Document promotion of match scrutinees due to guards

Borrowing part of a match scrutinee from within the guard can trigger
constant promotion even if that part of the scrutinee is later moved
into a variable.  Let's add a note about this.

Author: traviscross

src/expressions/match-expr.md

@@ -147,6 +147,23 @@ r[expr.match.guard.value]
 Only when the guard evaluates to true is the value moved, or copied, from the scrutinee into the variable.
 This allows shared borrows to be used inside guards without moving out of the scrutinee in case guard fails to match.
 
+> [!NOTE]
+> Due to this, borrowing within the guard can cause [constant promotion] of the part of the scrutinee borrowed, even if that part is later moved into a variable.
+>
+> ```rust
+> struct S;
+> let _y: &'static S;
+> match S {
+>     x if { _y = &x; true } => {}
+> //  ^      ^^^^^^^ 1. This happens first, before the move into `x`.
+> //  |
+> //  2. The move into `x` happens second, after `x` has been promoted.
+>     _ => (),
+> }
+> ```
+>
+> See [issue #145237](https://github.com/rust-lang/rust/issues/145237) for more details.
+
 r[expr.match.guard.no-mutation]
 Moreover, by holding a shared reference while evaluating the guard, mutation inside guards is also prevented.
 
@@ -163,6 +180,7 @@ r[expr.match.attributes.inner]
 [`cfg`]: ../conditional-compilation.md
 [attributes on block expressions]: block-expr.md#attributes-on-block-expressions
 [binding mode]: ../patterns.md#binding-modes
+[constant promotion]: destructors.scope.const-promotion
 [Inner attributes]: ../attributes.md
 [lint check attributes]: ../attributes/diagnostics.md#lint-check-attributes
 [pattern]: ../patterns.md