Auto merge of #102256 - cjgillot:let-under, r=lcnr

Introduce a no-op `PlaceMention` statement for `let _ =`.

Fixes rust-lang/rust#54003
Fixes rust-lang/rust#80059
Split from rust-lang/rust#101500

This PR introduces a new `PlaceMention` statement dedicated to matches that neither introduce bindings nor ascribe types.  Without this, all traces of the match would vanish from MIR, making it impossible to diagnose unsafety or use in #101500.

This allows to mark `let _ = <unsafe union access or dereference>` as requiring an unsafe block.
Nominating for lang team, as this introduces an extra error.

Author: bors

src/base.rs

@@ -819,6 +819,7 @@ fn codegen_stmt<'tcx>(
         | StatementKind::Nop
         | StatementKind::FakeRead(..)
         | StatementKind::Retag { .. }
+        | StatementKind::PlaceMention(..)
         | StatementKind::AscribeUserType(..) => {}
 
         StatementKind::Coverage { .. } => fx.tcx.sess.fatal("-Zcoverage is unimplemented"),

src/constant.rs

@@ -529,6 +529,7 @@ pub(crate) fn mir_operand_get_const_val<'tcx>(
                         | StatementKind::StorageDead(_)
                         | StatementKind::Retag(_, _)
                         | StatementKind::AscribeUserType(_, _)
+                        | StatementKind::PlaceMention(..)
                         | StatementKind::Coverage(_)
                         | StatementKind::ConstEvalCounter
                         | StatementKind::Nop => {}