Add clarifications

In the text as revised, it wasn't clear what rule prevented a mutable
reference from appearing in the final value of a constant item, so
let's add a rule specifically for that.

Similarly, it may not be immediately clear what rule prevents a
reference to a lifetime-extended interior mutable temporary created in
the initializer, so let's add an admonition section describing how
other rules apply to prevent this.

Author: traviscross

src/items/constant-items.md

@@ -48,6 +48,24 @@ const BITS_N_STRINGS: BitsNStrings<'static> = BitsNStrings {
 };
 ```
 
+r[items.const.final-value-immutable]
+The final value of a `const` item cannot contain a mutable reference.
+
+```rust
+static S: u8 = 0;
+const C1: &u8 = &mut S; // OK
+const C2: &mut u8 = &mut S; // ERROR not allowed
+```
+
+> [!NOTE]
+> We also disallow, in the final value, shared references to mutable statics created in the initializer for a separate reason. Consider:
+>
+> ```rust
+> const C: &AtomicU8 = &AtomicU8:new(0); // ERROR
+> ```
+>
+> Here, the `AtomicU8` is a temporary that is lifetime extended to `'static` (see [destructors.scope.lifetime-extension.static]), and references to lifetime-extended temporaries with interior mutability are not allowed in the final value of a constant expression (see [const-eval.const-expr.borrows]).
+
 r[items.const.expr-omission]
 The constant expression may only be omitted in a [trait definition].