Rollup merge of #97079 - SparrowLii:successors, r=lcnr

Change `Successors` to `impl Iterator<Item = BasicBlock>`

This PR fixes the FIXME in `compiler\rustc_middle\src\mir\mod.rs`.
This can omit several `&`, `*` or `cloned` operations on Successros' generated elements

compiler/rustc_mir_transform/src/coverage/debug.rs

@@ -701,7 +701,7 @@ pub(super) fn dump_coverage_graphviz<'tcx>(
         edge_labels.retain(|label| label != "unreachable");
         let edge_counters = from_terminator
             .successors()
-            .map(|&successor_bb| graphviz_data.get_edge_counter(from_bcb, successor_bb));
+            .map(|successor_bb| graphviz_data.get_edge_counter(from_bcb, successor_bb));
         iter::zip(&edge_labels, edge_counters)
             .map(|(label, some_counter)| {
                 if let Some(counter) = some_counter {

compiler/rustc_mir_transform/src/coverage/graph.rs

@@ -484,17 +484,17 @@ fn bcb_filtered_successors<'a, 'tcx>(
     body: &'tcx &'a mir::Body<'tcx>,
     term_kind: &'tcx TerminatorKind<'tcx>,
 ) -> Box<dyn Iterator<Item = BasicBlock> + 'a> {
-    let mut successors = term_kind.successors();
     Box::new(
         match &term_kind {
             // SwitchInt successors are never unwind, and all of them should be traversed.
-            TerminatorKind::SwitchInt { .. } => successors,
+            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.
-            _ => successors.next().into_iter().chain(&[]),
+            _ => term_kind.successors().next().into_iter().chain((&[]).into_iter().copied()),
         }
-        .copied()
         .filter(move |&successor| body[successor].terminator().kind != TerminatorKind::Unreachable),
     )
 }

compiler/rustc_mir_transform/src/inline.rs

@@ -450,7 +450,7 @@ impl<'tcx> Inliner<'tcx> {
             }
 
             if !is_drop {
-                for &succ in term.successors() {
+                for succ in term.successors() {
                     work_list.push(succ);
                 }
             }

compiler/rustc_mir_transform/src/remove_noop_landing_pads.rs

@@ -65,7 +65,7 @@ impl RemoveNoopLandingPads {
             | TerminatorKind::SwitchInt { .. }
             | TerminatorKind::FalseEdge { .. }
             | TerminatorKind::FalseUnwind { .. } => {
-                terminator.successors().all(|&succ| nop_landing_pads.contains(succ))
+                terminator.successors().all(|succ| nop_landing_pads.contains(succ))
             }
             TerminatorKind::GeneratorDrop
             | TerminatorKind::Yield { .. }

compiler/rustc_mir_transform/src/simplify.rs

@@ -81,7 +81,7 @@ impl<'a, 'tcx> CfgSimplifier<'a, 'tcx> {
 
         for (_, data) in traversal::preorder(body) {
             if let Some(ref term) = data.terminator {
-                for &tgt in term.successors() {
+                for tgt in term.successors() {
                     pred_count[tgt] += 1;
                 }
             }
@@ -235,8 +235,8 @@ impl<'a, 'tcx> CfgSimplifier<'a, 'tcx> {
         };
 
         let first_succ = {
-            if let Some(&first_succ) = terminator.successors().next() {
-                if terminator.successors().all(|s| *s == first_succ) {
+            if let Some(first_succ) = terminator.successors().next() {
+                if terminator.successors().all(|s| s == first_succ) {
                     let count = terminator.successors().count();
                     self.pred_count[first_succ] -= (count - 1) as u32;
                     first_succ