Rustfmt generates invalid code for associated type bounds with where-clause and comments

[taiheioki]

Input (Playground):

pub trait Trait {
    type I: Iterator<
        // This is an item
        Item = Self,
    >
    where
        Self: Copy;
}

Expected: rustfmt produces the same code as input.

Output: rustfmt produces the following invalid code

pub trait Trait {
    type I: Iterator<
        // This is an item
        Item = Self,
    >
    : Iterator<
    // This is an item
    Item = Self,
    >
    where
        Self: Copy;
}

Version

$ rustfmt --version
rustfmt 1.8.0-stable (ded5c06cf2 2025-12-08)

[ytmimi]

Thanks for the report. I can reproduce the issue with rustfmt 1.8.0-nightly (3f7e9cc 2026-01-01). The wrong span is being used to recover the comment and that's what's throwing things off here.

Input

pub trait Trait {
    type I: Iterator<
        Item = Self,
    >
    where
        Self: Copy;
}

Output

pub trait Trait {
    type I: Iterator<Item = Self>
    where
        Self: Copy;
}