Remove reachable coverage without counters

Remove reachable coverage without counters to maintain invariant that
either there is no coverage at all or there is a live coverage counter
left that provides the function source hash.

The motivating example would be a following closure:

```rust
    let f = |x: bool| {
        debug_assert!(x);
    };
```

Which, with span changes from rust-lang#93967, with disabled debug assertions,
after the final CFG simplifications but before removal of dead blocks,
gives rise to MIR:

```rust
fn main::{closure#0}(_1: &[[email protected]:2:13: 2:22], _2: bool) -> () {
    debug x => _2;
    let mut _0: ();

    bb0: {
        Coverage::Expression(4294967295) = 1 - 2;
        return;
    }

    ...
}
```

Author: tmiasko

compiler/rustc_mir_transform/src/simplify.rs

@@ -315,7 +315,7 @@ pub fn remove_dead_blocks<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
 /// with `0` executions.
 ///
 /// If there are no live `Counter` `Coverage` statements remaining, we remove
-/// dead `Coverage` statements along with the dead blocks. Since at least one
+/// `Coverage` statements along with the dead blocks. Since at least one
 /// counter per function is required by LLVM (and necessary, to add the
 /// `function_hash` to the counter's call to the LLVM intrinsic
 /// `instrprof.increment()`).
@@ -342,6 +342,16 @@ fn save_unreachable_coverage(
         }
     }
 
+    for block in &mut basic_blocks.raw[..first_dead_block] {
+        for statement in &mut block.statements {
+            let StatementKind::Coverage(_) = &statement.kind else { continue };
+            let instance = statement.source_info.scope.inlined_instance(source_scopes);
+            if !live.contains(&instance) {
+                statement.make_nop();
+            }
+        }
+    }
+
     if live.is_empty() {
         return;
     }

src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.inline-dead.txt

@@ -1,21 +1,28 @@
     1|       |// Regression test for issue #98833.
-    2|       |// compile-flags: -Zinline-mir
+    2|       |// compile-flags: -Zinline-mir -Cdebug-assertions=off
     3|       |
     4|      1|fn main() {
     5|      1|    println!("{}", live::<false>());
-    6|      1|}
-    7|       |
-    8|       |#[inline]
-    9|      1|fn live<const B: bool>() -> u32 {
-   10|      1|    if B {
-   11|      0|        dead()
-   12|       |    } else {
-   13|      1|        0
-   14|       |    }
-   15|      1|}
-   16|       |
-   17|       |#[inline]
-   18|      0|fn dead() -> u32 {
-   19|      0|    42
-   20|      0|}
+    6|      1|
+    7|      1|    let f = |x: bool| {
+    8|       |        debug_assert!(
+    9|       |            x
+   10|       |        );
+   11|      1|    };
+   12|      1|    f(false);
+   13|      1|}
+   14|       |
+   15|       |#[inline]
+   16|      1|fn live<const B: bool>() -> u32 {
+   17|      1|    if B {
+   18|      0|        dead()
+   19|       |    } else {
+   20|      1|        0
+   21|       |    }
+   22|      1|}
+   23|       |
+   24|       |#[inline]
+   25|      0|fn dead() -> u32 {
+   26|      0|    42
+   27|      0|}
 

src/test/run-make-fulldeps/coverage/inline-dead.rs

@@ -1,8 +1,15 @@
 // Regression test for issue #98833.
-// compile-flags: -Zinline-mir
+// compile-flags: -Zinline-mir -Cdebug-assertions=off
 
 fn main() {
     println!("{}", live::<false>());
+
+    let f = |x: bool| {
+        debug_assert!(
+            x
+        );
+    };
+    f(false);
 }
 
 #[inline]