Breaking change in macro_rules ty fragment parsing in version 1.68

[zakarumych]

I tried this code:

macro_rules! impl_primitive {
    ($ty:ty) => { impl_primitive!(impl $ty);};
    (impl $ty:ty) => { fn a(_: $ty) {} }
}

impl_primitive! { u8 }

link to playground

I expected to see this happen:
Macro is expanded to fn a(_: u8) {}

Instead, this happened:
On stable 1.67 it works as expected.
On beta 1.68 and nightly 1.69 it fails with the following error

error: expected a trait, found type
 --> src/lib.rs:6:19
  |
2 |     ($ty:ty) => { impl_primitive!(impl $ty);};
  |      ------ while parsing argument for this `ty` macro fragment
...
6 | impl_primitive! { u8 }
  |                   ^^

It is probably caused by ty fragment that now treats impl as a start of impl Trait type.
So makes sense. But breaks existing code :)

[compiler-errors]

searched nightlies: from nightly-2022-06-01 to nightly-2023-02-01
regressed nightly: nightly-2023-01-18
searched commit range: 4781233...3984bc5
regressed commit: 85357e3

bisected with cargo-bisect-rustc v0.6.4

Host triple: x86_64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc --access github --timeout 30 --start 2022-06-01 --end 2023-02-01 --regress error 

I think I see where the issue is, I'll fix it.

@rustbot claim

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-high

[apiraino]

IIRC issues with a backport pending should stay open to track the backport process (not 100% sure tho)

[apiraino]

beta backport in #108037