x/tools/gopls: support module-local implementation request

[litleleprikon]

What version of Go are you using (go version)?

$ go version
go version go1.12.6 darwin/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/esharifu/Library/Caches/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/esharifu/projects/go"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/Cellar/go/1.12.6/libexec"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.12.6/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/sh/vcq1w7n1547584p7d_kh_vf00000gn/T/go-build605958996=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

Trying to get implementations of interface in go

What did you expect to see?

gopls returns the list of places with structures which implements interface

What did you see instead?

textDocument/implementation request is not implemented

[litleleprikon]

As this feature is very valuable for my everyday development process I want to work on it if maintainers do not have any reasons against it.

[stamblerre]

@litleleprikon: If you are interested in working on this feature, please go ahead! We will be happy to review any CLs.

[gertcuykens]

@stamblerre that response was cutting a corner a bit :) because I think it gave a false sense of how difficult it is to implement :P But then again I could be totally wrong here

The good news I think is we can copy paste stuff from godoc src code who apparently has this feature already https://golang.org/lib/godoc/analysis/help.html

Would appreciate for example more specific references to parts in the gopls source code to take a look at that may help in the right direction.

Also some references in the source code where godoc analysis (see link above) does it's interface implement list magic.

And maybe some CL boilerplate so somebody calling this feature can already see "Hello world" appearing as ouput and basically has a crash course example how gopls works and know where to start by just looking at the boilerplate CL code provided

[stamblerre]

A lot of the code for this feature can actually be taken from guru, which has an implements feature (see https://github.com/golang/tools/blob/master/cmd/guru/implements.go).

I intend to compile a document that explains how to begin contributing to gopls, so I won't write in much detail here, but https://github.com/golang/tools/tree/master/internal/lsp/source is a good place to start looking to see how other language features are implemented in gopls. I would also suggest reading the LSP spec for textDocument/implementation, as well as taking a look here to see how to expose the feature via the LSP.

For those with specific questions, I'm available via Slack on the #gopls channel or via DM.