coverage: Re-enable UnreachablePropagation for coverage builds

Author: Zalathar

compiler/rustc_mir_transform/src/unreachable_prop.rs

@@ -14,11 +14,7 @@ pub struct UnreachablePropagation;
 impl MirPass<'_> for UnreachablePropagation {
     fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
         // Enable only under -Zmir-opt-level=2 as this can make programs less debuggable.
-
-        // FIXME(#116171) Coverage gets confused by MIR passes that can remove all
-        // coverage statements from an instrumented function. This pass can be
-        // re-enabled when coverage codegen is robust against that happening.
-        sess.mir_opt_level() >= 2 && !sess.instrument_coverage()
+        sess.mir_opt_level() >= 2
     }
 
     fn run_pass<'tcx>(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {

tests/coverage/unreachable.cov-map

@@ -14,11 +14,11 @@ Number of expressions: 0
 Number of file 0 mappings: 1
 - Code(Counter(0)) at (prev + 17, 1) to (start + 1, 37)
 
-Function name: unreachable::unreachable_intrinsic
-Raw bytes (9): 0x[01, 01, 00, 01, 01, 16, 01, 01, 2c]
+Function name: unreachable::unreachable_intrinsic (unused)
+Raw bytes (9): 0x[01, 01, 00, 01, 00, 16, 01, 01, 2c]
 Number of files: 1
 - file 0 => global file 1
 Number of expressions: 0
 Number of file 0 mappings: 1
-- Code(Counter(0)) at (prev + 22, 1) to (start + 1, 44)
+- Code(Zero) at (prev + 22, 1) to (start + 1, 44)