Don't remove const generic when using `#[feature(generic_const_items)]`

[ytmimi]

Fixes #5995

Added support for rewriting generics on const items.

[fmease]

Could you add a test for where-clauses, too? That would be great! Thanks in advance :)

[ytmimi]

@fmease happy to add the test case, but would you mind providing the code snippet. I'm not as familiar with this feature as you are.

[fmease]

Sure, for example:

pub const K<T>: Option<T> = None
where
    String: From<T>;

And:

pub trait Trait<T: ?Sized> {
    const C<'a>: &'a T
    where
        T: 'a + Eq;
}

[fmease]

Note that generic const items don't have an entry inside the Style Guide yet as you might or might not have noticed. However, the formatting should basically follow the new^1 formatting of type aliases and associated types (except that they don't need to deal with legacy-style leading where clauses, cc #5887).

[ytmimi]

Looks like there's still a little work to be done. Formatting with this branch on the snippets you provided removes the where clauses:

input:

pub const K<T>: Option<T> = None
where
    String: From<T>;

pub trait Trait<T: ?Sized> {
    const C<'a>: &'a T
    where
        T: 'a + Eq;
}

output:

pub const K<T>: Option<T> = None;

pub trait Trait<T: ?Sized> {
    const C<'a>: &'a T;
}

[fmease]

Yeah, seems to need rewrite_where_clause somewhere.

[ytmimi]

Just double checking, but where clauses are new to const items now that they can be generic with the #[feature(generic_const_items)] , right?

[fmease]

Yes, they are part of generic_const_items and don't exist outside of the feature.

[ytmimi]

Note to self, unlike the deprecated type alias syntax, it's not possible to place the where clause before the assignment. I put together this small snippet that compiles:

#![feature(generic_const_items)]

pub const K<T>: Option<T> = None
where
    String: From<T>,
    T: std::fmt::Debug;


#[derive(Debug)]
struct ToString;

impl From<ToString> for String {
    fn from(_value: ToString) -> Self {
        String::new()
    }
}

fn main() {
    println!("{:?}", K::<ToString>);
}

But when I switch the order of the where clause and the assignment I get the following compilation errors:

#![feature(generic_const_items)]

pub const K<T>: Option<T>
where
    String: From<T>,
    T: std::fmt::Debug,
 = None;

#[derive(Debug)]
struct ToString;

impl From<ToString> for String {
    fn from(_value: ToString) -> Self {
        String::new()
    }
}

fn main() {
    println!("{:?}", K::<ToString>);
}

error: where clauses are not allowed before const item bodies
 --> example.rs:5:1
  |
4 |   pub const K<T>: Option<T>
  |             - while parsing this const item
5 | / where
6 | |     String: From<T>,
7 | |     T: std::fmt::Debug,
  | |_______________________^ unexpected where clause
8 |   = None;
  |     ---- the item body
  |
help: move the body before the where clause
  |
5 ~ = None where
6 |     String: From<T>,
7 ~     T: std::fmt::Debug,;
  |

error: aborting due to 1 previous error