coverage: llvm-cov expects column numbers to be bytes, not code points

[Zalathar]

Normally the compiler emits column numbers as a 1-based number of Unicode code points.

But when we embed coverage mappings for -Cinstrument-coverage, those mappings will ultimately be read by the llvm-cov tool. That tool assumes that column numbers are 1-based numbers of bytes, and relies on that assumption when slicing up source code to apply highlighting (in HTML reports, and in text-based reports with colour).

For the very common case of all-ASCII source code, bytes and code points are the same, so the difference isn't noticeable. But for code that contains non-ASCII characters, emitting column numbers as code points will result in llvm-cov slicing strings in the wrong places, producing mangled output or fatal errors.

(See taiki-e/cargo-llvm-cov#275 as an example of what can go wrong.)

[Zalathar]

#119034 has landed, so now I can think about making this ready for review.

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[rustbot]

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[Zalathar]

r? compiler

[davidtwco]

r=me when CI passes

[oli-obk]

@bors delegate+

[bors]

✌️ @Zalathar, you can now approve this pull request!

If @oli-obk told you to "r=me" after making some further change, please make that change, then do @bors r=@oli-obk

[Zalathar]

@bors r=davidtwco

[bors]

📌 Commit 6971e93 has been approved by davidtwco

It is now in the queue for this repository.

[Zalathar]

@rustbot label +relnotes

This is a user-visible bug fix for taiki-e/cargo-llvm-cov#275.

Suggested summary: “Fix coverage instrumentation/reports for non-ASCII source code”