Fix #3850: recover ReadOnlySpan<T> array literals from the legacy lazy cache

[sailro]

Fixes #3850.

On target frameworks without RuntimeHelpers.CreateSpan (.NET Framework, or netstandard2.0 + System.Memory), Roslyn caches the backing array of a ReadOnlySpan<T> built from an array literal in a lazy <PrivateImplementationDetails> static field:

object obj = <PrivateImplementationDetails>.cache;
if (obj == null) {
    obj = new char[] { CR, LF };
    <PrivateImplementationDetails>.cache = (char[])obj;
}
... new ReadOnlySpan<char>((char[])obj) ...

The decompiled output referenced the compiler-synthesized <PrivateImplementationDetails> type, which is never declared and whose name is not expressible in C#, so the output failed to recompile (CS0400 in whole-project output).

Fix

A new CachedReadOnlySpanInitialization block transform collapses the lazy cache, mirroring CachedDelegateInitialization (which collapses the same lazy-static-field cache for anonymous-method delegates) and running right after it. Once the cache is collapsed, the existing array-initializer transforms recover the array literal, so the <PrivateImplementationDetails> reference disappears - the same result the modern RuntimeHelpers.CreateSpan path (net7+) already produces.

Before (does not recompile):

public static ReadOnlySpan<char> NewLine
{
    get
    {
        object obj = <PrivateImplementationDetails>.B7F5...7075_A1;
        if (obj == null)
        {
            obj = new char[2] { CR, LF };
            <PrivateImplementationDetails>.B7F5...7075_A1 = (char[])obj;
        }
        return new ReadOnlySpan<char>((char[])obj);
    }
}

After:

public static ReadOnlySpan<char> NewLine => new ReadOnlySpan<char>(new char[2] { CR, LF });

The transform only fires on a compiler-generated static cache field with this exact lazy-init shape, so it does not touch the existing <PrivateImplementationDetails> switch-on-string cache (covered by the existing CS1xSwitch_Debug test) or user code.

Test

ILPretty/CachedReadOnlySpanInitialization exercises the legacy lazy-cache IL and checks the recovered output.

Real-world occurrence: Markdig CodeInlineParser (net462).

[Copilot]

Pull request overview

This PR addresses a decompilation correctness issue on frameworks without RuntimeHelpers.CreateSpan by collapsing Roslyn’s legacy lazy static cache pattern used for ReadOnlySpan<T> array literals, preventing unexpressible/undeclared <PrivateImplementationDetails> references in the generated C#.

Changes:

• Add CachedReadOnlySpanInitialization block transform to collapse the legacy lazy-cache initialization pattern.
• Register the new transform immediately after CachedDelegateInitialization in the IL transform pipeline.
• Add an ILPretty regression test (CachedReadOnlySpanInitialization) with expected C# output.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
ICSharpCode.Decompiler/IL/Transforms/CachedReadOnlySpanInitialization.cs	New block transform that recognizes and collapses the legacy lazy cache pattern for ReadOnlySpan<T> array literals.
ICSharpCode.Decompiler/CSharp/CSharpDecompiler.cs	Inserts the new transform after CachedDelegateInitialization in the per-block transform pipeline.
ICSharpCode.Decompiler.Tests/TestCases/ILPretty/CachedReadOnlySpanInitialization.il	New ILPretty input reproducing the legacy cached-array pattern (using <PrivateImplementationDetails>).
ICSharpCode.Decompiler.Tests/TestCases/ILPretty/CachedReadOnlySpanInitialization.cs	Expected decompiled C# output fixture for the new ILPretty test.
ICSharpCode.Decompiler.Tests/ILPrettyTestRunner.cs	Adds a new NUnit test entry to run the new ILPretty case.
ICSharpCode.Decompiler.Tests/ICSharpCode.Decompiler.Tests.csproj	Includes the new .il test input in the project.

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[siegfriedpammer]

@sailro I think I have to apologize to you, after some investigation I noticed that the legacy pattern with the cache field was in-fact the correct choice of test. I have re-added a test. Sorry again.