Lint for #[must_use] on a function that always returns a #[must_use] type

[scottmcm]

I saw someone do this on a try_from today, so I figured I'd make an issue.

Repro:

#[must_use]
pub fn foo() -> Result<(), ()> { Ok(()) }

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=c03ca4925a9ac4ce9fdda4d2165e6892

The #[must_use] on the function there is unnecessary because it always returns Result, which is itself #[must_use]. So we might consider linting about that.

[joshtriplett]

This does seem worth linting about, but not just because it's redundant. Someone might write

#[must_use]
pub fn foo() -> Result<SomeType, SomeError> { ... }

and expect that the #[must_use] will apply to the SomeType, rather than just to the Result.

[cuviper]

It should probably only lint for #[must_use] without any message. In some cases, you might want to provide a contextual #[must_use = "this is important"] beyond the type's own attribute.