cmd/vet: warn on 3-clause loops where an iterator variable contains a lock type when GoVersion>=1.22

[timothy-king]

Proposal Details

Extend the copylock analyzer to emit a warning on the copy of the loop variable between iterations that occurs immediately before the loop's Post statement in Go >= 1.22. The requirements for the warning are when:

• a 3-clause for loop declares a loop scoped variable containing a lock type,
• the file's GoVersion is >= 1.22,
• the name of the variable is not _, and
• the Init statement of the for loop is not otherwise reported.

Example:

for _, mu := 0, (sync.Mutex{}); x < 10; x++ { // want "for loop iteration copies lock value to mu: sync.Mutex"
	_ = mu.TryLock()
}

Relevant part of the spec https://go.dev/ref/spec#For_clause :

The variable used by each subsequent iteration is declared implicitly before executing the post statement and initialized to the value of the previous iteration's variable at that moment.

[timothy-king]

Proposed implementation http://go.dev/cl/569955.

Related #66156. This is 'Case 1'.

[gopherbot]

Change https://go.dev/cl/569955 mentions this issue: go/analysis/passes/copylock: add support for ForStmt

[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]

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]

Based on the discussion above, this proposal seems like a likely accept.
— rsc for the proposal review group

The proposal details are in #66387 (comment).

[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 details are in #66387 (comment).

[zigo101]

Will this be merged to the main repo some day?

[timothy-king]

This is expected to go into tip during the next x/tools. Probably not more than a month. The first release it is expected to go into is 1.24.

[dmitshur]

This vet change 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 to track that.

[gopherbot]

Change https://go.dev/cl/632315 mentions this issue: doc/next: document copylock changes