ZhuUx

For now there are still some work need to do and some code is not well optimized to reduce work in merging #124255 . r? @ghost @rustbot label +A-code-coverage @rustbot...

I have pushed an impractical commit to show design for constructing decisions and mcdc branches of pattern matching. Mostly is in `MCDCDecisionBuilder`. I'd like to finish it after merging #124255...

Implementation for if-let has been drafted. Due to [llvm does not support nested decisions](https://github.com/rust-lang/rust/issues/124144#issuecomment-2102870506) yet tests for let-chains are not added. I should try to reduce coverage expressions and investigate...

Now I think this job is almost done. Two major changes are introduced: mcdc tests for refutable patterns (if-let) and branch coverage for match guards, as the commits history shows....

Change of `MCDCBranchSpan` introduces some duplicate coverage counter repressions apparently. I have managed to reduce them by [this commit](https://github.com/rust-lang/rust/commit/08ea3634bd058e835ceed67b4c61d41d668f3f33). However, as that commit shows, there have been many redundant expressions...

> Maybe we could have a separate PR that applies that change (or something like it), and then also applies some expression deduplication. Sure I do think so. Thus that...

Thanks to @Nadrieril now I could find a way to treat each arm in `match` statements as a decision. The basic idea is to update test vectors of every decision...

Changed a bit to fit with constant folding. The main goal is to find all `false block`s of branches and let the `false term` be sum of all counters in...

Upon compiling my projects with "--coverage-options=mcdc", I have found there are still some work to do with pattern matching expanded by macros. I would like to investigate whether we should...

Have refactored and tested much so that I'm a bit afraid there are too many details need to note in my implementation. I wonder is it possible to analyze the...