Const-generics don't work with constants

[Boscop]

This compiles:

use arrayvec::{Array, ArrayVec};

#[ext(pub, name = IterExt)]
impl<T: Iterator<Item = U>, U> T {
    fn collect_arr<const N: usize>(self) -> [U; N]
    where
        [U; N]: Array<Item = U>,
        ArrayVec<[U; N]>: Debug,
    {
        self.collect::<ArrayVec<[U; N]>>().into_inner().expect("collect_arr")
    }
}

(Using the extend and arrayvec crates.)

But using it with a constant doesn't compile:

Even though pub const CUE_POINT_COUNT: usize = 8; is in scope!
If I write .collect_arr::<8usize>() instead, it compiles.
Also, if I don't use the extension trait method but inline it (.collect::<ArrayVec<[_; CUE_POINT_COUNT]>>().into_inner().unwrap()) it works with the constant!
So for some reason rustc can't see that CUE_POINT_COUNT == 8 here.

[rodrimati1992]

Sorry for all the edits,apparently this only errors if the function is a method, when defined as a free function it doesn't error.

As a method

When it's defined as a method,this code:

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=cdf1b7f44fe0c7ae24ef2df775f841d8

#![feature(const_generics)]

use arrayvec::{Array, ArrayVec};
use std::fmt::Debug;

trait IterExt: Sized + Iterator {
    fn collect_arr<const N: usize>(self) -> [Self::Item; N]
    where
        [Self::Item; N]: Array<Item = Self::Item>,
        ArrayVec<[Self::Item; N]>: Debug,
    {
        self.into_iter()
            .collect::<ArrayVec<[Self::Item; N]>>()
            .into_inner()
            .expect("collect_arr")
    }
}

impl<This:Iterator> IterExt for This{}

fn main(){
    const N:usize=10;
    let arr:[u32;10]=(0..10).collect_arr::<N>();
}

Emits this error message

error[E0277]: the trait bound `[{integer}; _]: arrayvec::array::Array` is not satisfied
  --> src/main.rs:24:30
   |
24 |     let arr:[u32;10]=(0..10).collect_arr::<N>();
   |                              ^^^^^^^^^^^ the trait `arrayvec::array::Array` is not implemented for `[{integer}; _]`
   |
   = help: the following implementations were found:
             <[T; 0] as arrayvec::array::Array>
             <[T; 100] as arrayvec::array::Array>
             <[T; 1024] as arrayvec::array::Array>
             <[T; 10] as arrayvec::array::Array>
           and 53 others

error[E0308]: mismatched types
  --> src/main.rs:24:22
   |
24 |     let arr:[u32;10]=(0..10).collect_arr::<N>();
   |             -------- ^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `10usize`, found `N`
   |             |
   |             expected due to this
   |
   = note: expected array `[u32; 10]`
              found array `[{integer}; _]`

Curiously,by changing the method call line to
let arr:[u32;10]=IterExt::collect_arr::<N>((0..10));
it compiles without errors.
https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=7aff9f76b3d31e931ef1b66077d40e6f

As a free function

When it's defined as a free function no error happens:

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=9e7bae5d74089c7177aaa22226e1013c

#![feature(const_generics)]

use arrayvec::{Array, ArrayVec};
use std::fmt::Debug;

fn collect_arr<T, const N: usize>(this: T) -> [T::Item; N]
where
    T: IntoIterator,
    [T::Item; N]: Array<Item = T::Item>,
    ArrayVec<[T::Item; N]>: Debug,
{
    this.into_iter()
        .collect::<ArrayVec<[T::Item; N]>>()
        .into_inner()
        .expect("collect_arr")
}

fn main(){
    const N:usize=10;
    let arr:[u32;10]=collect_arr::<_,N>(0..10);
}

[rodrimati1992]

This is the most minimal code I could get to output a very similar error:
https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=f45606455c01151f0b16cd1484b3222c

#![feature(const_generics)]

trait IterExt: Sized + Iterator {
    fn default_for_size<const N: usize>(self) -> [Self::Item; N]
    where
        [Self::Item; N]: Default,
    {
        Default::default()
    }
}

impl<This:Iterator> IterExt for This{}

fn main(){
    const N:usize=10;
    let arr:[u32;10]=(0..10).default_for_size::<N>();
}

The error message for that code

error[E0277]: the trait bound `[{integer}; _]: std::default::Default` is not satisfied
  --> src/main.rs:16:30
   |
16 |     let arr:[u32;10]=(0..10).default_for_size::<N>();
   |                              ^^^^^^^^^^^^^^^^ the trait `std::default::Default` is not implemented for `[{integer}; _]`
   |
   = help: the following implementations were found:
             <&[T] as std::default::Default>
             <&mut [T] as std::default::Default>
             <[T; 0] as std::default::Default>
             <[T; 10] as std::default::Default>
           and 31 others

error[E0308]: mismatched types
  --> src/main.rs:16:22
   |
16 |     let arr:[u32;10]=(0..10).default_for_size::<N>();
   |             -------- ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `10usize`, found `N`
   |             |
   |             expected due to this
   |
   = note: expected array `[u32; 10]`
              found array `[{integer}; _]`

[Boscop]

@rodrimati1992 But your minimized example behaves differently when writing 10usize instead of N! Instead of compiling fine (like with my original example), the compiler panics:

error: internal compiler error: unexpected const parent in type_of_def_id(): Expr(expr(HirId { owner: DefIndex(10), local_id: 20 }: ::std::ops::Range{start: 0, end: 10,}.default_for_size::<>()))

error: internal compiler error: mir_const_qualif: MIR had errors
  --> src/main.rs:16:49
   |
16 |     let arr:[u32;10]=(0..10).default_for_size::<10usize>();
   |                                                 ^^^^^^^

error: internal compiler error: PromoteTemps: MIR had errors
  --> src/main.rs:16:49
   |
16 |     let arr:[u32;10]=(0..10).default_for_size::<10usize>();
   |                                                 ^^^^^^^

error: internal compiler error: broken MIR in DefId(0:13 ~ playground[a375]::main[0]::{{constant}}[1]) ("return type"): bad type [type error]
  --> src/main.rs:16:49
   |
16 |     let arr:[u32;10]=(0..10).default_for_size::<10usize>();
   |                                                 ^^^^^^^

error: internal compiler error: broken MIR in DefId(0:13 ~ playground[a375]::main[0]::{{constant}}[1]) (LocalDecl { mutability: Mut, local_info: Other, internal: false, is_block_tail: None, ty: [type error], user_ty: UserTypeProjections { contents: [] }, source_info: SourceInfo { span: src/main.rs:16:49: 16:56, scope: scope[0] } }): bad type [type error]
  --> src/main.rs:16:49
   |
16 |     let arr:[u32;10]=(0..10).default_for_size::<10usize>();
   |                                                 ^^^^^^^

thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', src/librustc_errors/lib.rs:355:17
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

error: internal compiler error: unexpected panic

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=2bbd3b7f2d16e643a8dedb82659aa0c4

[Boscop]

Btw, instead of writing 8usize in .collect_arr::<8usize>(), the compiler should be able to infer that 8 is an usize. IOW, this should compile: .collect_arr::<8>(). Is there already a ticket for this?