document where mutable references are allowed in const items

Author: tshepang

src/changelog.rst

@@ -33,6 +33,18 @@ Language changes in Rust 1.90.0
 
     - Above paragraph replaces :p:`fls_6g7c1kjrmfnr` and :p:`fls_hkbwa8xx2fwx`
 
+  * New paragraphs:
+
+    - :p:`fls_ooOYxhVh8hZo`
+
+    - :p:`fls_zkNFeBLy80UA`
+
+    - :p:`fls_VhzGfnWg7YrG`
+
+    - :p:`fls_ibYKKQdB2tDn`
+
+    - :p:`fls_dQdSxf8kOgbi`
+
 - `Allow volatile access to non-Rust memory, including address 0 <https://github.com/rust-lang/rust/pull/141260>`_
 
   - No change: lints are not part of the FLS
@@ -336,8 +348,6 @@ Language changes in Rust 1.83.0
 
   * Changed paragraphs: :p:`fls_to4e7imq2c0w`, :p:`fls_6g7c1kjrmfnr`, :p:`fls_hkbwa8xx2fwx`
 
-  * New paragraph: :p:`fls_ooOYxhVh8hZo`
-
   * Removed paragraph: :p:`fls_ox6sgl9n43g2`
 
 * `Allow creating references to statics in \`const\` initializers. <https://github.com/rust-lang/rust/pull/129759>`_

src/values.rst

@@ -95,9 +95,6 @@ The :t:`type specification` of a :t:`constant` shall have ``'static``
 The :t:`type` of a :t:`constant` shall implement the :std:`core::marker::Sized`
 :t:`trait`.
 
-:dp:`fls_ooOYxhVh8hZo`
-The type of a :t:`constant` cannot be a :t:`mutable reference type`.
-
 :dp:`fls_ndmfqxjpvsqy`
 A :t:`constant initializer` is a :t:`construct` that provides the :t:`value` of
 its related :t:`constant`.
@@ -114,6 +111,23 @@ The :t:`expression` of a :t:`constant initializer` shall be a
 The value of a :t:`constant` is determined by evaluating its
 :t:`constant initializer`.
 
+:dp:`fls_ooOYxhVh8hZo`
+After a :t:`constant initializer` is evaluated to a value of the declared type,
+the value of the constant, after :t:`type coercion`,
+cannot contain any :t:`[mutable reference]s`, except when
+
+- :dp:`fls_zkNFeBLy80UA`
+  The :t:`mutable reference` is contained within an :t:`external static`, or
+
+- :dp:`fls_VhzGfnWg7YrG`
+  The :t:`mutable reference` is contained within a :t:`mutable static`, or
+
+- :dp:`fls_ibYKKQdB2tDn`
+  The :t:`mutable reference` is contained within an :t:`union`, or
+
+- :dp:`fls_dQdSxf8kOgbi`
+  The :t:`referent` is a value of a :t:`zero-sized type`.
+
 .. rubric:: Dynamic Semantics
 
 :dp:`fls_xezt9hl069h4`