Fix perf regression caused by tracing

[Stypox]

See #143334, this is another alternative that may be worth benchmarking as suggested in #143334 (comment).

r? @RalfJung

[rustbot]

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

[Kobzol]

@bors2 try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[rust-bors]

⌛ Trying commit 57aa88e with merge 3ab9e25…

To cancel the try build, run the command @bors2 try cancel.

[RalfJung]

No, this is not the closure I meant. I don't think this one will help.

What I meant is an API of the form

fn with_trace_span<R>(_span: tracing::Span, f: impl FnOnce() -> R) -> R { f() }

I.e., the span ends when the closure returns.

[rust-bors]

☀️ Try build successful (CI)
Build commit: 3ab9e25 (3ab9e2528f3d5f3f77694dc747bed001f44bf378, parent: febb10d0a2d29278135676783f6a22eb83295981)

[rust-timer]

Queued 3ab9e25 with parent febb10d, future comparison URL.
There are currently 3 preceding artifacts in the queue.
It will probably take at least ~4.1 hours until the benchmark run finishes.