Make try_into const or don't suggest try_into in const scope

[estebank]

Given:

const fn foo() -> i32 {
    1
}

fn main() {
    [(); { foo() } ];
}

we suggest writing

#![feature(const_fn)]
use std::convert::TryInto;

const fn foo() -> i32 {
    1
}

fn main() {
    [(); { foo().try_into().unwrap() } ];
}

but when we do so, we complain about try_into not being const:

error[E0015]: calls in constants are limited to constant functions, tuple structs and tuple variants
 --> src/main.rs:9:12
  |
9 |     [(); { foo().try_into().unwrap() } ];
  |            ^^^^^^^^^^^^^^^^

error[E0015]: calls in constants are limited to constant functions, tuple structs and tuple variants
 --> src/main.rs:9:12
  |
9 |     [(); { foo().try_into().unwrap() } ];
  |            ^^^^^^^^^^^^^^^^^^^^^^^^^

error[E0080]: evaluation of constant value failed
 --> src/main.rs:9:12
  |
9 |     [(); { foo().try_into().unwrap() } ];
  |            ^^^^^^^^^^^^^^^^ calling non-const function `<i32 as std::convert::TryInto<usize>>::try_into`

[estebank]

We no longer suggest try_into:

error[E0308]: mismatched types
 --> src/main.rs:9:12
  |
9 |     [(); { foo() } ];
  |            ^^^^^ expected `usize`, found `i32`

[Trolldemorted]

Could we have a const try_into and unwrap though? It would provide a very clean way of expressing "please don't compile if into fails", which improves code reliability on unusual and small target platforms.

We do have panic in const contexts, but we can't use it in traits (since trait methods can't be const yet).