hex-literal: remove support for comments inside literals and migrate to CTFE

[newpavlov]

Potential breaking change for v1.0, see #814.

[newpavlov]

As suggested by @Nugine, I've migrated this crate from proc macro to CTFE. Initially I thought it would require Nightly, but looking closer at the const_str crate, it can work on stable as well. It results in a lighter crate, which likely should compile significantly faster.

[newpavlov]

@tarcieri
I think we can release these changes as v0.4. WDYT?

[tarcieri]

@newpavlov sure!

[triamazikamno]

Hi, this change seem to have broken the usage of hex! in the match patterns, i.e.

match buf {
hex!("0a0b") => {}

throws now:

error: expected pattern, found `{`
   |
60 |         hex!("0a0b") => {
   |         ^^^^^^^^^^^^^^^^
   |         |
   |         expected pattern
   |         in this macro invocation
   |         this macro call doesn't expand to a pattern

[newpavlov]

Unfortunately, I think it's inevitable with the CTFE approach until implementation of RFC 2920 will be stabilized. You can either continue to use v0.3, or introduce named constants and use them in patterns.

[triamazikamno]

I see, thank you. Will stick to the v0.3 for now

[tarcieri]

Another small regression (which is easy enough to work around, but was convenient before):

https://github.com/RustCrypto/SSH/actions/runs/4599840083/jobs/8125721544?pr=92#step:5:13

error[E0716]: temporary value dropped while borrowed
 --> ssh-encoding/tests/decode.rs:8:21
  |
8 |     let mut bytes = hex!("42").as_slice();
  |                     ^^^^^^^^^^           - temporary value is freed at the end of this statement
  |                     |
  |                     creates a temporary value which is freed while still in use
9 |     let ret = u8::decode(&mut bytes).unwrap();
  |                          ---------- borrow later used here

[newpavlov]

Yes, it's unfortunate. Though in such cases I usually use shorter let mut bytes = &hex!("..."); with hinted type, if necessary.

[Nugine]

This line may not be const-promoted (or const-evaluated), which may cause performance regression.