Add const blocks

Author: nbdd0121

src/const_eval.md

@@ -27,7 +27,7 @@ to be run.
 * [Tuple expressions].
 * [Array expressions].
 * [Struct] expressions.
-* [Block expressions], including `unsafe` blocks.
+* [Block expressions], including `unsafe` and `const` blocks.
     * [let statements] and thus irrefutable [patterns], including mutable bindings
     * [assignment expressions]
     * [compound assignment expressions]
@@ -59,6 +59,7 @@ A _const context_ is one of the following:
   * [statics]
   * [enum discriminants]
 * A [const generic argument]
+* A [const block]
 
 ## Const Functions
 
@@ -106,6 +107,7 @@ Conversely, the following are possible in a const function, but not in a const c
 [cast]:                 expressions/operator-expr.md#type-cast-expressions
 [closure expressions]:  expressions/closure-expr.md
 [comparison]:           expressions/operator-expr.md#comparison-operators
+[const block]:          expressions/block-expr.md#const-blocks
 [const functions]:      items/functions.md#const-functions
 [const generic argument]: items/generics.md#const-generics
 [const generic parameters]: items/generics.md#const-generics

src/expressions.md

@@ -35,6 +35,7 @@
 > &nbsp;&nbsp; [_OuterAttribute_]<sup>\*</sup>[†](#expression-attributes)\
 > &nbsp;&nbsp; (\
 > &nbsp;&nbsp; &nbsp;&nbsp; &nbsp;&nbsp; [_BlockExpression_]\
+> &nbsp;&nbsp; &nbsp;&nbsp; | [_ConstBlockExpression_]\
 > &nbsp;&nbsp; &nbsp;&nbsp; | [_UnsafeBlockExpression_]\
 > &nbsp;&nbsp; &nbsp;&nbsp; | [_LoopExpression_]\
 > &nbsp;&nbsp; &nbsp;&nbsp; | [_IfExpression_]\
@@ -311,6 +312,7 @@ They are never allowed before:
 [_ClosureExpression_]:            expressions/closure-expr.md
 [_ComparisonExpression_]:         expressions/operator-expr.md#comparison-operators
 [_CompoundAssignmentExpression_]: expressions/operator-expr.md#compound-assignment-expressions
+[_ConstBlockExpression_]:         expressions/block-expr.md#const-blocks
 [_ContinueExpression_]:           expressions/loop-expr.md#continue-expressions
 [_FieldExpression_]:              expressions/field-expr.md
 [_GroupedExpression_]:            expressions/grouped-expr.md

src/expressions/block-expr.md

@@ -117,6 +117,44 @@ loop {
 }
 ```
 
+## `const` blocks
+
+> **<sup>Syntax</sup>**\
+> _ConstBlockExpression_ :\
+> &nbsp;&nbsp; `const` _BlockExpression_
+
+A *const block* is a variant of a block expression which evaluates in the compile time instead of in the run time.
+
+Const blocks allows you to define a constant value without having to define new [constant items], and thus they are also sometimes referred as *inline consts*.
+It also supports type inference so there is no need to specify the type, unlike [constant items].
+
+Const blocks have ability to reference generic parameters in scope, unlike [free][free item] constant items.
+They are desugared to associated constant items with generic parameters in scope.
+For example, this code:
+
+```rust
+fn foo<T>() -> usize {
+    const { std::mem::size_of::<T>() + 1 }
+}
+```
+
+is equivalent to:
+
+```rust
+fn foo<T>() -> usize {
+    {
+        struct Const<T>(T);
+        impl<T> Const<T> {
+            const CONST: usize = std::mem::size_of::<T>() + 1;
+        }
+        Const::<T>::CONST
+    }
+}
+```
+
+This also means that const blocks are treated similarly to associated constants.
+For example, they are not guaranteed to be evaluated when the enclosing function is unused.
+
 ## `unsafe` blocks
 
 > **<sup>Syntax</sup>**\
@@ -181,6 +219,8 @@ fn is_unix_platform() -> bool {
 [array expressions]: array-expr.md
 [call expressions]: call-expr.md
 [capture modes]: ../types/closure.md#capture-modes
+[constant items]: ../items/constant-items.md
+[free item]: ../glossary.md#free-item
 [function]: ../items/functions.md
 [inner attributes]: ../attributes.md
 [method]: ../items/associated-items.md#methods

src/macros-by-example.md

@@ -146,6 +146,7 @@ For reasons of backwards compatibility, though `_` [is also an
 expression][_UnderscoreExpression_], a standalone underscore is not matched by
 the `expr` fragment specifier. However, `_` is matched by the `expr` fragment
 specifier when it appears as a subexpression.
+For the same reason, a standalone [const block] is not matched but it is matched when appearing as a subexpression.
 
 > **Edition Differences**: Starting with the 2021 edition, `pat` fragment-specifiers match top-level or-patterns (that is, they accept [_Pattern_]).
 >
@@ -492,6 +493,7 @@ expansions, taking separators into account. This means:
 
 For more detail, see the [formal specification].
 
+[const block]: expressions/block-expr.md#const-blocks
 [Hygiene]: #hygiene
 [IDENTIFIER]: identifiers.md
 [IDENTIFIER_OR_KEYWORD]: identifiers.md