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document const generics #901

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Jan 6, 2021
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document const generics #901

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1bd2340
mention const generics
lcnr Nov 11, 2020
1c55d27
Update src/const_eval.md
lcnr Dec 27, 2020
fec814c
Update src/items/generics.md
lcnr Dec 29, 2020
ce44ec2
Update src/items/generics.md
lcnr Dec 29, 2020
7c659ad
Update src/items/generics.md
lcnr Dec 29, 2020
49f9b34
review
lcnr Dec 29, 2020
210191a
generics
lcnr Dec 29, 2020
bd714f6
update generic arg docs
lcnr Dec 29, 2020
9fe55ae
asd
lcnr Dec 29, 2020
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asd
lcnr Dec 29, 2020
15ec5a6
Update _GenericArgsConsts_ grammar.
ehuss Jan 1, 2021
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2 changes: 1 addition & 1 deletion src/SUMMARY.md
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Expand Up @@ -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)

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2 changes: 2 additions & 0 deletions src/const_eval.md
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Expand Up @@ -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.
Expand Down Expand Up @@ -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
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59 changes: 49 additions & 10 deletions src/items/generics.md
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@@ -1,12 +1,13 @@
# Type and Lifetime Parameters
# Generic parameters
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> **<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_
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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.

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won't do this in that PR, as I don't know what it's expected to look like now.

>
> _LifetimeParams_ :\
> &nbsp;&nbsp; ( _LifetimeParam_ `,` )<sup>\*</sup> _LifetimeParam_<sup>?</sup>
Expand All @@ -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
Expand All @@ -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
Expand All @@ -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.

Expand Down Expand Up @@ -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
26 changes: 24 additions & 2 deletions src/paths.md
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Expand Up @@ -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>
>
Expand All @@ -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.
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in this case this section might not be needed


## Qualified paths

> **<sup>Syntax</sup>**\
Expand Down Expand Up @@ -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
Expand Down