[Cohosting] `@bind-<prop>` tag helper descriptors don't get created

[davidwengier]

I've spent an inordinate amount of time trying to work out why generating an event handler for @bind-Value:set doesn't work in our cohosting tests and I've finally gotten to the bottom of it.
This test:

razor/src/Razor/test/Microsoft.VisualStudio.LanguageServices.Razor.Test/Cohost/CodeActions/GenerateEventHandlerTests.cs

Line 59 in 8babb3a

[Fact(Skip = "@bind- attribute tag helper is not being found")]

In the regular tooling code we discover tag helpers by creating a TagHelperDescriptorProviderContext with no target symbol, here:

razor/src/Razor/src/Microsoft.CodeAnalysis.Razor.Workspaces/Extensions/ProjectExtensions.cs

Line 55 in 8babb3a

var context = new TagHelperDescriptorProviderContext(compilation, results)

In the source generator when finding tag helpers in references, the assembly symbol is used as the target symbol:

razor/src/Compiler/Microsoft.CodeAnalysis.Razor.Compiler/src/SourceGenerators/RazorSourceGenerator.cs

Line 214 in 8babb3a

tagHelperFeature.CollectDescriptors(assembly, results);

However, when the BindTagHelperDescriptorProvider runs, if the target symbol isn't null, it won't run unless the target symbol (ie assembly) is the same assembly as Microsoft.AspNetCore.Components.BindConverter is defined in. BindConverter is contained in Microsoft.AspNetCore.Components.dll and all of the standard components, like InputText etc., are in Microsoft.AspNetCore.Components.Web.dll.

Changing the source generator to pass null as the target symbol makes the test pass, but it also makes things take a lot longer. Removing the logic in BindTagHelperDescriptorProvider also works, but there is a test that specifically validates the behaviour:

razor/src/Compiler/Microsoft.CodeAnalysis.Razor/test/BindTagHelperDescriptorProviderTest.cs

Line 223 in 8babb3a

public void Execute_BindTagHelperReturnsEmptyWhenCompilationAssemblyTargetSymbol()

I'm guessing things work fine at runtime because the tag helpers are only in place to help tooling. I can't personally see any reason to keep the logic in BindTagHelperDescriptorProvider but maybe someone else has some more context. I removed it and checked that the generate methods tests pass, but I haven't run all of the rest of the compiler tests to know if there is any other impact.