Specify const block expressions

Author: Veykril

src/entities-and-resolution.rst

@@ -496,6 +496,10 @@ related :t:`let statement` appears.
 The :t:`binding` of a :t:`match arm` is :t:`in scope` within its related
 :t:`[expression]s` and related :t:`match arm guard`.
 
+:dp:`fls_eBacCVlDaKYK`
+A :t:`binding` declared outside of a :t:`const block expression` is not :t:`in
+scope` within such a :t:`const block expression`.
+
 .. _fls_ftphlagzd2te:
 
 Generic Parameter Scope

src/expressions.rst

@@ -20,6 +20,7 @@ Expressions
        OuterAttributeOrDoc* (
            AsyncBlockExpression
          | BlockExpression
+         | ConstBlockExpression
          | IfExpression
          | IfLetExpression
          | LoopExpression
@@ -316,6 +317,9 @@ A :t:`constant context` is a :t:`construct` that requires a
 * :dp:`fls_ib8p7dfwddx2`
   The :t:`static initializer` of a :t:`static`.
 
+* :dp:`fls_ucFupTeCyylb`
+  The :t:`block expression` of a :t:`const block expression`.
+
 :dp:`fls_ox6sgl9n43g2`
 It is a static error to create a :t:`mutable reference` in a
 :t:`constant context`.
@@ -784,6 +788,48 @@ the :t:`[capture target]s` of the :t:`async block expression`.
        42
    }
 
+.. _fls_G59PiNQkVUnQ:
+
+Const Blocks
+~~~~~~~~~~~~
+
+.. rubric:: Syntax
+
+.. syntax::
+
+   ConstBlockExpression ::=
+       $$const$$ BlockExpression
+
+.. rubric:: Legality Rules
+
+:dp:`fls_0lcunL4bo8ka`
+A :t:`const block expression` is a :t:`block expression` that is specified
+with :t:`keyword` ``const`` and encapsulates behavior which is evaluated
+statically.
+
+:dp:`fls_veEGzEbpT4ny`
+An :t:`const block expression` denotes a new :t:`control flow boundary`.
+
+:dp:`fls_PiUS1hF3dv9U`
+The :t:`block expression` of a :t:`const block expression` shall be a
+:t:`constant expression`.
+
+:dp:`fls_wuwb0SnpP6Zu`
+The :t:`type` of a :t:`const block expression` is the :t:`type` of the
+containing :t:`block expression`.
+
+:dp:`fls_2i7TD7VoQk4B`
+The :t:`value` of a :t:`const block expression` is the :t:`value` of the
+contained :t:`block expression`.
+
+.. rubric:: Examples
+
+.. code-block:: rust
+
+   const {
+       42
+   }
+
 .. _fls_0ybsR1hEo7wV:
 
 Named Blocks