x/tools/gopls: slow diagnostics after adding/removing an import

[findleyr]

I just noticed that in certain packages, diagnostics are very slow after adding or removing an import (or making any other metadata-affecting change).

It looks like I introduced an awaitLoaded(...) call into the critical path re-typechecking a package (via resolveImportGraph). When the package being edited is low level, this results in reloading its entire reverse-transitive closure.

As an invariant, we should always be able to operate on the current package without reloading everything.

• add a benchmark for this operation, which is common thanks to goimports on save
• fix the logic of resolveImportGraph

[earthboundkid]

I sometime see saving a file in VSCode take an extremely long time, and I've never been able to figure out why, but this sounds like a likely culprit. Good luck fixing it!

[findleyr]

Thanks @carlmjohnson. It should not actually be that difficult to fix. I'll be sure to ask you to test once it is done :)

[gopherbot]

Change https://go.dev/cl/510095 mentions this issue: gopls/internal/lsp: don't awaitLoaded inside resolveImportGraph