Adjust successor iterators.

Because rust-lang#148054 was a slight perf regression.

The problem was seemingly because this iterator structure:
```
slice_iter.chain(Option_iter.chain(Option_iter))
```
changed to this:
```
slice_iter.chain(Option_iter).chain(Option_iter)
```
The commit also tweaks the `slice_iter` part, changing `into_iter` to
`iter` and using `[]` instead of `(&[])`, for conciseness and
consistency.

Author: nnethercote

compiler/rustc_middle/src/mir/terminator.rs

@@ -501,30 +501,37 @@ pub use helper::*;
 
 mod helper {
     use super::*;
-    // Note: the methods below use a `slice.chain(Option).chain(Option)` pattern so that all paths
-    // produce an iterator with the same concrete type.
     pub type Successors<'a> = impl DoubleEndedIterator<Item = BasicBlock> + 'a;
 
+    // Note: this method ensures all paths below produce an iterator with the same concrete type.
+    #[inline]
+    #[define_opaque(Successors)]
+    fn mk_successors(
+        slice: &[BasicBlock],
+        option1: Option<BasicBlock>,
+        option2: Option<BasicBlock>,
+    ) -> Successors<'_> {
+        slice.iter().copied().chain(option1.into_iter().chain(option2))
+    }
+
     impl SwitchTargets {
         /// Like [`SwitchTargets::target_for_value`], but returning the same type as
         /// [`Terminator::successors`].
         #[inline]
-        #[define_opaque(Successors)]
         pub fn successors_for_value(&self, value: u128) -> Successors<'_> {
             let target = self.target_for_value(value);
-            (&[]).into_iter().copied().chain(Some(target)).chain(None)
+            mk_successors(&[], Some(target), None)
         }
     }
 
     impl<'tcx> TerminatorKind<'tcx> {
         #[inline]
-        #[define_opaque(Successors)]
         pub fn successors(&self) -> Successors<'_> {
             use self::TerminatorKind::*;
             match *self {
                 // 3-successors for async drop: target, unwind, dropline (parent coroutine drop)
                 Drop { target: ref t, unwind: UnwindAction::Cleanup(u), drop: Some(d), .. } => {
-                    slice::from_ref(t).into_iter().copied().chain(Some(u)).chain(Some(d))
+                    mk_successors(slice::from_ref(t), Some(u), Some(d))
                 }
                 // 2-successors
                 Call { target: Some(ref t), unwind: UnwindAction::Cleanup(u), .. }
@@ -533,7 +540,7 @@ mod helper {
                 | Drop { target: ref t, unwind: _, drop: Some(u), .. }
                 | Assert { target: ref t, unwind: UnwindAction::Cleanup(u), .. }
                 | FalseUnwind { real_target: ref t, unwind: UnwindAction::Cleanup(u) } => {
-                    slice::from_ref(t).into_iter().copied().chain(Some(u)).chain(None)
+                    mk_successors(slice::from_ref(t), Some(u), None)
                 }
                 // single successor
                 Goto { target: ref t }
@@ -543,7 +550,7 @@ mod helper {
                 | Drop { target: ref t, unwind: _, .. }
                 | Assert { target: ref t, unwind: _, .. }
                 | FalseUnwind { real_target: ref t, unwind: _ } => {
-                    slice::from_ref(t).into_iter().copied().chain(None).chain(None)
+                    mk_successors(slice::from_ref(t), None, None)
                 }
                 // No successors
                 UnwindResume
@@ -552,25 +559,17 @@ mod helper {
                 | Return
                 | Unreachable
                 | TailCall { .. }
-                | Call { target: None, unwind: _, .. } => {
-                    (&[]).into_iter().copied().chain(None).chain(None)
-                }
+                | Call { target: None, unwind: _, .. } => mk_successors(&[], None, None),
                 // Multiple successors
                 InlineAsm { ref targets, unwind: UnwindAction::Cleanup(u), .. } => {
-                    targets.iter().copied().chain(Some(u)).chain(None)
-                }
-                InlineAsm { ref targets, unwind: _, .. } => {
-                    targets.iter().copied().chain(None).chain(None)
-                }
-                SwitchInt { ref targets, .. } => {
-                    targets.targets.iter().copied().chain(None).chain(None)
+                    mk_successors(targets, Some(u), None)
                 }
+                InlineAsm { ref targets, unwind: _, .. } => mk_successors(targets, None, None),
+                SwitchInt { ref targets, .. } => mk_successors(&targets.targets, None, None),
                 // FalseEdge
-                FalseEdge { ref real_target, imaginary_target } => slice::from_ref(real_target)
-                    .into_iter()
-                    .copied()
-                    .chain(Some(imaginary_target))
-                    .chain(None),
+                FalseEdge { ref real_target, imaginary_target } => {
+                    mk_successors(slice::from_ref(real_target), Some(imaginary_target), None)
+                }
             }
         }