- Feature Name: `generic_named_constants`
- Start Date: (fill me in with today's date, YYYY-MM-DD)
- RFC PR: [rust-lang/rfcs#0000](https://github.com/rust-lang/rfcs/pull/0000)
- Rust Issue: [rust-lang/rust#0000](https://github.com/rust-lang/rust/issues/0000)

# Summary
[summary]: #summary

Allow associated and free constants to be generic.

# Motivation
[motivation]: #motivation

Unlike `const fn`, a named constant will always be fully evaluated at compile time, and are a better semantic representation of - well - constants.

| Thing | Generic in trait | Generic when used free |
| --- | --- | --- |
| fn | :heavy_check_mark: | :heavy_check_mark: |
| type | :heavy_check_mark: (only over `Self` without GATs) | :heavy_check_mark: (`type Foo<T>`) |
| const | :heavy_check_mark: (associated constants of generic types) | :x: |

A good example here would be the implementation of [`BTree`][1] in the standard library which currently uses the following constants:

const B: usize = 6;
pub const CAPACITY: usize = 2 * B - 1;
pub const MIN_LEN_AFTER_SPLIT: usize = B - 1;
const KV_IDX_CENTER: usize = B - 1;
const EDGE_IDX_LEFT_OF_CENTER: usize = B - 1;
const EDGE_IDX_RIGHT_OF_CENTER: usize = B;

Note that while `B` is currently a constant itself, it would potentially desirable to convert it to a generic parameter instead. Without generic named constants, the other constants would have to be redefined in some less ideal way.

In general, generic named constant tend to make sense when defining constants used in a generic context while not strictly bound to a specific type.

Generic named constants do not add much complexity to the language and improve its consistency as generic associated types and type aliases already exist.

# Guide-level explanation
[guide-level-explanation]: #guide-level-explanation

It is now possible to specify generic parameters and `where`-clauses for free and associated constants.

const MIN<N: usize>: [u32; N] = [0; N];
const MAX<N: usize>: [u32; N] = [0xFFFF_FFFF; N];

struct MyTree<T> {
    children: (Option<Box<MyTree<T>>>, Option<Box<MyTree<T>>>)
}

const LEAF<T>: (Option<Box<MyTree<T>>>, Option<Box<MyTree<T>>>) = (None, None);

MyTree { children: LEAF }

const SIZE_CAP<T>: usize = if mem::size_of::<T>() == 1 {
    8
} else {
    1
};

struct Foo<T>(T);

impl<T> Foo<T> {
    const ASSOC: usize where T: Trait = some_expr;
}

# Reference-level explanation
[reference-level-explanation]: #reference-level-explanation

Either uses the current system for gats or for associated consts of generic types, depending on what is applicable.

eval, when and how. -- i.e., will fully evaluate if not containing associating constants

# Drawbacks
[drawbacks]: #drawbacks

Why should we *not* do this?

# Rationale and alternatives
[rationale-and-alternatives]: #rationale-and-alternatives

- Why is this design the best in the space of possible designs?
- What other designs have been considered and what is the rationale for not choosing them?
- What is the impact of not doing this?

# Prior art
[prior-art]: #prior-art

Discuss prior art, both the good and the bad, in relation to this proposal.
A few examples of what this can include are:

- For language, library, cargo, tools, and compiler proposals: Does this feature exist in other programming languages and what experience have their community had?
- For community proposals: Is this done by some other community and what were their experiences with it?
- For other teams: What lessons can we learn from what other communities have done here?
- Papers: Are there any published papers or great posts that discuss this? If you have some relevant papers to refer to, this can serve as a more detailed theoretical background.

This section is intended to encourage you as an author to think about the lessons from other languages, provide readers of your RFC with a fuller picture.
If there is no prior art, that is fine - your ideas are interesting to us whether they are brand new or if it is an adaptation from other languages.

Note that while precedent set by other languages is some motivation, it does not on its own motivate an RFC.
Please also take into consideration that rust sometimes intentionally diverges from common language features.

# Unresolved questions
[unresolved-questions]: #unresolved-questions

- What parts of the design do you expect to resolve through the RFC process before this gets merged?
- What parts of the design do you expect to resolve through the implementation of this feature before stabilization?
- What related issues do you consider out of scope for this RFC that could be addressed in the future independently of the solution that comes out of this RFC?

# Future possibilities
[future-possibilities]: #future-possibilities

Think about what the natural extension and evolution of your proposal would
be and how it would affect the language and project as a whole in a holistic
way. Try to use this section as a tool to more fully consider all possible
interactions with the project and language in your proposal.
Also consider how this all fits into the roadmap for the project
and of the relevant sub-team.

This is also a good place to "dump ideas", if they are out of scope for the
RFC you are writing but otherwise related.

If you have tried and cannot think of any future possibilities,
you may simply state that you cannot think of anything.

Note that having something written down in the future-possibilities section
is not a reason to accept the current or a future RFC; such notes should be
in the section on motivation or rationale in this or subsequent RFCs.
The section merely provides additional information.

[1]: https://github.com/rust-lang/rust/blob/0bcacb391b28460f5a50fd627f01f670dfcfc7cc/library/alloc/src/collections/btree/node.rs#L42-L47