cmd/vet: consider reporting invalid/ignored Go versions in build tags

[griesemer]

Files may specify their own Go language version via a build tag. For instance:

//go:build go1.22

indicates that this file requires Go language version 1.22.

If the language version in the build tag contains dot-release information, say it's "go1.22.1", it is invalid.
The compiler simply ignores the version (it doesn't use, say "go1.22").

It seems (to me) that this shouldn't be silently ignored as it might be very confusing for users. go vet might be the right tool to report an error. Alternatively, the compiler could report an error, but generally, the compiler doesn't complain about errors in (pragma and build) comments.

[timothy-king]

go/analysis/passes/buildtag would be a good home for this check.

[Goclipse27]

@griesemer @ianlancetaylor - Do you consider handling tags including expression like go:build go1.21 && go1.22.3 or a single tag ?

[griesemer]

It should report any Go version string that is silently dropped and ignored.
Also, before anybody starts working on this: note this is a proposal and needs to be discussed by the proposal review committee first. Thanks.

[rsc]

We already have a //go:build line checker. It should definitely report these.

[rsc]

This proposal has been added to the active column of the proposals project
and will now be reviewed at the weekly proposal review meetings.
— rsc for the proposal review group

[rsc]

No change in consensus, so accepted. 🎉
This issue now tracks the work of implementing the proposal.
— rsc for the proposal review group

The proposal is for the ‘buildtag’ checker to report build tags of the form go1.X.Y since only go1.X is recognized in build tags.

[gopherbot]

Change https://go.dev/cl/597576 mentions this issue: go/analysis/passes/buildtag: report invalid go versions in build tags.

[dmitshur]

A new check added to vet seems like something that needs to be mentioned in Go 1.24 release notes, and it's not mentioned there yet. Reopening as a release blocker for tracking that.

[gopherbot]

Change https://go.dev/cl/632355 mentions this issue: doc/next: document buildtag changes