Document inferred const args (feature(generic_arg_infer))

src/expressions/array-expr.md

@@ -35,7 +35,18 @@ r[expr.array.length-operand]
 The expression after the `;` is called the *length operand*.
 
 r[expr.array.length-restriction]
-It must have type `usize` and be a [constant expression], such as a [literal] or a [constant item].
+The length operand must either be an [inferred const] or be a [constant expression] of type `usize` (e.g. a [literal] or a [constant item]).
+
+```rust
+const C: usize = 1;
+let _: [u8; C] = [0; 1]; // Literal.
+let _: [u8; C] = [0; C]; // Constant item.
+let _: [u8; C] = [0; _]; // Inferred const.
+let _: [u8; C] = [0; (((_)))]; // Inferred const.
+```
+
+> [!NOTE]
+> In an array expression, an [inferred const] is parsed as an [expression][Expression] but then semantically treated as a separate kind of [const generic argument].
 
 r[expr.array.repeat-behavior]
 An array expression of this form creates an array with the length of the value of the length operand with each element being a copy of the repeat operand.
@@ -111,8 +122,10 @@ The array index expression can be implemented for types other than arrays and sl
 [IndexMut]: std::ops::IndexMut
 [Index]: std::ops::Index
 [array]: ../types/array.md
+[const generic argument]: items.generics.const.argument
 [constant expression]: ../const_eval.md#constant-expressions
 [constant item]: ../items/constant-items.md
+[inferred const]: items.generics.const.inferred
 [literal]: ../tokens.md#literals
 [memory location]: ../expressions.md#place-expressions-and-value-expressions
 [panic]: ../panic.md

src/items/generics.md

@@ -146,29 +146,55 @@ r[items.generics.const.argument]
 A const argument in a [path] specifies the const value to use for that item.
 
 r[items.generics.const.argument.const-expr]
-The argument must be a [const expression] of the type ascribed to the const
-parameter. The const expression must be a [block expression][block]
-(surrounded with braces) unless it is a single path segment (an [IDENTIFIER])
-or a [literal] (with a possibly leading `-` token).
+The argument must either be an [inferred const] or be a [const expression] of the type ascribed to the const parameter. The const expression must be a [block expression][block] (surrounded with braces) unless it is a single path segment (an [IDENTIFIER]) or a [literal] (with a possibly leading `-` token).
 
 > [!NOTE]
 > This syntactic restriction is necessary to avoid requiring infinite lookahead when parsing an expression inside of a type.
 
 ```rust
-fn double<const N: i32>() {
-    println!("doubled: {}", N * 2);
-}
+struct S<const N: i64>;
+const C: i64 = 1;
+fn f<const N: i64>() -> S<N> { S }
+
+let _ = f::<1>(); // Literal.
+let _ = f::<-1>(); // Negative literal.
+let _ = f::<{ 1 + 2 }>(); // Constant expression.
+let _ = f::<C>(); // Single segment path.
+let _ = f::<{ C + 1 }>(); // Constant expression.
+let _: S<1> = f::<_>(); // Inferred const.
+let _: S<1> = f::<(((_)))>(); // Inferred const.
+```
+
+> [!NOTE]
+> In a generic argument list, an [inferred const] is parsed as an [inferred type][InferredType] but then semantically treated as a separate kind of [const generic argument].
 
-const SOME_CONST: i32 = 12;
+r[items.generics.const.inferred]
+Where a const argument is expected, an `_` (optionally surrounding by any number of matching parentheses), called the *inferred const* ([path rules][paths.expr.complex-const-params], [array expression rules][expr.array.length-restriction]), can be used instead. This asks the compiler to infer the const argument if possible based on surrounding information.
 
-fn example() {
-    // Example usage of a const argument.
-    double::<9>();
-    double::<-123>();
-    double::<{7 + 8}>();
-    double::<SOME_CONST>();
-    double::<{ SOME_CONST + 5 }>();
+```rust
+fn make_buf<const N: usize>() -> [u8; N] {
+    [0; _]
+    //  ^ Infers `N`.
 }
+let _: [u8; 1024] = make_buf::<_>();
+//                             ^ Infers `1024`.
+```
+
+> [!NOTE]
+> An [inferred const] is not semantically an [expression][Expression] and so is not accepted within braces.
+>
+> ```rust,compile_fail
+> fn f<const N: usize>() -> [u8; N] { [0; _] }
+> let _: [_; 1] = f::<{ _ }>();
+> //                    ^ ERROR `_` not allowed here
+> ```
+
+r[items.generics.const.inferred.constraint]
+The inferred const cannot be used in item signatures.
+
+```rust,compile_fail
+fn f<const N: usize>(x: [u8; N]) -> [u8; _] { x }
+//                                       ^ ERROR not allowed
 ```
 
 r[items.generics.const.type-ambiguity]
@@ -293,6 +319,7 @@ struct Foo<#[my_flexible_clone(unbounded)] H> {
 [block]: ../expressions/block-expr.md
 [const contexts]: ../const_eval.md#const-context
 [const expression]: ../const_eval.md#constant-expressions
+[const generic argument]: items.generics.const.argument
 [const item]: constant-items.md
 [enumerations]: enumerations.md
 [functions]: functions.md
@@ -301,6 +328,7 @@ struct Foo<#[my_flexible_clone(unbounded)] H> {
 [generic parameter scopes]: ../names/scopes.md#generic-parameter-scopes
 [higher-ranked lifetimes]: ../trait-bounds.md#higher-ranked-trait-bounds
 [implementations]: implementations.md
+[inferred const]: items.generics.const.inferred
 [item declarations]: ../statements.md#item-declarations
 [item]: ../items.md
 [literal]: ../expressions/literal-expr.md

src/paths.md

@@ -91,8 +91,30 @@ The order of generic arguments is restricted to lifetime arguments, then type
 arguments, then const arguments, then equality constraints.
 
 r[paths.expr.complex-const-params]
-Const arguments must be surrounded by braces unless they are a
-[literal] or a single segment path.
+Const arguments must be surrounded by braces unless they are a [literal], an [inferred const], or a single segment path. An [inferred const] may not be surrounded by braces.
+
+```rust
+mod m {
+    pub const C: usize = 1;
+}
+const C: usize = m::C;
+fn f<const N: usize>() -> [u8; N] { [0; N] }
+
+let _ = f::<1>(); // Literal.
+let _: [_; 1] = f::<_>(); // Inferred const.
+let _: [_; 1] = f::<(((_)))>(); // Inferred const.
+let _ = f::<C>(); // Single segment path.
+let _ = f::<{ m::C }>(); // Multi-segment path must be braced.
+```
+
+```rust,compile_fail
+fn f<const N: usize>() -> [u8; N] { [0; _] }
+let _: [_; 1] = f::<{ _ }>();
+//                    ^ ERROR `_` not allowed here
+```
+
+> [!NOTE]
+> In a generic argument list, an [inferred const] is parsed as an [inferred type][InferredType] but then semantically treated as a separate kind of [const generic argument].
 
 r[paths.expr.impl-trait-params]
 The synthetic type parameters corresponding to `impl Trait` types are implicit,
@@ -480,10 +502,12 @@ mod without { // crate::without
 [`Self` scope]: names/scopes.md#self-scope
 [`use`]: items/use-declarations.md
 [attributes]: attributes.md
+[const generic argument]: items.generics.const.argument
 [enumeration]: items/enumerations.md
 [expressions]: expressions.md
 [extern prelude]: names/preludes.md#extern-prelude
 [implementation]: items/implementations.md
+[inferred const]: items.generics.const.inferred
 [macro transcribers]: macros-by-example.md
 [macros]: macros.md
 [mbe]: macros-by-example.md