Fix #3827: keep the initializer on a ref local hoisted out of a for-loop

[sailro]

Fixes #3827.

A ref local that is used after a for-loop has its declaration hoisted in front
of the loop, but its only initialization is the for-initializer ref-assignment.
DeclareVariables then emitted the declaration without an initializer
(ref T x;), which does not compile (CS8174).

Fix

When a by-ref-like local's matching assignment is the first for-initializer, move
the ref-assignment's value up into the declaration (ref T x = ref expr;) and drop
the for-initializer.

Before:

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

After:

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

Test

Pretty/RefLocalsAndReturns.RefLocalUsedAfterForLoop round-trips the hoisted
declaration form.

This is the same symptom as the long-closed #1630 (closed by the stale-bot without
a fix) for the for-loop / used-after-loop case. Real-world occurrence: Enums.NET
(net461) hash-bucket walk.

Note: this covers the for-initializer case; 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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