document const generics

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 +19,34 @@
 > &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.
+
+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].
+They can be used freely outside of [const contexts].
+
+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]);
 ```
 
 [References], [raw pointers], [arrays], [slices][arrays], [tuples], and
@@ -65,7 +80,7 @@ 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 +127,15 @@ 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