Format long numeric literals

[tesuji]

I got this warning from clippy:

warning: long literal lacking separators
  --> src/elf/gnu_hash.rs:38:43
   |
38 |         assert_eq!(hash("flapenguin.me"), 0x8ae9f18e);
   |                                           ^^^^^^^^^^ help: consider: `0x8ae9_f18e`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unreadable_literal

It would be better if rustfmt can handle it automatically.

[sinkuu]

unreadable_literal emits a machine-applicable suggestion, which would (hopefully) become auto-fixable by cargo fix with clippy integration in the future.

[scampi]

The rustfmt tool is not made for fixing such things

[tesuji]

What I mean is not fixing those clippy lints. But rustfmt could format those long
numeric literals while running?

[scampi]

@lzutao Given the assert expression in the description, what would you expect the formatting to be ?

[calebcartwright]

This is something rustfix is great for, but is not something that fits in rustfmt's purview and thus is not something we'll implement