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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -1,12 +1,13 @@ | ||
| # Generic parameters | ||
| > **<sup>Syntax</sup>**\ | ||
| > _Generics_ :\ | ||
| > `<` _GenericParams_ `>` | ||
| > | ||
| > _GenericParams_ :\ | ||
| > _LifetimeParams_\ | ||
| > | ( _LifetimeParam_ `,` )<sup>\*</sup> _TypeParams_\ | ||
| > | ( _LifetimeParam_ `,` )<sup>\*</sup> ( _TypeParam_ `,` )<sup>\*</sup> _ConstParams_ | ||
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Contributor There was a problem hiding this comment. I believe this grammar is no longer correct (see #785). The order of the parameters doesn't matter syntactically. You don't have to fix it if you don't want to (although that would be nice), I just wanted to make a note of it. ContributorAuthor There was a problem hiding this comment. won't do this in that PR, as I don't know what it's expected to look like now. | ||
| > | ||
| > _LifetimeParams_ :\ | ||
| > ( _LifetimeParam_ `,` )<sup>\*</sup> _LifetimeParam_<sup>?</sup> | ||
| @@ -18,20 +19,55 @@ | ||
| > ( _TypeParam_ `,` )<sup>\*</sup> _TypeParam_<sup>?</sup> | ||
| > | ||
| > _TypeParam_ :\ | ||
| > [_OuterAttribute_]<sup>?</sup> [IDENTIFIER]( `:` [_TypeParamBounds_]<sup>?</sup> )<sup>?</sup> ( `=` [_Type_] )<sup>?</sup> | ||
| > | ||
| > _ConstParams_:\ | ||
| > ( _ConstParam_ `,` )<sup>\*</sup> _ConstParam_<sup>?</sup> | ||
| > | ||
| > _ConstParam_:\ | ||
| > [_OuterAttribute_]<sup>?</sup> `const` [IDENTIFIER] `:` [_Type_] | ||
| Functions, type aliases, structs, enumerations, unions, traits, and | ||
| 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`. | ||
| 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. | ||
| > _ForLifetimes_<sup>?</sup> [_Type_] `:` [_TypeParamBounds_]<sup>?</sup> | ||
| > | ||
| > _ForLifetimes_ :\ | ||
| > `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 | ||
| 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 | ||
| [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 | ||
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