Rollup merge of rust-lang#153018 - WaffleLapkin:must_use_improvements, r=jdonszelmann

`unused_must_use` lint improvements

The main purpose of this is to allow clippy to use `is_ty_must_use` from rustc, rather than making their own (outdated) copy. This also adds changes from rust-lang#148214 under a function flag, so clippy can start linting with the new semantics in mind, while rustc doesn't yet (see rust-lang#148214 (comment)).

I've also done some refactoring while I was at it.

r? @jdonszelmann

Author: JonathanBrouwer

compiler/rustc_lint/src/lib.rs

@@ -78,7 +78,7 @@ mod transmute;
 mod types;
 mod unit_bindings;
 mod unqualified_local_imports;
-mod unused;
+pub mod unused;
 mod utils;
 
 use async_closures::AsyncClosureUsage;
@@ -125,6 +125,7 @@ use transmute::CheckTransmutes;
 use types::*;
 use unit_bindings::*;
 use unqualified_local_imports::*;
+use unused::must_use::*;
 use unused::*;
 
 #[rustfmt::skip]