Add a lint for unnecessary trailing return

[huonw]

fn foo() -> int {
    return 1; // warning: unnecessary `return`.
}

It would be extremely neat if it could handle

fn foo() -> int {
    if bar() {
        return 1;
    } else {
        return 2;
    }
}

(This is purely stylistic: how many style lints do we want/need?)

[alexcrichton]

If this is added, I think it should definitely be allow by default. It's a pretty cool idea, but I personally prefer to write return foo at the end of long functions instead of just foo (may just be me though).

[jesseray]

I omit return when working with a function/method definition or match expression that can fit comfortably on a single line, but I use return in all other situations. When I depart from this style, the code looks weird to me. (shrugs)

[Kimundi]

My personal experience is that you very quickly drop writing return everywhere except for early return after you've learned to accept last-express-return-value as natural.
If you look at the Rust source code, the majority of functions are written without return, so even if this lint would be off per default, it certainly makes sense to have it enabled for Rust to catch styling issues.

[steveklabnik]

triage: no change

personal note: would love to see this.

[sanxiyn]

This is implemented as needless_return in Clippy. It handles both of @huonw's examples and more. Check the test.