Don't specialize on maybe-const

[clarfonthey]

Related: #148200

Was mentioned at a T-types meeting. Immediately an issue with this due to pointers being not const-comparable, but wanted to at least open a PR with the change to discuss.

Note that currently the following impls are notable:

• const trait BytewiseEq<Rhs = Self>: [const] PartialEq<Rhs> + Sized
• const trait AlwaysApplicableOrd: [const] SliceOrd + [const] Ord {}
• const unsafe trait UnsignedBytewiseOrd: [const] Ord {}
• const unsafe trait TrustedStep: [const] Step + Copy {}

[rustbot]

r? @Mark-Simulacrum

rustbot has assigned @Mark-Simulacrum.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: libs
• libs expanded to 12 candidates
• Random selection from JohnTitor, Mark-Simulacrum, joboet

[clarfonthey]

Line of shame that makes this change not work atm.

View changes since the review

[Mark-Simulacrum]

Maybe this should be reviewed by Types? It's not very clear to me (a) what problems this solves or (b) how we avoid them elsewhere -- presumably we should have the compiler stop allowing the pattern? Are there any other traits affected in std?

but wanted to at least open a PR with the change to discuss.

Is this waiting on review then? What specifically is being sought for discussion here?

(Maybe this will become clear once @jackh726 gets an opportunity to write up a summary of the discussion on the linked issue?)

[clarfonthey]

Yes, apologies for assigning this to you; it should really be:

@rustbot label -T-libs +T-types
r? types

[clarfonthey]

Apologies for this @-mentioning so many people. Will bother more people in the nominated thread as needed.

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[rust-log-analyzer]

The job test-aarch64-gnu-llvm-21-1 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

[RUSTC-TIMING] build_script_build test:false 0.318
error[E0277]: pointers cannot be reliably compared during const eval
   --> library/core/src/slice/cmp.rs:257:24
    |
255 | / macro_rules! always_applicable_ord {
256 | |     ($([$($p:tt)*] $t:ty,)*) => {
257 | |         $(impl<$($p)*> AlwaysApplicableOrd for $t {})*
    | |                        ^^^^^^^^^^^^^^^^^^^
258 | |     }
259 | | }
    | |_- in this expansion of `always_applicable_ord!`
260 |
261 | / always_applicable_ord! {
262 | |     [] u8, [] u16, [] u32, [] u64, [] u128, [] usize,
263 | |     [] i8, [] i16, [] i32, [] i64, [] i128, [] isize,
264 | |     [] bool, [] char,
...   |
268 | |     [T: AlwaysApplicableOrd] Option<T>,
269 | | }
    | |_- in this macro invocation
    |
    = note: see issue #53020 <https://github.com/rust-lang/rust/issues/53020> for more information
note: required by a bound in `AlwaysApplicableOrd`
---

error[E0277]: the trait bound `*const T: const SliceOrd` is not satisfied
   --> library/core/src/slice/cmp.rs:257:24
    |
255 | / macro_rules! always_applicable_ord {
256 | |     ($([$($p:tt)*] $t:ty,)*) => {
257 | |         $(impl<$($p)*> AlwaysApplicableOrd for $t {})*
    | |                        ^^^^^^^^^^^^^^^^^^^
258 | |     }
259 | | }
    | |_- in this expansion of `always_applicable_ord!`
260 |
261 | / always_applicable_ord! {
262 | |     [] u8, [] u16, [] u32, [] u64, [] u128, [] usize,
263 | |     [] i8, [] i16, [] i32, [] i64, [] i128, [] isize,
264 | |     [] bool, [] char,
...   |
268 | |     [T: AlwaysApplicableOrd] Option<T>,
269 | | }
    | |_- in this macro invocation
    |
note: required by a bound in `AlwaysApplicableOrd`
   --> library/core/src/slice/cmp.rs:253:34
    |
253 | const trait AlwaysApplicableOrd: const SliceOrd + const Ord {}
    |                                  ^^^^^^^^^^^^^^ required by this bound in `AlwaysApplicableOrd`

error[E0277]: the trait bound `*mut T: const SliceOrd` is not satisfied
   --> library/core/src/slice/cmp.rs:257:24
    |
255 | / macro_rules! always_applicable_ord {
256 | |     ($([$($p:tt)*] $t:ty,)*) => {
257 | |         $(impl<$($p)*> AlwaysApplicableOrd for $t {})*
    | |                        ^^^^^^^^^^^^^^^^^^^
258 | |     }
259 | | }
    | |_- in this expansion of `always_applicable_ord!`
260 |
261 | / always_applicable_ord! {
262 | |     [] u8, [] u16, [] u32, [] u64, [] u128, [] usize,
263 | |     [] i8, [] i16, [] i32, [] i64, [] i128, [] isize,
264 | |     [] bool, [] char,
...   |
268 | |     [T: AlwaysApplicableOrd] Option<T>,
269 | | }
    | |_- in this macro invocation
    |
note: required by a bound in `AlwaysApplicableOrd`
   --> library/core/src/slice/cmp.rs:253:34