coverage: Remove all unstable support for MC/DC instrumentation
Preliminary support for a partial implementation of “Modified Condition/Decision Coverage” instrumentation was added behind the unstable flag `-Zcoverage-options=mcdc` in 2024. These are the most substantial PRs involved:
- rust-lang/rust#123409
- rust-lang/rust#126733
At the time, I accepted these PRs with relatively modest scrutiny, because I did not want to stand in the way of independent work on MC/DC instrumentation. My hope was that ongoing work by interested contributors would lead to the code becoming clearer and more maintainable over time.
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However, that MC/DC code has proven itself to be a major burden on overall maintenance of coverage instrumentation, and a major obstacle to other planned improvements, such as internal changes needed for proper support of macro expansion regions.
I have also become reluctant to accept any further MC/DC-related changes that would increase this burden.
That tension has resulted in an unhappy impasse. On one hand, the present MC/DC implementation is not yet complete, and shows little sign of being complete at an acceptable level of code quality in the foreseeable future. On the other hand, the continued existence of this partial MC/DC implementation is imposing serious maintenance burdens on every other aspect of coverage instrumentation, and is preventing some of the very improvements that would make it easier to accept expanded MC/DC support in the future.
While I know this will be disappointing to some, I think the healthy way forward is accept that I made the wrong call in accepting the current implementation, and to remove it entirely from the compiler.
get rid of some false negatives in rustdoc::broken_intra_doc_links
rustdoc will not try to do intra-doc linking if the "path" of a link looks too much like a "real url".
however, only inline links (`[text](url)`) can actually contain a url, other types of links (reference links, shortcut links) contain a *reference* which is later resolved to an actual url.
the "path" in this case cannot be a url, and therefore it should not be skipped due to looking like a url.
fixes https://github.com/rust-lang/rust/issues/54191
to minimize the number of false positives that will be introduced, the following heuristic is used:
If there's no backticks, be lenient revert to old behavior.
This is to prevent churn by linting on stuff that isn't meant to be a link.
only shortcut links have simple enough syntax that they
are likely to be written accidentlly, collapsed and reference links
need 4 metachars, and reference links will not usually use
backticks in the reference name.
therefore, only shortcut syntax gets the lenient behavior.
here's a truth table for how link kinds that cannot be urls are handled:
| | is shortcut link | not shortcut link |
|--------------|--------------------|-------------------|
| has backtick | never ignore | never ignore |
| no backtick | ignore if url-like | never ignore |
Rollup of 3 pull requests
Successful merges:
- rust-lang/rust#144657 (fix: Only "close the window" when its the last annotated file)
- rust-lang/rust#144665 (Re-block SRoA on SIMD types)
- rust-lang/rust#144713 (`rustc_middle::ty` cleanups)
r? `@ghost`
`@rustbot` modify labels: rollup
coverage: Re-land "Enlarge empty spans during MIR instrumentation"
This allows us to assume that coverage spans will only be discarded during codegen in very unusual situations.
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This seemingly-simple change has a rather messy history:
- rust-lang/rust#140847
- rust-lang/rust#141650
- rust-lang/rust#144298
- rust-lang/rust#144480
Since then, a number of related changes have landed that should make it reasonable to try again:
- rust-lang/rust#144530
- rust-lang/rust#144560
- rust-lang/rust#144616
In particular, we have multiple fixes/mitigations, and a confirmed regression test for the original bug that is not triggered by re-landing the changes in this PR.
Fix Box allocator drop elaboration
New version of rust-lang/rust#131146.
Clearing Box's drop flag after running its destructor can cause it to skip dropping its allocator, so just don't. Its cleared by the drop ladder code afterwards already.
Unlike the last PR this also handles other types with destructors properly, in the event that we can have open drops on them in the future (by partial initialization or DerefMove or something).
Finally, I also added tests for the interaction with async drop here but I discovered rust-lang/rust#143658, so one of the tests has a `knownbug` annotation. Not sure if it should be in this PR at all though.
Fixesrust-lang/rust#131082
r? wesleywiser - prev. reviewer
Rename impl_of_method and trait_of_item
This PR used to tweak the implementation of impl_of_method, but that introduced a perf regression.
Rename impl_of_method and trait_of_item to impl_of_assoc and trait_of_assoc respectively. This reflects how the two functions are closely related. And it reflects the behavior more accurately as the functions check whether the input is an associated item.
Allow more MIR SROA
This removes some guards on SROA that are no longer needed:
- With https://github.com/rust-lang/compiler-team/issues/838 it no longer needs to check for SIMD
- With https://github.com/rust-lang/compiler-team/issues/807 it no longer needs to check for niches
- This means that `Wrapper(char)` and `Pin<&mut T>` can get SRoA'd now, where previously they weren't because the check was banning SRaA for anything with a niche -- not just things with `#[rustc_layout_scalar_valid_range_*]`.
- Technically rust-lang/rust#133652 isn't complete yet, but `NonZero` and `NonNull` have already moved over, so this is fine. At worst this will mean that LLVM gets less `!range` metadata on something that wasn't already fixed by rust-lang/rust#133651 or rust-lang/rust#135236, but that's still sound, and unblocking general SRoA is worth that tradeoff.
coverage: Enlarge empty spans during MIR instrumentation, not codegen
This re-lands the part of rust-lang/rust#140847 that was (hopefully) not responsible for the coverage-instrumentation regressions that caused that PR to be reverted.
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Enlarging empty spans was historically performed during MIR instrumentation, but had to be moved to codegen as part of larger changes in rust-lang/rust#134497, leading to the status quo. But now there should be no reason not to move that step back to its more logical home in instrumentaion.
Debug impls for DropElaborators
It's a little weird that these just have a completely empty Debug impl. Now they're `ElaborateDropsCtxt { .. }` and `DropShimElaborator { .. }`.
