rustfmt rewrites valid `impl<T: /*comment*/>` into invalid syntax with `,,`

[Marsman1996]

Summary

I tried to format this code:

trait SpinGuardian {}

struct RwLockUpgradeableGuard<'a, T, G: SpinGuardian>(&'a T, G);

// Compile fail after rustfmt
impl<T: /*?Sized*/, G: SpinGuardian> RwLockUpgradeableGuard<'_, T, G> {}

Expected behavior

I expected the formated code to be compiled normally.

Actual behavior

Instead, this happened:

trait SpinGuardian {}

struct RwLockUpgradeableGuard<'a, T, G: SpinGuardian>(&'a T, G);

// Compile fail after rustfmt
impl<T /*?Sized*/,, G: SpinGuardian> RwLockUpgradeableGuard<'_, T, G> {}

And I get a compile error:

error: expected one of `#`, `>`, `const`, identifier, or lifetime, found `,`
 --> /playground/src/main.rs:6:19
  |
6 | impl<T /*?Sized*/,, G: SpinGuardian> RwLockUpgradeableGuard<'_, T, G> {}
  |                   ^ expected one of `#`, `>`, `const`, identifier, or lifetime

Configuration

No extra configuration.

Reproduction Steps

Just run rustfmt and then compile.

You can reproduce this in the Rust Playground:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=358af121c5c0572cf0663c013de3948e

Meta

rustfmt --version:

rustfmt 1.9.0-nightly (562dee4820 2026-03-21)

[ytmimi]

Confirming I can reproduce with rustfmt 1.9.0-nightly (0660429 2026-03-20)