cmd/go: fail if the user tries to modify the GOMOD environment variable

[mvdan]

See #51171 (comment). The user likely wants to either use -modfile, or cd into the right directory to enter or leave a module. Right now, a command like GOMOD=foo.mod go list will happily ignore GOMOD=foo.mod.

cc @bcmills @matloob

[mvdan]

Open question; if one does the equivalent of source <(go env) to import all of go env into the current shell environment (aka go env >env.txt && source env.txt), one can end up with the equivalent of GOMOD=${PWD}/go.mod. So perhaps we shouldn't fail if GOMOD is set but the path is precisely the same as the go.mod file we are meant to be using.

In other words: to ensure we don't error out when we don't really have to, we could only error if GOMOD is set to a path that means the user is trying to change what go.mod file we're meant to be using.

[bcmills]

Perhaps!

I have also argued in the past that we should actually allow the user to actually change the “main module” for a go invocation by setting GOMOD explicitly.
(That would be a bit, but not entirely, redundant with the -C flag proposed in #50332.)

[mvdan]

I'd also be fine with that :)

[bstncartwright]

Is this available to be worked on? I'd love to pick this up as a new contributor. 😃

[gopherbot]

Change https://go.dev/cl/395915 mentions this issue: cmd/go: fail if the user tries to modify the GOMOD environment variable