New lint: let _ binding of must use function

[flip1995]

Extracted from #4655

A lint that detects let _ bindings of functions, that are must_use:

#[must_use]
fn foo() -> u32 { 42 }

fn bar() -> Result<(), ()> { Ok(()) }

let _ = foo();
let _ = bar();

On how to detect must use #4655 (comment):

We have a check for #[must_use] types in functions.rs already (IIRC is_must_use_ty(cx, ty), we best move this to utils/mod.rs), that you can call with the result of an expr_ty(cx, _) on the initializer.

Originally posted by @flip1995 in #4655 (comment)

[basil-cow]

I would like to tackle this.

[llogiq]

Great, go ahead. Tell us if you need anything.

[basil-cow]

Should we not lint Result<T, !> and Result<T, Infallible>? Also, is #[must_use] something like a type modifier, like mut?

[llogiq]

No, #[must_use] is an annotation that is only used by rustc's unused_must_use lint. And it's easy enough to unwrap() those results; in fact it shouldn't even generate code (I think we should be able to check if the Err type is uninhabited and give a suitable suggestion in those cases).

[basil-cow]

I just realized I assumed a bunch of things. 1. This issue does not talk about Result, even though it's parent does. If we do lint Result, do we lint against let _ = <expr_with_result_ty> or against let _ = <fn_call_returning_result>?. 2. Same goes for #[must_use], issue talks about against let _ = <fn_call_returning_result>, we could also lint against let _ = <expr_with_must_use_ty>

[llogiq]

Exactly. The lint checks hir::Locals (= let ..) with a wildcard pat and an init expression whose type (via expr_ty) has a must_use annotation.

This will catch, among other things, Result and MutexGuard.

If we want, we could extend the lint to also cover functions/methods directly marked as #[must_use].