cmd/vet: know what builtin functions have no side effects

[mvdan]

Reminder issue to continue the work in https://go-review.googlesource.com/c/go/+/79536.

For example, len and cap never have any side effects, and it would be useful to know that i == len(x) || i == len(x) is a suspicious expression.

More broadly, this could be extended to automatically detect what functions are free of side effects. But this would require having access to the full source with full type information, and may be complex and costly, so I'm not entirely sure it's a right fit for vet.

A simpler version of the above would be to also add standard library functions, such as strings.Contains or path.Join. I don't know if it is OK for vet to treat standard library packages differently, though.

[gopherbot]

Change https://golang.org/cl/97775 mentions this issue: cmd/vet: teach vet that len and cap are pure funcs

[gopherbot]

Change https://golang.org/cl/112177 mentions this issue: cmd/vet: assume that no builtin funcs are pure

[mvdan]

I've abandoned the CL I had sent for this, since it required more work and it had gotten considerable merge conflicts. Perhaps someone else will pick up this issue in the future.