cmd/go: provide some way to get the gofmt binary path

[josharian]

We can ask cmd/go for the path to cmd/compile:

$ go tool -n compile
/Users/josh/go/tip/pkg/tool/darwin_amd64/compile

There does not seem to be any way to ask cmd/go where the gofmt binary is.

$ go tool -n fmt
go tool: no such tool "fmt"
$ go fmt -n
can't load package: package .: found packages main (w.go) and p (x.go) in /Users/josh/go/tip/src

cc @rsc @bcmills

[rsc]

$(go env GOROOT)/bin/gofmt?

[zolotov]

@rsc as I said in the related issue, OS-specific distributions (like deb or rpm packages) doesn't always contain gofmt binary in GOROOT/bin.

[bcmills]

Compare go fix, go vet, and go doc:

$ GO111MODULE=off go fix -n
flag provided but not defined: -n
usage: go fix [packages]
Run 'go help fix' for details.

$ GO111MODULE=off go vet -n
can't load package: package .: no Go files in /tmp/tmp.L5dpPjzEzQ

$ GO111MODULE=off go doc -n
flag provided but not defined: -n
Usage of [go] doc:
        go doc
        go doc <pkg>
        go doc <sym>[.<method>]
        go doc [<pkg>].<sym>[.<method>]
        go doc <pkg> <sym>[.<method>]
For more information run
        go help doc

Flags:
  -all
        show all documentation for package
  -c    symbol matching honors case (paths not affected)
  -cmd
        show symbols with package docs even if package is a command
  -src
        show source code for symbol
  -u    show unexported symbols as well as exported
exit status 2

Arguably those should all be consistent, in that they invoke other tools with flags set by cmd/go.

[josharian]

@bcmills maybe I'm missing something, but I think the thing that should be consistent is go tool -n <toolname>. fix, vet, and doc currently at least pass that test:

$ go tool -n fix
/Users/josh/go/tip/pkg/tool/darwin_amd64/fix
$ go tool -n vet
/Users/josh/go/tip/pkg/tool/darwin_amd64/vet
$ go tool -n doc
/Users/josh/go/tip/pkg/tool/darwin_amd64/doc