bytes inside implicitly const-promoted expressions are immutable

Author: RalfJung

src/behavior-considered-undefined.md

@@ -49,7 +49,7 @@ Please read the [Rustonomicon] before writing unsafe code.
   All this also applies when values of these
   types are passed in a (nested) field of a compound type, but not behind
   pointer indirections.
-* Mutating immutable bytes. All bytes inside a [`const`] item are immutable.
+* Mutating immutable bytes. All bytes inside a [`const`] item or within an implicitly [const-promoted] expression are immutable.
   The bytes owned by an immutable binding or immutable `static` are immutable, unless those bytes are part of an [`UnsafeCell<U>`].
 
   Moreover, the bytes [pointed to] by a shared reference, including transitively through other references (both shared and mutable) and `Box`es, are immutable; transitivity includes those references stored in fields of compound types.
@@ -189,3 +189,4 @@ reading uninitialized memory is permitted are inside `union`s and in "padding"
 [project-field]: expressions/field-expr.md
 [project-tuple]: expressions/tuple-expr.md#tuple-indexing-expressions
 [project-slice]: expressions/array-expr.md#array-and-slice-indexing-expressions
+[const-promoted]: destructors.md#constant-promotion