`#[tracing::instrument]` may be discarding some spans

[xd009642]

rust-lang/rust#131119

There's a relative rust compiler issue. But essentially when using the llvm instrumentation based coverage often tracing::instrument annotated functions are completely excluded. This apparently doesn't seem to be a compiler thing but rather somewhere spans are being discarded when they should be kept.

[mladedav]

Just to clarify, it seems to be discarding rustc spans, i.e. what should be highlighted by a compiler for errors and such. It's not discarding tracing spans.