Report coverage 0 of dead blocks

Fixes: #84018

With `-Z instrument-coverage`, coverage reporting of dead blocks
(for example, blocks dropped because a conditional branch is dropped,
based on const evaluation) is now supported.

If `instrument-coverage` is enabled, `simplify::remove_dead_blocks()`
finds all dropped coverage `Statement`s and adds their `code_region`s as
`Unreachable` coverage `Statement`s to the `START_BLOCK`, so they are
still included in the coverage map.

Check out the resulting changes in the test coverage reports in this PR.

compiler/rustc_mir/src/transform/simplify.rs

@@ -29,6 +29,7 @@
 
 use crate::transform::MirPass;
 use rustc_index::vec::{Idx, IndexVec};
+use rustc_middle::mir::coverage::*;
 use rustc_middle::mir::visit::{MutVisitor, MutatingUseContext, PlaceContext, Visitor};
 use rustc_middle::mir::*;
 use rustc_middle::ty::TyCtxt;
@@ -46,9 +47,9 @@ impl SimplifyCfg {
     }
 }
 
-pub fn simplify_cfg(body: &mut Body<'_>) {
+pub fn simplify_cfg(tcx: TyCtxt<'tcx>, body: &mut Body<'_>) {
     CfgSimplifier::new(body).simplify();
-    remove_dead_blocks(body);
+    remove_dead_blocks(tcx, body);
 
     // FIXME: Should probably be moved into some kind of pass manager
     body.basic_blocks_mut().raw.shrink_to_fit();
@@ -59,9 +60,9 @@ impl<'tcx> MirPass<'tcx> for SimplifyCfg {
         Cow::Borrowed(&self.label)
     }
 
-    fn run_pass(&self, _tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+    fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
         debug!("SimplifyCfg({:?}) - simplifying {:?}", self.label, body.source);
-        simplify_cfg(body);
+        simplify_cfg(tcx, body);
     }
 }
 
@@ -286,7 +287,7 @@ impl<'a, 'tcx> CfgSimplifier<'a, 'tcx> {
     }
 }
 
-pub fn remove_dead_blocks(body: &mut Body<'_>) {
+pub fn remove_dead_blocks(tcx: TyCtxt<'tcx>, body: &mut Body<'_>) {
     let reachable = traversal::reachable_as_bitset(body);
     let num_blocks = body.basic_blocks().len();
     if num_blocks == reachable.count() {
@@ -306,6 +307,11 @@ pub fn remove_dead_blocks(body: &mut Body<'_>) {
         }
         used_blocks += 1;
     }
+
+    if tcx.sess.instrument_coverage() {
+        save_unreachable_coverage(basic_blocks, used_blocks);
+    }
+
     basic_blocks.raw.truncate(used_blocks);
 
     for block in basic_blocks {
@@ -315,6 +321,32 @@ pub fn remove_dead_blocks(body: &mut Body<'_>) {
     }
 }
 
+fn save_unreachable_coverage(
+    basic_blocks: &mut IndexVec<BasicBlock, BasicBlockData<'_>>,
+    first_dead_block: usize,
+) {
+    // retain coverage info for dead blocks, so coverage reports will still
+    // report `0` executions for the uncovered code regions.
+    let mut dropped_coverage = Vec::new();
+    for dead_block in first_dead_block..basic_blocks.len() {
+        for statement in basic_blocks[BasicBlock::new(dead_block)].statements.iter() {
+            if let StatementKind::Coverage(coverage) = &statement.kind {
+                if let Some(code_region) = &coverage.code_region {
+                    dropped_coverage.push((statement.source_info, code_region.clone()));
+                }
+            }
+        }
+    }
+    for (source_info, code_region) in dropped_coverage {
+        basic_blocks[START_BLOCK].statements.push(Statement {
+            source_info,
+            kind: StatementKind::Coverage(box Coverage {
+                kind: CoverageKind::Unreachable,
+                code_region: Some(code_region),
+            }),
+        })
+    }
+}
 pub struct SimplifyLocals;
 
 impl<'tcx> MirPass<'tcx> for SimplifyLocals {