cmd/go: set the Go Version for command-line files to be the containing module's Go Version

[samthanawalla]

Current behavior

The Go version for a file passed in via command-line uses the local version.
e.g. $ go vet -x main.go
will set the Go Version to be the local version.

This happens in 3 places (maybe more):

1. vet
2. compiler
3. init.go

This ignores whether or not the file has a containing module.
e.g. current go version running is 1.23
-> main.go
-> go.mod (1.21)

$ go vet -x main.go
will set the Go Version to be 1.23

New behavior

As discussed offline and in https://go-review.googlesource.com/c/go/+/567435 we have come to the decision to set the Go version to use the containing go.mod's version.
In the above example, $ go vet -x main.go
will set the Go Version to be 1.21

[gabyhelp]

Similar Issues

• cmd/go: always pass a GoVersion to cmd/vet #65612
• proposal: cmd/go: refine the default language version based on context #36875
• cmd/go: inconsistent behavior when 'go' directive is missing from 'go.mod' #44976
• cmd/go: go directive is insufficiently documented for module authors to be able to make a decision about its value #30791
• cmd/go: always pass -lang to cmd/compile #66092

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[samthanawalla]

Thanks Gaby. Duplicate of #36875

[rsc]

I closed the proposal since it was so stale and mostly done. Reopening this issue.