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document const generics #901

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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
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Update src/items/generics.md
lcnr Dec 29, 2020
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Update src/items/generics.md
lcnr Dec 29, 2020
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review
lcnr Dec 29, 2020
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generics
lcnr Dec 29, 2020
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update generic arg docs
lcnr Dec 29, 2020
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lcnr Dec 29, 2020
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Update _GenericArgsConsts_ grammar.
ehuss Jan 1, 2021
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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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36 changes: 27 additions & 9 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 +19,34 @@
> &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
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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.
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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].
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I find these sentences a little difficult to understand. Can you add more detail, along with some examples?

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