error[internal]: left behind trailing whitespace (full repro with GitHub link to offending repo)

Toolchain Version

Everything is installed using rustup.

$ cargo --version
cargo 1.89.0 (c24e10642 2025-06-23)

Observed Behavior

When running cargo fmt, I get the following errors:

error[internal]: left behind trailing whitespace
   --> /home/manish/work/hedgehog/src/dataplane/pkt-meta/src/flow_table/table.rs:371:371:1
    |
371 |                     
    | ^^^^^^^^^^^^^^^^^^^^
    |

error[internal]: left behind trailing whitespace
   --> /home/manish/work/hedgehog/src/dataplane/pkt-meta/src/flow_table/table.rs:380:380:1
    |
380 |                     
    | ^^^^^^^^^^^^^^^^^^^^
    |

Expected Behavior

cargo fmt removes the trailing whitespace without internal error.

How to Reproduce

On GitHub checkout the code from https://github.com/githedgehog/dataplane/tree/bug-report/rust-1.89/rustfmt-internal-error. After checkout, don't forget to switch to the bug-report/rust-1.89/rustfmt-internal-error branch.

Using the Rust 1.89 toolchain, run cargo fmt to reproduce the issue.

Check out the linked dataplane repository's bug-report/rust-1.89/rustfmt-internal-error branch and run cargo fmt with Rust 1.89. Inspect dataplane/pkt-meta/src/flow_table/table.rs at lines 371 and 380, where trailing whitespace triggers the internal errors. Done means cargo fmt removes that whitespace without reporting an internal error.