regression: error: path separator must be a double colon (in where clause)

[cuviper]

• https://crater-reports.s3.amazonaws.com/beta-1.87-rustdoc-2/beta-2025-04-20/gh/wahid122.Rust-tool-testing/log.txt

[INFO] [stdout] error: path separator must be a double colon
[INFO] [stdout]   --> src/lib.rs:68:12
[INFO] [stdout]    |
[INFO] [stdout] 68 |     where a:AsRef<str>{
[INFO] [stdout]    |            ^
[INFO] [stdout]    |
[INFO] [stdout] help: use a double colon instead
[INFO] [stdout]    |
[INFO] [stdout] 68 |     where a::AsRef<str>{
[INFO] [stdout]    |             +

Version it worked on

It most recently worked on: 1.86.0

Version with regression

rustc 1.87.0-beta.5 (386abeb93 2025-04-19) in crater #139827.

@rustbot modify labels: +regression-from-stable-to-beta -regression-untriaged

[cuviper]

I minimized it to this example:

pub fn upper<T>() where T:Copy {}
pub fn lower<t>() where t:Copy {}

The upper is fine, but lower gives the error. It's also fine if you add a space, where t: Copy.

On nightly, it also mentions type ascription:

error: path separator must be a double colon
 --> src/lib.rs:2:26
  |
2 | pub fn lower<t>() where t:Copy {}
  |                          ^
  |
  = note: if you meant to annotate an expression with a type, the type ascription syntax has been removed, see issue #101728 <https://github.com/rust-lang/rust/issues/101728>
help: use a double colon instead
  |
2 | pub fn lower<t>() where t::Copy {}
  |                           +

edit: that was with rustc 1.88.0-nightly (78f2104e3 2025-04-16), but not after I updated to 6bc57c6bf 2025-04-22.

[fmease]

Regressed in commit 500a686 (PR #136808)

[fmease]

Also regressed:

fn f<t>(_: impl a<t:u>) {}

trait a { type t; }
trait u {}

[cuviper]

To be clear, all of those also warn that you should use CamelCase, but that's a style lint, not a language requirement.

[apiraino]

Assigning priority (discussion on Zulip).

@rustbot label -I-prioritize +P-critical

[fmease]

Fixed in #140228.
Backported in #140269.