Comma gets duplicated when obstructed by inline comments

[bczhc]

Input:

fn foo() {
    struct A {
        a: i32 /* i32 */, /* type */
    }

    let a = A {
        a: 123 /* default */, /* value */
    };

    let b = match () {
        () => 123 /* some */, /* comment */
    };

    match todo!() {
        Ok(0) => break /* EOF */,
        Ok(r) => read += r,
        Err(e) if e.kind() == ErrorKind::Interrupted => continue,
        Err(e) => return Err(e),
    }
}

// https://github.com/rust-lang/rustfmt/issues/3876#issuecomment-619383696
fn f<S>(s: S)
where
    S: Send /* */,
             //
{
}

Output:

fn foo() {
    struct A {
        a: i32, /* i32 */, /* type */
    }

    let a = A {
        a: 123, /* default */, /* value */
    };

    let b = match () {
        () => 123, /* some */, /* comment */
    };

    match todo!() {
        Ok(0) => break, /* EOF */,
        Ok(r) => read += r,
        Err(e) if e.kind() == ErrorKind::Interrupted => continue,
        Err(e) => return Err(e),
    }
}

// https://github.com/rust-lang/rustfmt/issues/3876#issuecomment-619383696
fn f<S>(s: S)
where
    S: Send, /* */,
             //
{
}

Note the difference on /* */ lines! An unexpected comma was added.

Expected:

Might be something like a: i32, /* i32 */ /* type */ or Ok(0) => break, /* EOF */? By the code style convention comma gets moved (but not unexpectedly duplicated) before the inline comments.

Maybe an uncovered case or regression of #3876? I took the example in #3876 (comment) - also reproducible using the current rustfmt tool.

---

Version

❯ cargo run --bin rustfmt -- --version
rustfmt 1.8.0-nightly (86261bfb87 2025-12-08)