allow constants to refer to mutable/external memory, but reject such constants as patterns

Author: RalfJung

src/items/constant-items.md

@@ -48,9 +48,6 @@ const BITS_N_STRINGS: BitsNStrings<'static> = BitsNStrings {
 };
 ```
 
-r[items.const.final-value-immutable]
-The final value of a `const` item cannot contain references to anything mutable.
-
 r[items.const.expr-omission]
 The constant expression may only be omitted in a [trait definition].
 

src/patterns.md

@@ -975,6 +975,9 @@ r[patterns.const.generic]
 In particular, the value of `C` must be known at pattern-building time (which is pre-monomorphization).
 This means that associated consts that involve generic parameters cannot be used as patterns.
 
+r[patterns.const.immutable]
+Finally, the value of `C` must not contain any references to mutable statics (`static mut` or interior mutable) or `extern` statics.
+
 r[patterns.const.translation]
 After ensuring all conditions are met, the constant value is translated into a pattern, and now behaves exactly as-if that pattern had been written directly.
 In particular, it fully participates in exhaustiveness checking.