Make inline const work in range patterns

[spastorino]

Fixes #78108 which is a follow up of #77124

r? @petrochenkov

[petrochenkov]

r=me with additional test cases.

[spastorino]

@petrochenkov check it out again because the half open ranges solutions seems a bit hacky to me.

[camelid]

It might be a good idea to add some UI check-fail tests for cases like:

match x {
    1 .. const 2 => {}
}

[spastorino]

match x {
    1 .. const 2 => {}
}

It's interesting how this case fails ...

error[E0586]: inclusive range with no end
 --> src/test/ui/inline-const/const-match-pat-fail.rs:5:11
  |
5 |         1 ..= const 2 => {}
  |           ^^^ help: use `..` instead
  |
  = note: inclusive ranges must be bounded at the end (`..=b` or `a..=b`)

error: expected one of `=>`, `if`, or `|`, found keyword `const`
 --> src/test/ui/inline-const/const-match-pat-fail.rs:5:15
  |
5 |         1 ..= const 2 => {}
  |               ^^^^^ expected one of `=>`, `if`, or `|`

error[E0658]: half-open range patterns are unstable
 --> src/test/ui/inline-const/const-match-pat-fail.rs:5:9
  |
5 |         1 ..= const 2 => {}
  |         ^^^^^
  |
  = note: see issue #67264 <https://github.com/rust-lang/rust/issues/67264> for more information
  = help: add `#![feature(half_open_range_patterns)]` to the crate attributes to enable

In the parser we are looking for const { not just const so it asummes that it's a half-open range with no end. Unsure what would be ok to do with this case but the error doesn't sound great.
Anyway it's the same that happens if instead of const you use something else like let.

error[E0586]: inclusive range with no end
 --> src/test/ui/inline-const/const-match-pat-fail.rs:5:11
  |
5 |         1 ..= let 2 => {}
  |           ^^^ help: use `..` instead
  |
  = note: inclusive ranges must be bounded at the end (`..=b` or `a..=b`)

error: expected one of `=>`, `if`, or `|`, found keyword `let`
 --> src/test/ui/inline-const/const-match-pat-fail.rs:5:15
  |
5 |         1 ..= let 2 => {}
  |               ^^^ expected one of `=>`, `if`, or `|`

error[E0658]: half-open range patterns are unstable
 --> src/test/ui/inline-const/const-match-pat-fail.rs:5:9
  |
5 |         1 ..= let 2 => {}
  |         ^^^^^
  |
  = note: see issue #67264 <https://github.com/rust-lang/rust/issues/67264> for more information
  = help: add `#![feature(half_open_range_patterns)]` to the crate attributes to enable

[camelid]

In the parser we are looking for const { not just const so it asummes that it's a half-open range with no end. Unsure what would be ok to do with this case but the error doesn't sound great.

I'm trying to work on this in #78168.

[petrochenkov]

r=me after rebasing and squashing commits.

[spastorino]

@bors r=petrochenkov

[bors]

📌 Commit 5656a41 has been approved by petrochenkov