the inability to comment every non-obnoxious token without leftover whitespace before or after

Take any piece of Rust code, split by non-obnoxious tokens on each line, for each line add a space after the token, a newline, and a comment.
non-obnoxious token would mean tokens that make sense to be commented, e.g. all tokens other than commas, double colons :: in paths or dots in field access or method calls ., or < and > in generic argument syntax etc.

for example this code:

pub trait Clean<Inner, Outer>: Dirty + 'static {
    const ASSOC: usize = 0;
}

would become

pub 
// comment
trait 
// comment
Clean< 
// comment
Inner, 
// comment
Outer>: 
// comment
Dirty + 
// comment
'static { 
// comment
const 
// comment
ASSOC: 
// comment
usize = 
// comment
0; 
// comment
}

(note the whitespace before newlines)

the reason we do this is to see which tokens leave leftover whitespace after them when passed through rustfmt. these are all locations a programmer would possibly want to comment on, for example, one could need to add a comment between pub and trait to denote why Clean needs to be a trait and not a standalone constant (first error), or simply comment on the type of a constant (8th error) etc.

in contemporary rustfmt this cannot be formatted due to left behind trailing whitespace errors:

eurydice@fedora:~/coreign/thread_local_rng$ cargo fmt
error[internal]: left behind trailing whitespace
  --> /home/eurydice/coreign/thread_local_rng/src/lib.rs:22:22:4
   |
22 | pub 
   |    ^
   |

error[internal]: left behind trailing whitespace
  --> /home/eurydice/coreign/thread_local_rng/src/lib.rs:24:24:6
   |
24 | trait 
   |      ^
   |

...

warning: rustfmt has failed to format. See previous 11 errors.

Reproduce the issue with the Rust snippet in the report and run cargo fmt to observe the trailing-whitespace errors. Then trace rustfmt's handling of whitespace around comments and token boundaries. Done means the described comments can be inserted without leftover whitespace errors while formatting remains valid.