Cannot infer type using `as _` while indexing

[panpanpro888]

When trying to index an array with another type that is not usize that can be converted to it using as _ inference fails.

Example code:

fn main() {
    let array = [0, 1, 1, 2, 3, 5];
    let index: u32 = 1;
    array[index as _];
}

Error:

error[E0282]: type annotations needed
 --> src/main.rs:4:20
  |
4 |     array[index as _];
  |                    ^ cannot infer type

[asquared31415]

The index type could be any of:

• usize
• any of the Range family of types, where the range is over usize
• (Bound<usize>, Bound<usize>)

Because there's only a generic restriction on the type of the index (impl SliceIndex<[i32]>), the compiler cannot determine which type you want. In theory there's only one as cast that actually succeeds, but the set of as casts that succeed are not currently considered in type inference in this case.

[panpanpro888]

It would be really useful if there was only an impl Index<usize> or even an impl From<usize> (only a single implementation of a type that can be converted to using as) for the compiler to infer the type you want when using as _ or ::from, but I don't know if this would bring any compatibility issues with any code and how complex it would be to implement.