cmd/vet: warn when ignoring the return value of slices.Delete and friends

[Deleplace]

Calling slices.Delete while ignoring the returned slice value is almost certainly a bug, doing some element shifting but losing track of which elements are kept/discarded.

See sample: https://go.dev/play/p/2jV918bTDAX

Suggestion

vet warning:

./prog.go:11:14: ignoring the return value of slices.Delete

Same for DeleteFunc, Compact and CompactFunc.

[ianlancetaylor]

One of the criteria for vet checks is frequency. How often do people make this mistake?

[bcmills]

(Compare #20803 (comment).)

[timothy-king]

@ianlancetaylor There is already an x/tools/go/analysis/passes/unusedresult checker. These functions would fit into that I believe. The additional [computational] cost of adding these there would be low. This partially addresses the vet frequency requirement.

[Deleplace]

Frequency: looking at the results of this GitHub search, the rate of incorrect usage seems low. Almost all of the matched lines are using either return, or =, or :=.

[Deleplace]

The search also proves the mistake does sometimes happen:

[seankhliao]

doesn't appear to be too common https://github.com/search?type=code&q=language%3AGo+%2F%5E%5Cs%2Bslices.Delete%5C%28%2F&p=1