-Zdont-buffer-diagnostics
It was added in #54232. It seems like it was aimed at NLL development, which is well in the past. Also, it looks like `-Ztreat-err-as-bug` can be used to achieve the same effect. So it doesn't seem necessary.
DiagCtxt
EarlyDiagCtxt
DiagnosticBuilder
is_lint
Level::Error
Emitter
SubDiagnostic::render_span
_ == _ || _ == _
matches!(_, _ | _)