Background and Motivation

We have customers today (Razor) who use metadata references as an input to their pipeline. Today it is possible to get these from the compilation object, then use a custom comparer to decide if they have changed, but this is inefficient and requires code per-key press in the IDE, even though the compiler knows they havn't changed.

Proposed API

namespace Microsoft.CodeAnalysis
{
    public readonly struct IncrementalGeneratorInitializationContext
    {
+     public IncrementalValuesProvider<MetadataReference> MetadataReferencesProvider => ...
    }
}

Usage Examples

 var generator = new PipelineCallbackGenerator(ctx => {
                   ctx.RegisterSourceOutput(ctx.MetadataReferencesProvider, (spc, r) => ...);
                 });