coverage: Unbox and simplify `bcb_filtered_successors`

This function already has access to the MIR body, so instead of taking a
reference to a terminator, it's simpler and easier to pass in a basic block
index.

There is no need to box the returned iterator if we instead add appropriate
lifetime captures, since `short_circuit_preorder` is now generic over the type
of iterator it expects.

We can also greatly simplify the function's implementation by observing that
the only difference between its two cases is whether we take all of a BB's
successors, or just the first one.

compiler/rustc_mir_transform/src/coverage/graph.rs

@@ -3,7 +3,7 @@ use rustc_data_structures::graph::dominators::{self, Dominators};
 use rustc_data_structures::graph::{self, GraphSuccessors, WithNumNodes, WithStartNode};
 use rustc_index::bit_set::BitSet;
 use rustc_index::{IndexSlice, IndexVec};
-use rustc_middle::mir::{self, BasicBlock, Terminator, TerminatorKind};
+use rustc_middle::mir::{self, BasicBlock, TerminatorKind};
 
 use std::cmp::Ordering;
 use std::ops::{Index, IndexMut};
@@ -37,9 +37,8 @@ impl CoverageGraph {
                 }
                 let bcb_data = &bcbs[bcb];
                 let mut bcb_successors = Vec::new();
-                for successor in
-                    bcb_filtered_successors(&mir_body, &bcb_data.terminator(mir_body).kind)
-                        .filter_map(|successor_bb| bb_to_bcb[successor_bb])
+                for successor in bcb_filtered_successors(&mir_body, bcb_data.last_bb())
+                    .filter_map(|successor_bb| bb_to_bcb[successor_bb])
                 {
                     if !seen[successor] {
                         seen[successor] = true;
@@ -316,11 +315,6 @@ impl BasicCoverageBlockData {
     pub fn last_bb(&self) -> BasicBlock {
         *self.basic_blocks.last().unwrap()
     }
-
-    #[inline(always)]
-    pub fn terminator<'a, 'tcx>(&self, mir_body: &'a mir::Body<'tcx>) -> &'a Terminator<'tcx> {
-        &mir_body[self.last_bb()].terminator()
-    }
 }
 
 /// Represents a successor from a branching BasicCoverageBlock (such as the arms of a `SwitchInt`)
@@ -362,26 +356,28 @@ impl std::fmt::Debug for BcbBranch {
     }
 }
 
-// Returns the `Terminator`s non-unwind successors.
+// Returns the subset of a block's successors that are relevant to the coverage
+// graph, i.e. those that do not represent unwinds or unreachable branches.
 // FIXME(#78544): MIR InstrumentCoverage: Improve coverage of `#[should_panic]` tests and
 // `catch_unwind()` handlers.
 fn bcb_filtered_successors<'a, 'tcx>(
     body: &'a mir::Body<'tcx>,
-    term_kind: &'a TerminatorKind<'tcx>,
-) -> Box<dyn Iterator<Item = BasicBlock> + 'a> {
-    Box::new(
-        match &term_kind {
-            // SwitchInt successors are never unwind, and all of them should be traversed.
-            TerminatorKind::SwitchInt { ref targets, .. } => {
-                None.into_iter().chain(targets.all_targets().into_iter().copied())
-            }
-            // For all other kinds, return only the first successor, if any, and ignore unwinds.
-            // NOTE: `chain(&[])` is required to coerce the `option::iter` (from
-            // `next().into_iter()`) into the `mir::Successors` aliased type.
-            _ => term_kind.successors().next().into_iter().chain((&[]).into_iter().copied()),
-        }
-        .filter(move |&successor| body[successor].terminator().kind != TerminatorKind::Unreachable),
-    )
+    bb: BasicBlock,
+) -> impl Iterator<Item = BasicBlock> + Captures<'a> + Captures<'tcx> {
+    let terminator = body[bb].terminator();
+
+    let take_n_successors = match terminator.kind {
+        // SwitchInt successors are never unwinds, so all of them should be traversed.
+        TerminatorKind::SwitchInt { .. } => usize::MAX,
+        // For all other kinds, return only the first successor (if any), ignoring any
+        // unwind successors.
+        _ => 1,
+    };
+
+    terminator
+        .successors()
+        .take(take_n_successors)
+        .filter(move |&successor| body[successor].terminator().kind != TerminatorKind::Unreachable)
 }
 
 /// Maintains separate worklists for each loop in the BasicCoverageBlock CFG, plus one for the
@@ -558,7 +554,7 @@ fn short_circuit_preorder<'a, 'tcx, F, Iter>(
     filtered_successors: F,
 ) -> impl Iterator<Item = BasicBlock> + Captures<'a> + Captures<'tcx>
 where
-    F: Fn(&'a mir::Body<'tcx>, &'a TerminatorKind<'tcx>) -> Iter,
+    F: Fn(&'a mir::Body<'tcx>, BasicBlock) -> Iter,
     Iter: Iterator<Item = BasicBlock>,
 {
     let mut visited = BitSet::new_empty(body.basic_blocks.len());
@@ -570,7 +566,7 @@ where
                 continue;
             }
 
-            worklist.extend(filtered_successors(body, &body[bb].terminator().kind));
+            worklist.extend(filtered_successors(body, bb));
 
             return Some(bb);
         }

compiler/rustc_mir_transform/src/coverage/tests.rs

@@ -241,7 +241,7 @@ fn print_coverage_graphviz(
                         "    {:?} [label=\"{:?}: {}\"];\n{}",
                         bcb,
                         bcb,
-                        bcb_data.terminator(mir_body).kind.name(),
+                        mir_body[bcb_data.last_bb()].terminator().kind.name(),
                         basic_coverage_blocks
                             .successors(bcb)
                             .map(|successor| { format!("    {:?} -> {:?};", bcb, successor) })