go/types, types2: CheckExpr / Eval may mutate type checked objects (=> data race) [1.24 backport]

[gopherbot]

@findleyr requested issue #71817 to be considered for backport to the next 1.24 minor release.

@gopherbot please backport this to 1.24. It is causing test flakes.

[findleyr]

@griesemer what do you think about this backport?

It will not be a clean cherry-pick, since the current implementation relies on VarKind. It also introduces map reads/writes replacing struct field access, which may have performance implications (but seemed not to matter significantly in benchmarking).

The problem is that a recent use of CheckExpr within gopls is causing a lot of test flakes on arbitrary tests, and there's no easy way to eliminate those flakes.

@adonovan one possibility is to live with this, writing a suitable watchflakes expression to at least consolidate the flakes into a single issue. Not great, but possible.

[griesemer]

Let's do the backport CL and see how bad the diffs are.
I'm not worried about the map access time.
The respective 1.25 change was very controlled and limited in complexity (fairly easy to believe that it was correct).
If the backport CL is equally limited (with amendments for VarKind), I am fine with it as it seems low risk.

[gopherbot]

Change https://go.dev/cl/657695 mentions this issue: [release-branch.go1.24] go/types,types2: allocate the used* maps in initFiles

[gopherbot]

Change https://go.dev/cl/657675 mentions this issue: [release-branch.go1.24] go/types,types2: externalize used objects

[findleyr]

@golang/release if this is approved for cherry-pick, we'll need to merge BOTH of the CLs above (the second fixes a bug in the first). If you'd prefer, I can squash them into a single CL. Please let me know what's best.