Show values that lead to an arithmetic overflow when computing constants

[shepmaster]

In Rust 1.42, this code correctly fails due to a misuse of bitwise-XOR as exponentiation:

const N: usize = 10;
const N_CELLS: usize = (N ^ 2) - 50;

error: any use of this value will cause an error
 --> src/lib.rs:2:24
  |
2 | const N_CELLS: usize = (N ^ 2) - 50;
  | -----------------------^^^^^^^^^^^^-
  |                        |
  |                        attempt to subtract with overflow
  |
  = note: `#[deny(const_err)]` on by default

However, the error message could be enhanced to show the values that lead to the overflow. One rough idea:

2 | const N_CELLS: usize = (N ^ 2) - 50;
  |                        ^^^^^^^   ^^
  |                        |         |
  |                        evaluated to 8
  |                                  |
  |                                  evaluated to 50

This originated from a Stack Overflow question: How can I guarantee that overflow will not occur in const variables?

/cc @oli-obk