Const generics: Incorrect error "wrong number of const arguments" with struct constructor

[andreytkachenko]

this works fine:

#![feature(const_generics)]
pub struct Array<T, const N: usize> ([T; {N}]);
impl <T, const N: usize> Array<T, {N}> {
    #[inline]
    pub fn cap(&self) -> usize {
        {N}
    }
}

fn main() {
    let arr = Array([0u32; 8]);

    println!("{}", arr.cap());
}

but this fails to compile:

#![feature(const_generics)]
pub struct Array<T, const N: usize> {
    data: [T; {N}]
}
impl <T, const N: usize> Array<T, {N}> {
    #[inline]
    pub fn cap(&self) -> usize {
        {N}
    }
}

fn main() {
    let arr = Array { data: [0u32; 8] };

    println!("{}", arr.cap());
}

here is the error:

error[E0107]: wrong number of const arguments: expected 1, found 0
  --> src/main.rs:28:15
   |
28 |     let arr = Array { data: [0u32; 8] };
   |               ^^^^^ expected 1 const argument

error: aborting due to previous error

[Centril]

Reduced:

#![feature(const_generics)]

pub struct Array<T, const N: usize> {
    data: [T; {N}]
}

fn main() {
    let _ = Array { data: [0u32; 8] };
}

[Centril]

cc @varkor @yodaldevoid

[varkor]

I wonder whether the right fix is removing:

let infer_consts = position != GenericArgPosition::Type && arg_counts.consts == 0;

and just using infer_types instead. However, this leads to a different error:

error: internal compiler error: src/librustc_codegen_llvm/context.rs:851: failed to get layout for `[type error]`: the type `[type error]` has an unknown layout

which might be "more correct", but is an ICE. However, making this change does fix #60724.

[varkor]

Interestingly, the following works.

#![feature(const_generics)]

pub struct Array<T, const N: usize>([T; N]);

fn main() {
    let _ = Array([0u32; 8]);
}

So it's not a problem with tuple structs.

[varkor]

Additionally, this works.

#![feature(const_generics)]

pub struct Array<T, const N: usize> { x: [T; N] }

fn main() {
    let _ = Array::<u32, 8> { x: [0u32; 8] };
}

So there's a problem with the inference.