compiletest seems to be generating its own macro backtrace diagnostics?

What on earth is going on here

https://github.com/rust-lang/rust/blob/096277e989d6de11c3077472fc05778e261e7b8e/src/tools/compiletest/src/json.rs#L305-L321

@BoxyUwU found out that compiletest seemingly emits diagnostics that are not in rustc's diagnostics. Like

error[E0425]: cannot find value `ident` in this scope
  --> $DIR/const_arg_trivial_macro_expansion-1.rs:11:9
   |
LL |         ident
   |         ^^^^^ not found in this scope
...
LL |     fn array_0() -> [(); unbraced_unbraced_ident!()] { loop {} }
   |                          -------------------------- in this macro invocation
   |
   = note: this error originates in the macro `unbraced_ident` which comes from the expansion of the macro `unbraced_unbraced_ident` (in Nightly builds, run with -Z macro-backtrace for more info)

like, compiletest is generating its own macro backtrace diagnostics or something funny like that?

Start with src/tools/compiletest/src/json.rs lines 305-321 and the const_arg_trivial_macro_expansion-1.rs diagnostic shown in the issue. Compare compiletest's emitted output with rustc's diagnostics to determine whether the macro-backtrace text is generated or reformatted by compiletest. Done means identifying the source of the extra diagnostic and documenting or fixing the discrepancy.