Rollup merge of #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]` (https://github.com/rust-lang/libs-team/issues/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 #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

compiler/rustc_trait_selection/src/error_reporting/infer/nice_region_error/find_anon_type.rs

@@ -77,7 +77,7 @@ impl<'tcx> Visitor<'tcx> for FindNestedTypeVisitor<'tcx> {
         match arg.kind {
             hir::TyKind::BareFn(_) => {
                 self.current_index.shift_in(1);
-                intravisit::walk_ty(self, arg);
+                let _ = intravisit::walk_ty(self, arg);
                 self.current_index.shift_out(1);
                 return ControlFlow::Continue(());
             }
@@ -85,7 +85,7 @@ impl<'tcx> Visitor<'tcx> for FindNestedTypeVisitor<'tcx> {
             hir::TyKind::TraitObject(bounds, ..) => {
                 for bound in bounds {
                     self.current_index.shift_in(1);
-                    self.visit_poly_trait_ref(bound);
+                    let _ = self.visit_poly_trait_ref(bound);
                     self.current_index.shift_out(1);
                 }
             }