coverage: Build an "expansion tree" and use it to unexpand raw spans

[Zalathar]

Historically and currently, coverage instrumentation assumes that all of a function's spans are in the same file and have the same syntax context. The spans extracted directly from MIR don't satisfy that assumption, so there is an “unexpansion” step that walks up each span's expansion-call-site tree to find a suitable span in the same context as the function's body span.

(That unexpansion step is what allows us to have somewhat reasonable coverage instrumentation for macros like println!, and for syntax like for and ? that undergo desugaring expansion.)

The current unexpansion code mostly works fine in that “flat” single-file single-context world. But it's not suitable for incremental work towards proper expansion-aware coverage instrumentation, which would allow a function's coverage spans to encompass multiple expansion contexts and multiple files.

This PR therefore replaces the current unexpansion code with a more sophisticated system that uses the raw MIR spans to reconstruct an “expansion tree”, and then uses that tree to help perform most of the unexpansion work.

Building the tree is “overkill” for current unexpansion needs (though it does give some minor edge-case improvements), but my hope is that having the explicit tree available will be a big help when taking the next steps towards proper expansion-region support.

[rustbot]

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[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[Zalathar]

(Note that we currently don't have any benchmarks for -Cinstrument-coverage, so a perf build is not going to reveal anything useful.)

[Zalathar]

Previously, the span for this println!(..) was discarded early, because it covers the whole body span of its surrounding closure. In the new implementation, the span gets truncated to just the println! part before the body-span check, so it is retained, giving slightly better instrumentation.

This wasn't an intentional improvement, but I'll take it.

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.