Unable to match against secondary labels

It seems there is no way to match label text other than error-pattern :/
I might fix that separately later hehe

Originally posted by @jdonszelmann in https://github.com/rust-lang/rust/pull/141891#discussion_r2120567611

Example stderr

error[E0596]: cannot borrow value as mutable, as it is not declared as mutable
  --> $DIR/borrowck-error-in-macro.rs:6:1
   |
LL | a::ice! {}
   | ^^^^^^^^^^
   | |
   | cannot borrow as mutable
   | a call in this macro requires a mutable binding due to mutable borrow of `d`
   |
   = note: this error originates in the macro `a::ice` (in Nightly builds, run with -Z macro-backtrace for more info)
help: consider changing this to be mutable
  --> $DIR/auxiliary/borrowck-error-in-macro.rs:6:17
   |
LL |             let mut c = || *d += 1;
   |                 +++

error: aborting due to 1 previous error

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

AFAICT it's not yet possible to match cannot borrow as mutable with a NOTE 🤔

The payload names no source file or test. Start by tracing the existing error-pattern matching behavior against the provided stderr, focusing on how secondary labels such as NOTE are handled. Done means a test can match “cannot borrow as mutable” as a NOTE rather than requiring an error-pattern match.