`unreachable_pattern` could get more specific if the preempting pattern resembles a `const`

[vDorst]

matchallways picks the first arm if matching value is not defined/unknown.
I was using constto define the values for the arms.
But the const were places inside the main() and the match expression was in a separate function.

I tried this code:

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=4b8fce2e782604f738c4fb9229b2a29b

Uncommenting the first 2 const gives the wanted results.

I expected to see this happen:

That the compiler finds the const values or stop compiling because the matching values are unknown.

Instead, this happened:

It just compiles.
But it gives some warnings like warning: unreachable pattern.
It just runs but with the wrong values.

See play link. See the behavior on all the versions "Stable", "Beta" and "Nightly".

[workingjubilee]

This is because without the constant in scope, it perceives the pattern given not as a constant but as a binding to an identifier: https://doc.rust-lang.org/reference/patterns.html#identifier-patterns

It does not seem plausible to change this, but we could lint on it more specifically than just "unreachable pattern", because SCREAMING_SNAKE_CASE suggests a constant.

[workingjubilee]

Though, item scoping definitely isn't going to meaningfully change. We could try to find the items for diagnostics purposes, depending on how acceptable that is.

And yes, match arms are indeed ordered.

[workingjubilee]

also see: #84078

[abonander]

Elevating unreachable_patterns to deny-by-default in a new edition seems reasonable. Very rarely is it actually desirable.

[scottmcm]

I don't think we should make unreachable_patterns all the way to deny, especially because things like if a _ => arm is unreachable that's completely fine.

Much better, in my opinion, to add new deny-by-default lints that are extra-specific to the known issue. For example, we did this a few releases ago for this blatant "I bet you meant a variant not a binding here": https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=c753f94223a1e275d6dc0544b9165ebf

error[E0170]: pattern binding `Less` is named the same as one of the variants of the type `std::cmp::Ordering`
 --> src/lib.rs:3:9
  |
3 |         Less => "fewer",
  |         ^^^^ help: to match on the variant, qualify the path: `std::cmp::Ordering::Less`
  |
  = note: `#[deny(bindings_with_variant_name)]` on by default

So I think we should find a similar "hey, we see specifically what probably happened here" lint that we could add at deny-by-default. Maybe, for example, a "hey, this matches everything and only introduces weirdly-cased bindings, but isn't just _" lint. (Probably needs more workshopping on exactly what the rule would be.) Notably, it would be good to lint for such things even if there are no unreachable patterns because of it.

[workingjubilee]

I'll leave L-unreachable_patterns here since in a sense if the solution is a new lint, it will be a sublint of unreachable_patterns (morally even if not actually).