MC/DC Coverage re-introduction (1/3): THIR data gathering

[RenjiSann]

Tracking issue: rust-lang/goals#638

Identify MC/DC decision in the AST form, to allow for incoming instrumentation and mappings creation.

1. Introduce a -Z coverage-options=mcdc flag
2. Detect boolean expressions in THIR to:
   1. Gather sufficient data to construct MC/DC decisions;
   2. Emit block markers in MIR to track the expressions and instrument them

r? @davidtwco

[rustbot]

Some changes occurred in match lowering

cc @Nadrieril

Some changes occurred in coverage instrumentation.

cc @Zalathar

Some changes occurred in coverage instrumentation.

cc @Zalathar

[rust-log-analyzer]

The job tidy failed! Check out the build log: (web) (plain enhanced) (plain)

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fmt: checked 7163 files
tidy check
tidy [rustdoc_json (src)]: `rustdoc-json-types` modified, checking format version
tidy: Skipping binary file check, read-only filesystem
tidy [style (compiler)]: /checkout/compiler/rustc_middle/src/mir/coverage.rs:235: 14-line comment block with odd number of backticks
tidy [style (compiler)]: FAIL
removing old virtual environment
creating virtual environment at '/checkout/obj/build/venv' using 'python3.12' and 'venv'
Requirement already satisfied: pip in ./build/venv/lib/python3.12/site-packages (24.0)
Collecting pip
  Downloading pip-26.2.1-py3-none-any.whl.metadata (4.6 kB)
---
typechecking javascript files
tidy: The following check failed: style (compiler)
Bootstrap failed while executing `test src/tools/tidy tidyselftest --extra-checks=py,cpp,js,spellcheck`
Currently active steps:
test::Tidy {  } at src/bootstrap/src/core/build_steps/test.rs:1661
Command `/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-tools-bin/rust-tidy --root-path=/checkout --cargo-path=/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo --output-dir=/checkout/obj/build --concurrency=4 --npm-path=/node/bin/yarn --ci=true --extra-checks=py,cpp,js,spellcheck` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/tool.rs:1626:23
Executed at: src/bootstrap/src/core/build_steps/test.rs:1743:29

Command has failed. Rerun with -v to see more details.
Build completed unsuccessfully in 0:02:39
  local time: Mon Aug 17 16:33:29 UTC 2026
  network time: Mon, 17 Aug 2026 16:33:29 GMT
##[error]Process completed with exit code 1.
##[group]Run echo "disk usage:"