Rollup merge of rust-lang#137449 - compiler-errors:control-flow, r=Amanieu,lnicola

Denote `ControlFlow` as `#[must_use]`

I've repeatedly hit bugs in the compiler due to `ControlFlow` not being marked `#[must_use]`. There seems to be an accepted ACP to make the type `#[must_use]` (rust-lang/libs-team#444), so this PR implements that part of it.

Most of the usages in the compiler that trigger this new warning are "root" usages (calling into an API that uses control-flow internally, but for which the callee doesn't really care) and have been suppressed by `let _ = ...`, but I did legitimately find one instance of a missing `?` and one for a never-used `ControlFlow` value in rust-lang#137448.

Presumably this needs an FCP too, so I'm opening this and nominating it for T-libs-api.

This PR also touches the tools (incl. rust-analyzer), but if this went into FCP, I'd split those out into separate PRs which can land before this one does.

r? libs-api
`@rustbot` label: T-libs-api I-libs-api-nominated

Author: matthiaskrgr

core/src/ops/control_flow.rs

@@ -80,6 +80,7 @@ use crate::{convert, ops};
 /// [`Continue`]: ControlFlow::Continue
 #[stable(feature = "control_flow_enum_type", since = "1.55.0")]
 #[rustc_diagnostic_item = "ControlFlow"]
+#[must_use]
 // ControlFlow should not implement PartialOrd or Ord, per RFC 3058:
 // https://rust-lang.github.io/rfcs/3058-try-trait-v2.html#traits-for-controlflow
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]