Infer const generics properly

[CohenArthur]

let u: [u32; _] = [15u32];

this is hidden behind #![feature(generic_arg_infer)]

[philberty]

So i added this PR does most of the work we need for type checking to solve this:

#3889

But we need to support this in the parser i think we currently get:

test.rs:2:18: error: use of ‘_’ is not allowed on the right-side of an assignment
    2 |     let u: [u32; _] = [15u32];
      |                  ^
test.rs:2:19: error: failed to parse size expression in array type
    2 |     let u: [u32; _] = [15u32];
      |                   ^
test.rs:2:19: error: failed to parse type in let statement
test.rs:2:5: error: failed to parse statement or expression in block expression
    2 |     let u: [u32; _] = [15u32];
      |     ^~~
test.rs:3:1: error: unrecognised token ‘}’ for start of item
    3 | }
      | ^
test.rs:3:1: error: failed to parse item in crat

So we need some kind of way of representing wildcard or infer on the capacity expr not sure how rustc represents this but doing this inference is like really easy to do during the infer rules now

[philberty]

Ah i see there is an issue open for the parser side so yeah once thats fixed i can fix this

[powerboat9]

https://doc.rust-lang.org/reference/types/array.html
https://doc.rust-lang.org/reference/expressions/underscore-expr.html
It looks like it's considered a WildcardExpression parsing-wise