add restrictions on constant expressions

Author: tshepang

src/changelog.rst

@@ -29,6 +29,7 @@ Language changes in Rust 1.90.0
 
   - This lifted restriction was not specified in the FLS
   - The restriction on patterns is documented in :p:`fls_wJ9f906BlBvg`
+  - New paragraph: :p:`fls_zyuxqty09SDO`
 
 - `Allow volatile access to non-Rust memory, including address 0 <https://github.com/rust-lang/rust/pull/141260>`_
 

src/expressions.rst

@@ -312,6 +312,12 @@ A :t:`constant context` is a :t:`construct` that requires a
 * :dp:`fls_ucFupTeCyylb`
   The :t:`block expression` of a :t:`const block expression`.
 
+* :dp:`fls_zyuxqty09SDO`
+  All forms of :t:`[borrow]s` except those of expressions that would be subject to
+  :t:`drop scope extension`,
+  and which are either :t:`[mutable borrow]s`
+  or borrows of expressions that result in values with :t:`interior mutability`.
+
 :dp:`fls_od0h3v40kjp6`
 An invocation of the :std:`core::ptr::addr_of` :t:`macro` expands to a
 :t:`constant expression` allowed in any :t:`constant context` and