Function rustc_mir_transform::simplify::save_unreachable_coverage

fn save_unreachable_coverage(
    basic_blocks: &mut IndexVec<BasicBlock, BasicBlockData<'_>>, 
    first_dead_block: usize
)

Some MIR transforms can determine at compile time that a sequences of statements will never be executed, so they can be dropped from the MIR. For example, an if or else block that is guaranteed to never be executed because its condition can be evaluated at compile time, such as by const evaluation: if false { ... }.

Those statements are bypassed by redirecting paths in the CFG around the dead blocks; but with -Z instrument-coverage, the dead blocks usually include Coverage statements representing the Rust source code regions to be counted at runtime. Without these Coverage statements, the regions are lost, and the Rust source code will show no coverage information.

What we want to show in a coverage report is the dead code with coverage counts of 0. To do this, we need to save the code regions, by injecting Unreachable coverage statements. These are non-executable statements whose code regions are still recorded in the coverage map, representing regions with 0 executions.