Auto merge of #124255 - RenjiSann:renji/mcdc-nested-expressions, r=Zalathar

MCDC coverage: support nested decision coverage

#123409 provided the initial MCDC coverage implementation.

As referenced in #124144, it does not currently support "nested" decisions, like the following example :

```rust
fn nested_if_in_condition(a: bool, b: bool, c: bool) {
    if a && if b || c { true } else { false } {
        say("yes");
    } else {
        say("no");
    }
}
```

Note that there is an if-expression (`if b || c ...`) embedded inside a boolean expression in the decision of an outer if-expression.

This PR proposes a workaround for this cases, by introducing a Decision context stack, and by handing several `temporary condition bitmaps` instead of just one.
When instrumenting boolean expressions, if the current node is a leaf condition (i.e. not a `||`/`&&` logical operator nor a `!` not operator), we insert a new decision context, such that if there are more boolean expressions inside the condition, they are handled as separate expressions.

On the codegen LLVM side, we allocate as many `temp_cond_bitmap`s as necessary to handle the maximum encountered decision depth.

compiler/rustc_middle/src/mir/coverage.rs

@@ -132,7 +132,7 @@ pub enum CoverageKind {
     ///
     /// If this statement does not survive MIR optimizations, the condition would never be
     /// taken as evaluated.
-    CondBitmapUpdate { id: ConditionId, value: bool },
+    CondBitmapUpdate { id: ConditionId, value: bool, decision_depth: u16 },
 
     /// Marks the point in MIR control flow represented by a evaluated decision.
     ///
@@ -140,7 +140,7 @@ pub enum CoverageKind {
     ///
     /// If this statement does not survive MIR optimizations, the decision would never be
     /// taken as evaluated.
-    TestVectorBitmapUpdate { bitmap_idx: u32 },
+    TestVectorBitmapUpdate { bitmap_idx: u32, decision_depth: u16 },
 }
 
 impl Debug for CoverageKind {
@@ -151,11 +151,17 @@ impl Debug for CoverageKind {
             BlockMarker { id } => write!(fmt, "BlockMarker({:?})", id.index()),
             CounterIncrement { id } => write!(fmt, "CounterIncrement({:?})", id.index()),
             ExpressionUsed { id } => write!(fmt, "ExpressionUsed({:?})", id.index()),
-            CondBitmapUpdate { id, value } => {
-                write!(fmt, "CondBitmapUpdate({:?}, {:?})", id.index(), value)
+            CondBitmapUpdate { id, value, decision_depth } => {
+                write!(
+                    fmt,
+                    "CondBitmapUpdate({:?}, {:?}, depth={:?})",
+                    id.index(),
+                    value,
+                    decision_depth
+                )
             }
-            TestVectorBitmapUpdate { bitmap_idx } => {
-                write!(fmt, "TestVectorUpdate({:?})", bitmap_idx)
+            TestVectorBitmapUpdate { bitmap_idx, decision_depth } => {
+                write!(fmt, "TestVectorUpdate({:?}, depth={:?})", bitmap_idx, decision_depth)
             }
         }
     }
@@ -269,6 +275,9 @@ pub struct FunctionCoverageInfo {
     pub mcdc_bitmap_bytes: u32,
     pub expressions: IndexVec<ExpressionId, Expression>,
     pub mappings: Vec<Mapping>,
+    /// The depth of the deepest decision is used to know how many
+    /// temp condbitmaps should be allocated for the function.
+    pub mcdc_max_decision_depth: u16,
 }
 
 /// Branch information recorded during THIR-to-MIR lowering, and stored in MIR.
@@ -319,6 +328,7 @@ pub struct MCDCBranchSpan {
     pub condition_info: Option<ConditionInfo>,
     pub true_marker: BlockMarkerId,
     pub false_marker: BlockMarkerId,
+    pub decision_depth: u16,
 }
 
 #[derive(Copy, Clone, Debug)]
@@ -334,4 +344,5 @@ pub struct MCDCDecisionSpan {
     pub span: Span,
     pub conditions_num: usize,
     pub end_markers: Vec<BlockMarkerId>,
+    pub decision_depth: u16,
 }

compiler/rustc_middle/src/mir/pretty.rs

@@ -496,20 +496,27 @@ fn write_coverage_branch_info(
         )?;
     }
 
-    for coverage::MCDCBranchSpan { span, condition_info, true_marker, false_marker } in
-        mcdc_branch_spans
+    for coverage::MCDCBranchSpan {
+        span,
+        condition_info,
+        true_marker,
+        false_marker,
+        decision_depth,
+    } in mcdc_branch_spans
     {
         writeln!(
             w,
-            "{INDENT}coverage mcdc branch {{ condition_id: {:?}, true: {true_marker:?}, false: {false_marker:?} }} => {span:?}",
+            "{INDENT}coverage mcdc branch {{ condition_id: {:?}, true: {true_marker:?}, false: {false_marker:?}, depth: {decision_depth:?} }} => {span:?}",
             condition_info.map(|info| info.condition_id)
         )?;
     }
 
-    for coverage::MCDCDecisionSpan { span, conditions_num, end_markers } in mcdc_decision_spans {
+    for coverage::MCDCDecisionSpan { span, conditions_num, end_markers, decision_depth } in
+        mcdc_decision_spans
+    {
         writeln!(
             w,
-            "{INDENT}coverage mcdc decision {{ conditions_num: {conditions_num:?}, end: {end_markers:?} }} => {span:?}"
+            "{INDENT}coverage mcdc decision {{ conditions_num: {conditions_num:?}, end: {end_markers:?}, depth: {decision_depth:?} }} => {span:?}"
         )?;
     }