trait bounds with comments are not formatted

This compiles file:

pub trait A {}
pub trait B {}
pub trait C {}

pub trait Foo:
// A and C
A + C
// and B
    + B
{}

but rustfmt formats it into this:

pub trait Foo: A + C + B // A and C
A
    + C
// and B
    + B {
}

try this live (just click on Format in the playground)

which fails to compile with:

   Compiling playground v0.0.1 (file:///playground)
error: expected one of `(`, `+`, `::`, `<`, `where`, or `{`, found `A`
 --> src/main.rs:6:1
  |
5 | trait Foo: A + C + B // A and C:
  |                     - expected one of `(`, `+`, `::`, `<`, `where`, or `{` here
6 | A + C
  | ^ unexpected token

Start with the Rust reproducer in the issue and run it through rustfmt, or use the linked Rust Playground example. Compare the formatted output with the compiler error and trace the rustfmt path responsible for trait bounds and comments. Done means formatting preserves the comments without producing code that fails to compile.