Merge pull request #901 from lcnr/const-generics

document const generics

Author: ehuss

src/SUMMARY.md

@@ -35,7 +35,7 @@
     - [Traits](items/traits.md)
     - [Implementations](items/implementations.md)
     - [External blocks](items/external-blocks.md)
-    - [Type and lifetime parameters](items/generics.md)
+    - [Generic parameters](items/generics.md)
     - [Associated Items](items/associated-items.md)
     - [Visibility and Privacy](visibility-and-privacy.md)
 

src/const_eval.md

@@ -20,6 +20,7 @@ also constant expressions and do not cause any [`Drop::drop`][destructors] calls
 to be run.
 
 * [Literals].
+* [Const parameters].
 * [Paths] to [functions] and [constants].
   Recursively defining constants is not allowed.
 * Paths to [statics]. These are only allowed within the initializer of a static.
@@ -112,6 +113,7 @@ Conversely, the following are possible in a const function, but not in a const c
 [comparison]:           expressions/operator-expr.md#comparison-operators
 [const functions]:      items/functions.md#const-functions
 [constants]:            items/constant-items.md
+[Const parameters]:     items/generics.md
 [dereference operator]: expressions/operator-expr.md#the-dereference-operator
 [destructors]:          destructors.md
 [enum discriminants]:   items/enumerations.md#custom-discriminant-values-for-fieldless-enumerations

src/items/generics.md

@@ -1,12 +1,13 @@
-# Type and Lifetime Parameters
+# Generic parameters
 
 > **<sup>Syntax</sup>**\
 > _Generics_ :\
 > &nbsp;&nbsp; `<` _GenericParams_ `>`
 >
 > _GenericParams_ :\
 > &nbsp;&nbsp; &nbsp;&nbsp; _LifetimeParams_\
-> &nbsp;&nbsp; | ( _LifetimeParam_ `,` )<sup>\*</sup> _TypeParams_
+> &nbsp;&nbsp; | ( _LifetimeParam_ `,` )<sup>\*</sup> _TypeParams_\
+> &nbsp;&nbsp; | ( _LifetimeParam_ `,` )<sup>\*</sup> ( _TypeParam_ `,` )<sup>\*</sup> _ConstParams_
 >
 > _LifetimeParams_ :\
 > &nbsp;&nbsp; ( _LifetimeParam_ `,` )<sup>\*</sup> _LifetimeParam_<sup>?</sup>
@@ -18,20 +19,55 @@
 > &nbsp;&nbsp; ( _TypeParam_ `,` )<sup>\*</sup> _TypeParam_<sup>?</sup>
 >
 > _TypeParam_ :\
-> &nbsp;&nbsp; [_OuterAttribute_]<sup>?</sup> [IDENTIFIER] ( `:` [_TypeParamBounds_]<sup>?</sup> )<sup>?</sup> ( `=` [_Type_] )<sup>?</sup>
+> &nbsp;&nbsp; [_OuterAttribute_]<sup>?</sup> [IDENTIFIER]( `:` [_TypeParamBounds_]<sup>?</sup> )<sup>?</sup> ( `=` [_Type_] )<sup>?</sup>
+>
+> _ConstParams_:\
+> &nbsp;&nbsp; ( _ConstParam_ `,` )<sup>\*</sup> _ConstParam_<sup>?</sup>
+>
+> _ConstParam_:\
+> &nbsp;&nbsp; [_OuterAttribute_]<sup>?</sup> `const` [IDENTIFIER] `:` [_Type_]
 
 Functions, type aliases, structs, enumerations, unions, traits, and
-implementations may be *parameterized* by types and lifetimes. These parameters
-are listed in angle <span class="parenthetical">brackets (`<...>`)</span>,
+implementations may be *parameterized* by types, constants, and lifetimes. These
+parameters are listed in angle <span class="parenthetical">brackets (`<...>`)</span>,
 usually immediately after the name of the item and before its definition. For
 implementations, which don't have a name, they come directly after `impl`.
-Lifetime parameters must be declared before type parameters. Some examples of
-items with type and lifetime parameters:
+The order of generic parameters is restricted to lifetime parameters, then type parameters, and then const parameters.
+
+Some examples of items with type, const, and lifetime parameters:
 
 ```rust
 fn foo<'a, T>() {}
 trait A<U> {}
 struct Ref<'a, T> where T: 'a { r: &'a T }
+struct InnerArray<T, const N: usize>([T; N]);
+```
+
+The only allowed types of const parameters are `u8`, `u16`, `u32`, `u64`, `u128`, `usize`
+`i8`, `i16`, `i32`, `i64`, `i128`, `isize`, `char` and `bool`.
+
+Const parameters may only be be used as standalone arguments inside
+of [types] and [repeat expressions] but may be freely used elsewhere:
+
+```rust,compile_fail
+// ok: standalone argument
+fn foo<const N: usize>() -> [u8; N] { todo!() }
+
+// ERROR: generic const operation
+fn bar<const N: usize>() -> [u8; N + 1] { todo!() }
+```
+
+Unlike type and lifetime parameters, const parameters of types can be used without
+being mentioned inside of a parameterized type:
+
+```rust,compile_fail
+// ok
+struct Foo<const N: usize>;
+enum Bar<const M: usize> { A, B }
+
+// ERROR: unused parameter
+struct Baz<T>;
+struct Biz<'a>;
 ```
 
 [References], [raw pointers], [arrays], [slices][arrays], [tuples], and
@@ -55,7 +91,7 @@ referred to with path syntax.
 > &nbsp;&nbsp; _ForLifetimes_<sup>?</sup> [_Type_] `:` [_TypeParamBounds_]<sup>?</sup>
 >
 > _ForLifetimes_ :\
-> &nbsp;&nbsp; `for` `<` [_LifetimeParams_](#type-and-lifetime-parameters) `>`
+> &nbsp;&nbsp; `for` `<` [_LifetimeParams_](#generic-parameters) `>`
 
 *Where clauses* provide another way to specify bounds on type and lifetime
 parameters as well as a way to specify bounds on types that aren't type
@@ -65,7 +101,7 @@ Bounds that don't use the item's parameters or higher-ranked lifetimes are
 checked when the item is defined. It is an error for such a bound to be false.
 
 [`Copy`], [`Clone`], and [`Sized`] bounds are also checked for certain generic
-types when defining the item. It is an error to have `Copy` or `Clone`as a
+types when defining the item. It is an error to have `Copy` or `Clone` as a
 bound on a mutable reference, [trait object] or [slice][arrays] or `Sized` as a
 bound on a trait object or slice.
 
@@ -112,12 +148,15 @@ struct Foo<#[my_flexible_clone(unbounded)] H> {
 [_TypeParamBounds_]: ../trait-bounds.md
 
 [arrays]: ../types/array.md
+[const contexts]: ../const_eval.md#const-context
 [function pointers]: ../types/function-pointer.md
-[references]: ../types/pointer.md#shared-references-
 [raw pointers]: ../types/pointer.md#raw-pointers-const-and-mut
+[references]: ../types/pointer.md#shared-references-
+[repeat expressions]: ../expressions/array-expr.md
 [`Clone`]: ../special-types-and-traits.md#clone
 [`Copy`]: ../special-types-and-traits.md#copy
 [`Sized`]: ../special-types-and-traits.md#sized
 [tuples]: ../types/tuple.md
 [trait object]: ../types/trait-object.md
+[types]: ../types.md
 [attributes]: ../attributes.md

src/paths.md

@@ -52,18 +52,32 @@ mod m {
 > &nbsp;&nbsp; &nbsp;&nbsp; `<` `>`\
 > &nbsp;&nbsp; | `<` _GenericArgsLifetimes_ `,`<sup>?</sup> `>`\
 > &nbsp;&nbsp; | `<` _GenericArgsTypes_ `,`<sup>?</sup> `>`\
+> &nbsp;&nbsp; | `<` _GenericArgsConsts_ `,`<sup>?</sup> `>`\
 > &nbsp;&nbsp; | `<` _GenericArgsBindings_ `,`<sup>?</sup> `>`\
-> &nbsp;&nbsp; | `<` _GenericArgsTypes_ `,` _GenericArgsBindings_ `,`<sup>?</sup> `>`\
 > &nbsp;&nbsp; | `<` _GenericArgsLifetimes_ `,` _GenericArgsTypes_ `,`<sup>?</sup> `>`\
+> &nbsp;&nbsp; | `<` _GenericArgsLifetimes_ `,` _GenericArgsConsts_ `,`<sup>?</sup> `>`\
 > &nbsp;&nbsp; | `<` _GenericArgsLifetimes_ `,` _GenericArgsBindings_ `,`<sup>?</sup> `>`\
-> &nbsp;&nbsp; | `<` _GenericArgsLifetimes_ `,` _GenericArgsTypes_ `,` _GenericArgsBindings_ `,`<sup>?</sup> `>`
+> &nbsp;&nbsp; | `<` _GenericArgsLifetimes_ `,` _GenericArgsTypes_ `,` _GenericArgsConsts_ `,`<sup>?</sup> `>`\
+> &nbsp;&nbsp; | `<` _GenericArgsLifetimes_ `,` _GenericArgsTypes_ `,` _GenericArgsBindings_ `,`<sup>?</sup> `>`\
+> &nbsp;&nbsp; | `<` _GenericArgsLifetimes_ `,` _GenericArgsConsts_ `,` _GenericArgsBindings_ `,`<sup>?</sup> `>`\
+> &nbsp;&nbsp; | `<` _GenericArgsTypes_ `,` _GenericArgsConsts_ `,` _GenericArgsBindings_ `,`<sup>?</sup> `>`\
+> &nbsp;&nbsp; | `<` _GenericArgsLifetimes_ `,` _GenericArgsTypes_ `,` _GenericArgsConsts_ `,` _GenericArgsBindings_ `,`<sup>?</sup> `>`
 >
 > _GenericArgsLifetimes_ :\
 > &nbsp;&nbsp; [_Lifetime_] (`,` [_Lifetime_])<sup>\*</sup>
 >
 > _GenericArgsTypes_ :\
 > &nbsp;&nbsp; [_Type_] (`,` [_Type_])<sup>\*</sup>
 >
+> _GenericArgsConsts_ :\
+> &nbsp;&nbsp; _GenericArgsConst_ (`,` _GenericArgsConst_)<sup>\*</sup>
+>
+> _GenericArgsConst_ :\
+> &nbsp;&nbsp; &nbsp;&nbsp; [_BlockExpression_]\
+> &nbsp;&nbsp; | [_LiteralExpression_]\
+> &nbsp;&nbsp; | `-` [_LiteralExpression_]\
+> &nbsp;&nbsp; | [_SimplePathSegment_]
+>
 > _GenericArgsBindings_ :\
 > &nbsp;&nbsp; _GenericArgsBinding_ (`,` _GenericArgsBinding_)<sup>\*</sup>
 >
@@ -81,6 +95,9 @@ ambiguity with the less-than operator. This is colloquially known as "turbofish"
 Vec::<u8>::with_capacity(1024);
 ```
 
+Const arguments must be surrounded by braces unless they are a
+[literal] or a single segment path.
+
 ## Qualified paths
 
 > **<sup>Syntax</sup>**\
@@ -364,9 +381,14 @@ mod without { // ::without
 # fn main() {}
 ```
 
+[_BlockExpression_]: expressions/block-expr.md
+[_Expression_]: expressions.md
 [_GenericArgs_]: #paths-in-expressions
 [_Lifetime_]: trait-bounds.md
+[_LiteralExpression_]: expressions/literal-expr.md
+[_SimplePathSegment_]: #simple-paths
 [_Type_]: types.md#type-expressions
+[literal]: expressions/literal-expr.md
 [item]: items.md
 [variable]: variables.md
 [implementations]: items/implementations.md