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[siegfriedpammer]

Thank you for your contribution!