DllImportGenerator should error if attribute is applied to an extern

[danmoseley]

Bug #62616 was caused by applying [GeneratedDllImport] to an extern rather than partial method. The compilation succeeded, but the analyzer ignored it, and there was a failure at runtime.

1. The generator should error if it is applied to any method declaration that does not lead the generator to actually run.
2. Optionally, I suggest trimming (no RVA). from this message:
   IDS_CLASSLOAD_MISSINGMETHODRVA "Could not load type '%1' from assembly '%2' because the method '%3' has no implementation (no RVA)."

because it's what you see in this case, and it seems a rather technical distraction - not sure how relative virtual address is helpful here, the essential fact is that the implementation is missing.

[danmoseley]

That seems different, it concerns the emitted code. This is about an error while enumerating the attributes

[elinor-fung]

We have an analyzer that should be warning when the attribute is used on a method without partial:

runtime/src/libraries/System.Runtime.InteropServices/gen/DllImportGenerator/Analyzers/GeneratedDllImportAnalyzer.cs

Lines 82 to 83 in 55fecfb

	// Must be marked partial
	context.ReportDiagnostic(methodSymbol.CreateDiagnostic(GeneratedDllImportMissingModifiers, methodSymbol.Name));

We'll have to check why that didn't seem to do its job in this case.

That analyzer only looks for static and partial right now, so it should also be updated to handle other cases where the generator wouldn't run (for example, if the method already has a body).

[danmoseley]

Will this analyzer always run when the generator is used? A rule like this should always fire.

[elinor-fung]

Yeah, it should always run when the generator is used. It lives in the same assembly as the generator, so the analyzer reference to that assembly should pull in both the generator and analyzer.