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

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)

Start by running rustfmt on the Rust Playground reproducer and comparing the input with the invalid formatted output. Trace the rustfmt handling of comments in generic bounds, then add regression coverage showing that the formatted impl remains valid Rust and no longer produces the extra comma.