[meziantou]

MA0008 (UseStructLayoutAttributeAnalyzer) crashed the compiler process with a stack overflow on invalid code where a struct's fields cycle back to the struct (CS0523). A fuzzing campaign found it with this input:

internal struct Data
{
    publicainer
{
 =   internal Data _data;
}

Cause

IsBlittableType from Meziantou.Framework.Roslyn recurses into struct fields with no limit. A struct that contains itself, directly or through other structs, makes it recurse forever. A generic struct such as struct D<T> { D<D<T>> d; } creates a new constructed type at every level, so remembering visited types wouldn't stop it either.

Change

• MA0008 now uses its own copy of the blittable check, with a depth limit of 32. A struct nested deeper than that is treated as not blittable, so no diagnostic is reported. MA0008 is the only rule that called IsBlittableType.
• Added WithCyclicStruct_ShouldNotReportDiagnostic. It covers a struct that contains itself, two structs that contain each other, and the generic case, and expects CS0523 and no MA0008.

Testing

• Before the fix, the new test crashed the test host with "Stack overflow".
• After the fix, all 20 UseStructLayoutAttributeAnalyzerTests pass on roslyn5.9 and roslyn4.8.
• The original fuzzer input passes through the analyzer in a temporary test, which isn't committed.
• dotnet run --project src/DocumentationGenerator changes no files.

Note for reviewers: test restore currently fails on main with NU1902/NU1903 (System.Security.Cryptography.Pkcs 9.0.6 advisory, pulled in by the Roslyn testing packages). I ran the tests locally with -p:NuGetAudit=false. This PR doesn't change anything about that.