document if let guards

src/changelog.rst

@@ -23,6 +23,31 @@ Language changes in Rust 1.95.0
 -------------------------------
 
 - `Stabilize if let guards on match arms <https://github.com/rust-lang/rust/pull/141295>`_
+
+  Changed syntax: :s:`MatchArmGuard`
+
+  New syntax:
+
+  - :s:`MatchArmGuardChain`
+  - :s:`MatchArmGuardCondition`
+  - :s:`MatchArmGuardExpression`
+  - :s:`MatchArmGuardLetPattern`
+
+  New paragraphs:
+
+  - :p:`fls_UlxLrpyPlVmv`
+  - :p:`fls_XADcpJBUxSfv`
+  - :p:`fls_gfHe2Cy6WXsK`
+  - :p:`fls_QQep7FKA1EQX`
+  - :p:`fls_Wepy5R7FZQPU`
+  - :p:`fls_imEIc7PUUO1x`
+  - :p:`fls_fs4ZpXjt0Wqt`
+  - :p:`fls_DT4N2rr6wpvZ`
+  - :p:`fls_AAuyKfxLgJ43`
+  - :p:`fls_uCDQMkWx5OMS`
+
+  Removed paragraph: :p:`fls_sbtx1l6n2tp2`
+
 - `irrefutable_let_patterns lint no longer lints on let chains <https://github.com/rust-lang/rust/pull/146832>`_
 
   - Lints are outside the scope of the FLS.

src/expressions.rst

@@ -4645,7 +4645,35 @@ Match Expressions
        OuterAttributeOrDoc* Pattern MatchArmGuard?
 
    MatchArmGuard ::=
-       $$if$$ Operand
+       $$if$$ (Operand | MatchArmGuardChain)
+
+   MatchArmGuardChain ::=
+       MatchArmGuardCondition ($$&&$$ MatchArmGuardCondition)*
+
+   MatchArmGuardCondition ::=
+       (MatchArmGuardExpression | OuterAttributeOrDoc* MatchArmGuardLetPattern)
+
+   MatchArmGuardLetPattern ::=
+       $$let$$ Pattern $$=$$ MatchArmGuardExpression
+
+   MatchArmGuardExpression ::=
+       Expression
+
+:dp:`fls_UlxLrpyPlVmv`
+A :dt:`match arm guard expression` is any :t:`expression` in category :s:`Expression`, except:
+
+- :dp:`fls_XADcpJBUxSfv`
+  :s:`AssignmentExpression`
+- :dp:`fls_gfHe2Cy6WXsK`
+  :s:`CompoundAssignmentExpression`
+- :dp:`fls_QQep7FKA1EQX`
+  :s:`LazyBooleanExpression`
+- :dp:`fls_Wepy5R7FZQPU`
+  :s:`RangeFromExpression`
+- :dp:`fls_imEIc7PUUO1x`
+  :s:`RangeFromToExpression`
+- :dp:`fls_fs4ZpXjt0Wqt`
+  :s:`RangeInclusiveExpression`
 
 .. rubric:: Legality Rules
 
@@ -4679,6 +4707,9 @@ A :t:`match arm body` is the :t:`operand` of a :t:`match arm`.
 A :t:`match arm guard` is a :t:`construct` that provides additional filtering to
 a :t:`match arm matcher`.
 
+:dp:`fls_DT4N2rr6wpvZ`
+A :dt:`match arm guard chain` is a set of conditions that must each evaluate to ``true`` in the case of :t:`[match arm guard expression]s`, or must each produce a positive match in the case of a :t:`[match arm guard let pattern]s` for the :t:`match arm` to be selected.
+
 :dp:`fls_RPMOAaZ6lflI`
 :t:`[Binding]s` introduced in the :t:`pattern` of a :t:`match arm matcher` are
 :t:`immutable` in the :t:`match arm guard`.
@@ -4715,6 +4746,12 @@ match the :t:`[subject expression]'s` :t:`type`.
 The :t:`value` of a :t:`match expression` is the :t:`value` of the :t:`operand`
 of the selected :t:`match arm`.
 
+:dp:`fls_AAuyKfxLgJ43`
+A :dt:`match arm guard let pattern` is evaluated when its :t:`match arm guard expression` matches the specified :t:`pattern`.
+
+:dp:`fls_uCDQMkWx5OMS`
+Each :t:`let binding` introduced in a :t:`match arm guard let pattern` is :t:`in scope` for the rest of the :t:`match arm guard` as well as the :t:`match arm body`.
+
 .. rubric:: Dynamic Semantics
 
 :dp:`fls_g551l8r8yh6d`
@@ -4767,11 +4804,6 @@ The :t:`evaluation` of a :t:`match arm matcher` proceeds as follows:
 #. :dp:`fls_yk8l9zjh7i0d`
    Otherwise the :t:`match arm matcher` fails.
 
-:dp:`fls_sbtx1l6n2tp2`
-The :t:`evaluation` of a :t:`match arm guard` evaluates its :t:`operand`. A
-:t:`match arm guard` evaluates to ``true`` when its :t:`operand` evaluates to
-``true``, otherwise it evaluates to ``false``.
-
 .. rubric:: Examples
 
 .. code-block:: rust