error[E0596]: cannot borrow data in an index of `HashMap<String, Vec<String>>` as mutable
  --> $DIR/issue-41726.rs:5:9
   |
LL |         things[src.as_str()].sort();
   |         ^^^^^^^^^^^^^^^^^^^^ cannot borrow as mutable
   |
   = help: trait `IndexMut` is required to modify indexed content, but it is not implemented for `HashMap<String, Vec<String>>`
   = help: to modify a `HashMap<String, Vec<String>>`, use `.get_mut()`, `.insert()` or the entry API

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0596`.