Rollup merge of rust-lang#122764 - Zalathar:loopy, r=oli-obk

coverage: Remove incorrect assertions from counter allocation

These assertions detect situations where a BCB node (in the coverage graph) would have both a physical counter and one or more in-edge counters/expressions.

For most BCBs that situation would indicate an implementation bug. However, it's perfectly fine in the case of a BCB having an edge that loops back to itself.

Given the complexity and risk involved in fixing the assertions, and the fact that nothing relies on them actually being true, this patch just removes them instead.

Fixes rust-lang#122738.

`````@rustbot````` label +A-code-coverage

Author: jhpratt

compiler/rustc_mir_transform/src/coverage/counters.rs

@@ -1,13 +1,12 @@
+use std::fmt::{self, Debug};
+
 use rustc_data_structures::captures::Captures;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::graph::WithNumNodes;
-use rustc_index::bit_set::BitSet;
 use rustc_index::IndexVec;
-use rustc_middle::mir::coverage::*;
+use rustc_middle::mir::coverage::{CounterId, CovTerm, Expression, ExpressionId, Op};
 
-use super::graph::{BasicCoverageBlock, CoverageGraph, TraverseCoverageGraphWithLoops};
-
-use std::fmt::{self, Debug};
+use crate::coverage::graph::{BasicCoverageBlock, CoverageGraph, TraverseCoverageGraphWithLoops};
 
 /// The coverage counter or counter expression associated with a particular
 /// BCB node or BCB edge.
@@ -18,10 +17,6 @@ pub(super) enum BcbCounter {
 }
 
 impl BcbCounter {
-    fn is_expression(&self) -> bool {
-        matches!(self, Self::Expression { .. })
-    }
-
     pub(super) fn as_term(&self) -> CovTerm {
         match *self {
             BcbCounter::Counter { id, .. } => CovTerm::Counter(id),
@@ -60,10 +55,6 @@ pub(super) struct CoverageCounters {
     /// We currently don't iterate over this map, but if we do in the future,
     /// switch it back to `FxIndexMap` to avoid query stability hazards.
     bcb_edge_counters: FxHashMap<(BasicCoverageBlock, BasicCoverageBlock), BcbCounter>,
-    /// Tracks which BCBs have a counter associated with some incoming edge.
-    /// Only used by assertions, to verify that BCBs with incoming edge
-    /// counters do not have their own physical counters (expressions are allowed).
-    bcb_has_incoming_edge_counters: BitSet<BasicCoverageBlock>,
     /// Table of expression data, associating each expression ID with its
     /// corresponding operator (+ or -) and its LHS/RHS operands.
     expressions: IndexVec<ExpressionId, Expression>,
@@ -83,7 +74,6 @@ impl CoverageCounters {
             counter_increment_sites: IndexVec::new(),
             bcb_counters: IndexVec::from_elem_n(None, num_bcbs),
             bcb_edge_counters: FxHashMap::default(),
-            bcb_has_incoming_edge_counters: BitSet::new_empty(num_bcbs),
             expressions: IndexVec::new(),
         };
 
@@ -122,14 +112,6 @@ impl CoverageCounters {
     }
 
     fn set_bcb_counter(&mut self, bcb: BasicCoverageBlock, counter_kind: BcbCounter) -> BcbCounter {
-        assert!(
-            // If the BCB has an edge counter (to be injected into a new `BasicBlock`), it can also
-            // have an expression (to be injected into an existing `BasicBlock` represented by this
-            // `BasicCoverageBlock`).
-            counter_kind.is_expression() || !self.bcb_has_incoming_edge_counters.contains(bcb),
-            "attempt to add a `Counter` to a BCB target with existing incoming edge counters"
-        );
-
         if let Some(replaced) = self.bcb_counters[bcb].replace(counter_kind) {
             bug!(
                 "attempt to set a BasicCoverageBlock coverage counter more than once; \
@@ -146,19 +128,6 @@ impl CoverageCounters {
         to_bcb: BasicCoverageBlock,
         counter_kind: BcbCounter,
     ) -> BcbCounter {
-        // If the BCB has an edge counter (to be injected into a new `BasicBlock`), it can also
-        // have an expression (to be injected into an existing `BasicBlock` represented by this
-        // `BasicCoverageBlock`).
-        if let Some(node_counter) = self.bcb_counter(to_bcb)
-            && !node_counter.is_expression()
-        {
-            bug!(
-                "attempt to add an incoming edge counter from {from_bcb:?} \
-                when the target BCB already has {node_counter:?}"
-            );
-        }
-
-        self.bcb_has_incoming_edge_counters.insert(to_bcb);
         if let Some(replaced) = self.bcb_edge_counters.insert((from_bcb, to_bcb), counter_kind) {
             bug!(
                 "attempt to set an edge counter more than once; from_bcb: \

tests/coverage/let_else_loop.cov-map

@@ -0,0 +1,30 @@
+Function name: let_else_loop::_if (unused)
+Raw bytes (19): 0x[01, 01, 00, 03, 00, 16, 01, 01, 0c, 00, 02, 09, 00, 10, 00, 02, 09, 00, 10]
+Number of files: 1
+- file 0 => global file 1
+Number of expressions: 0
+Number of file 0 mappings: 3
+- Code(Zero) at (prev + 22, 1) to (start + 1, 12)
+- Code(Zero) at (prev + 2, 9) to (start + 0, 16)
+- Code(Zero) at (prev + 2, 9) to (start + 0, 16)
+
+Function name: let_else_loop::_loop_either_way (unused)
+Raw bytes (19): 0x[01, 01, 00, 03, 00, 0f, 01, 01, 14, 00, 01, 1c, 00, 23, 00, 01, 05, 00, 0c]
+Number of files: 1
+- file 0 => global file 1
+Number of expressions: 0
+Number of file 0 mappings: 3
+- Code(Zero) at (prev + 15, 1) to (start + 1, 20)
+- Code(Zero) at (prev + 1, 28) to (start + 0, 35)
+- Code(Zero) at (prev + 1, 5) to (start + 0, 12)
+
+Function name: let_else_loop::loopy
+Raw bytes (19): 0x[01, 01, 00, 03, 01, 09, 01, 01, 14, 00, 01, 1c, 00, 23, 05, 01, 01, 00, 02]
+Number of files: 1
+- file 0 => global file 1
+Number of expressions: 0
+Number of file 0 mappings: 3
+- Code(Counter(0)) at (prev + 9, 1) to (start + 1, 20)
+- Code(Zero) at (prev + 1, 28) to (start + 0, 35)
+- Code(Counter(1)) at (prev + 1, 1) to (start + 0, 2)
+

tests/coverage/let_else_loop.coverage

@@ -0,0 +1,35 @@
+   LL|       |#![feature(coverage_attribute)]
+   LL|       |//@ edition: 2021
+   LL|       |
+   LL|       |// Regression test for <https://github.com/rust-lang/rust/issues/122738>.
+   LL|       |// These code patterns should not trigger an ICE when allocating a physical
+   LL|       |// counter to a node and also one of its in-edges, because that is allowed
+   LL|       |// when the node contains a tight loop to itself.
+   LL|       |
+   LL|      1|fn loopy(cond: bool) {
+   LL|      1|    let true = cond else { loop {} };
+                                         ^0
+   LL|      1|}
+   LL|       |
+   LL|       |// Variant that also has `loop {}` on the success path.
+   LL|       |// This isn't needed to catch the original ICE, but might help detect regressions.
+   LL|      0|fn _loop_either_way(cond: bool) {
+   LL|      0|    let true = cond else { loop {} };
+   LL|      0|    loop {}
+   LL|       |}
+   LL|       |
+   LL|       |// Variant using regular `if` instead of let-else.
+   LL|       |// This doesn't trigger the original ICE, but might help detect regressions.
+   LL|      0|fn _if(cond: bool) {
+   LL|      0|    if cond {
+   LL|      0|        loop {}
+   LL|       |    } else {
+   LL|      0|        loop {}
+   LL|       |    }
+   LL|       |}
+   LL|       |
+   LL|       |#[coverage(off)]
+   LL|       |fn main() {
+   LL|       |    loopy(true);
+   LL|       |}
+

tests/coverage/let_else_loop.rs

@@ -0,0 +1,33 @@
+#![feature(coverage_attribute)]
+//@ edition: 2021
+
+// Regression test for <https://github.com/rust-lang/rust/issues/122738>.
+// These code patterns should not trigger an ICE when allocating a physical
+// counter to a node and also one of its in-edges, because that is allowed
+// when the node contains a tight loop to itself.
+
+fn loopy(cond: bool) {
+    let true = cond else { loop {} };
+}
+
+// Variant that also has `loop {}` on the success path.
+// This isn't needed to catch the original ICE, but might help detect regressions.
+fn _loop_either_way(cond: bool) {
+    let true = cond else { loop {} };
+    loop {}
+}
+
+// Variant using regular `if` instead of let-else.
+// This doesn't trigger the original ICE, but might help detect regressions.
+fn _if(cond: bool) {
+    if cond {
+        loop {}
+    } else {
+        loop {}
+    }
+}
+
+#[coverage(off)]
+fn main() {
+    loopy(true);
+}