proposal: cmd/vet: warn on unused AppendFormat return value

[1f604]

Currently the compiler will throw an error if the return value of append is not used, but it does not error or even warn if the return value of time.AppendFormat is not used, even though AppendFormat has the same semantics as append.

https://go.dev/play/p/IpxJrcX3FkT

If someone calls AppendFormat without using the return value, that is always an error, because it does nothing.

I looked for linters that would catch this issue and could not find any.

EDIT: I tried to add it to go vet unusedresult but I couldn't get it to work...is it that unusedresult still only works for pure functions as stated in #14972 ?

[bcmills]

(See also #20803.)

[timothy-king]

It seems reasonable to me to include time.AppendFormat in unusedresult's list of functions to warn on: funcs. The function does seem directly analogous to append. This function could be called just for a side-effect to write into a slice with sufficient capacity (just like append), in which case we are requesting users write _ = time.AppendFormat(b, ...) (again just like append).

I think it is unlikely adding time.AppendFormat to the list would cause a noticeable performance problem. This covers most of the frequency concern (README).