x/tools/gopls: implement type hierarchy support

[findleyr]

Recently, @adonovan a I were discussing the problem of finding interfaces/implementations for function types, and we (re)encountered a critical limitation of the textDocument/implementations query: there's no way to go up the type hierarchy (which matters a lot for Go!). Currently, gopls (by convention) serves implementations when asking for implementation of a concrete type. But if you're on type HandlerFunc func(ResponseWriter, *Request), there's no way to query both interfaces implemented by this type (http.Handler, in this case), and functions that implement it (all of the literal handlers in your codebase).

From microsoft/language-server-protocol#2037 (comment), it sounds like the only option is to implement the textDocument/typeHierarchy request, though unfortunately this is not supported by many clients. Nevertheless, we should implement this capability.

[gabyhelp]

Related Issues

• x/tools/gopls: support showing all interfaces that a given type implements #35550 (closed)
• x/tools/gopls: support "go to implementation" (textDocument/implementation) for function types #56572

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