Use a lock-free datastructure for source_span follow up to the perf regression in https://github.com/rust-lang/rust/pull/105462 The main regression is likely the CStore, but let's evaluate the perf impact of this on its own
rustc_data_structures
ref
source_span
if $c:expr { Some($r:expr) } else { None }
$c.then(|| $r)
uninlined_format_args