coverage: Extract CoverageGraph::bcb_has_multiple_in_edges

This was previously a helper method in `MakeBcbCounters`, but putting it in the
graph lets us call it from `BcbBranch`, and gives us a more fine-grained
borrow.

Author: Zalathar

compiler/rustc_mir_transform/src/coverage/counters.rs

@@ -339,7 +339,7 @@ impl<'a> MakeBcbCounters<'a> {
         // A BCB with only one incoming edge gets a simple `Counter` (via `make_counter()`).
         // Also, a BCB that loops back to itself gets a simple `Counter`. This may indicate the
         // program results in a tight infinite loop, but it should still compile.
-        let one_path_to_target = self.bcb_has_one_path_to_target(bcb);
+        let one_path_to_target = !self.basic_coverage_blocks.bcb_has_multiple_in_edges(bcb);
         if one_path_to_target || self.bcb_predecessors(bcb).contains(&bcb) {
             let counter_kind = self.coverage_counters.make_counter();
             if one_path_to_target {
@@ -508,11 +508,4 @@ impl<'a> MakeBcbCounters<'a> {
             self.coverage_counters.bcb_counters[to_bcb].as_ref()
         }
     }
-
-    /// Returns true if the BasicCoverageBlock has zero or one incoming edge. (If zero, it should be
-    /// the entry point for the function.)
-    #[inline]
-    fn bcb_has_one_path_to_target(&self, bcb: BasicCoverageBlock) -> bool {
-        self.bcb_predecessors(bcb).len() <= 1
-    }
 }

compiler/rustc_mir_transform/src/coverage/graph.rs

@@ -199,6 +199,20 @@ impl CoverageGraph {
     pub fn cmp_in_dominator_order(&self, a: BasicCoverageBlock, b: BasicCoverageBlock) -> Ordering {
         self.dominators.as_ref().unwrap().cmp_in_dominator_order(a, b)
     }
+
+    /// Returns true if the given node has 2 or more in-edges, i.e. 2 or more
+    /// predecessors.
+    ///
+    /// This property is interesting to code that assigns counters to nodes and
+    /// edges, because if a node _doesn't_ have multiple in-edges, then there's
+    /// no benefit in having a separate counter for its in-edge, because it
+    /// would have the same value as the node's own counter.
+    ///
+    /// FIXME: That assumption might not be true for [`TerminatorKind::Yield`]?
+    #[inline(always)]
+    pub(super) fn bcb_has_multiple_in_edges(&self, bcb: BasicCoverageBlock) -> bool {
+        self.predecessors[bcb].len() > 1
+    }
 }
 
 impl Index<BasicCoverageBlock> for CoverageGraph {
@@ -334,7 +348,7 @@ impl BcbBranch {
         to_bcb: BasicCoverageBlock,
         basic_coverage_blocks: &CoverageGraph,
     ) -> Self {
-        let edge_from_bcb = if basic_coverage_blocks.predecessors[to_bcb].len() > 1 {
+        let edge_from_bcb = if basic_coverage_blocks.bcb_has_multiple_in_edges(from_bcb) {
             Some(from_bcb)
         } else {
             None