Reduce the time spent in MA0182

[meziantou]

Why

MA0182 (AvoidUnusedInternalTypesAnalyzer) shows up as one of the slowest rules. For every operation that references a type of the analyzed assembly (invocation, member reference, object creation, ...), the rule called SemanticModel.GetEnclosingSymbol to ignore the references a type makes to itself. That call costs 10-100 µs each and accounted for half or more of the time spent in the callbacks of the rule.

It is worst for the types that are only referenced by themselves, as they are never added to the processed types: a public class that calls its own helpers (test classes, controllers) pays it on every reference to one of its own members.

What

The operation callbacks now use OperationAnalysisContext.ContainingSymbol, which Roslyn already provides, instead of querying the semantic model. The lazy ReferenceLocation struct is not needed anymore, so it is removed.

Measurements

Measured by running the analyzer on the test project of this repository (Roslyn 5.9) with CompilationWithAnalyzers and logAnalyzerExecutionTime. The machine was under heavy load, so the medians are noisy, but the direction was the same in every run.

Measurement	Before	After
MA0182 alone, reported analyzer time (median)	190-330 ms	87-245 ms
MA0182 alone, reported analyzer time (best iteration)	78-181 ms	46-65 ms
MA0182 with all the analyzers (median)	349 ms	179 ms
Time inside the callbacks, sequential, best case	~16 ms	~7.5 ms

Behavior change

A type that references itself only from an attribute applied to a parameter of its own primary constructor is now reported. It was considered as used before, because GetEnclosingSymbol returns the containing namespace or type for that position:

internal class NotUsed([Sample(typeof(NotUsed))] int value) { }

This is consistent with the same attribute applied to the type itself, which was already reported (InternalClassAttributeTypeOfOwnClass_Diagnostic).

I compared the old and the new reference location on every operation of the analyzer project and of the test project (about 79,000 operations). The only difference is the base call of a primary constructor, which has no effect on the diagnostics. The diagnostics reported on both projects are the same before and after.

Tests

• 4 new tests: a type referencing itself in a lambda, a local function and a primary constructor, and the matching cases where the type is used from another type. InternalClassReferencingItselfInPrimaryConstructor_Diagnostic fails without the change.
• AvoidUnusedInternalTypesAnalyzerTests passes with every supported Roslyn version (119 tests with 4.8 and 4.14, 125 with 5.0, 5.6 and 5.9). The full test suite was not run locally.
• dotnet run --project src/DocumentationGenerator produces no change.