testing: enforce the documented restriction on fuzz target result parameters

[findleyr]

The documentation for F.Fuzz states that "ff must be a function with no return value", but in practice this is not enforced. For example, the following invocation is accepted:

func FuzzFoo(f *testing.F) {
	f.Fuzz(func(t *testing.T, i int) int {
		return 0
	})
}

We should enforce the documented restriction. If we don't, users will likely depend on the current behavior. We can always relax this restriction in the future.

Marking as a release blocker as this is an API change.

CC @golang/release @bcmills

[findleyr]

(CC @julieqiu as well).

[gopherbot]

Change https://go.dev/cl/386175 mentions this issue: testing: panic in Fuzz if the function returns a value

[bcmills]

Marking as a release blocker as this is an API change.

Since the API is already documented to require “no return value” it's not technically an API change, but I agree that it should be a release-blocker. (Otherwise, it's too easy for users to accidentally rely on behavior counter to the documentation.)

[findleyr]

Since the API is already documented to require “no return value” it's not technically an API change

Indeed, I probably should have said "observable change" :)