Use liballoc's specialised in-place vec collection

liballoc already specialises in-place vector collection, so manually
reimplementing it in `IdFunctor::try_map_id` was superfluous.

Author: eggyal

compiler/rustc_data_structures/src/functor.rs

@@ -34,43 +34,11 @@ impl<T> IdFunctor for Vec<T> {
     type Inner = T;
 
     #[inline]
-    fn try_map_id<F, E>(self, mut f: F) -> Result<Self, E>
+    fn try_map_id<F, E>(self, f: F) -> Result<Self, E>
     where
         F: FnMut(Self::Inner) -> Result<Self::Inner, E>,
     {
-        struct HoleVec<T> {
-            vec: Vec<mem::ManuallyDrop<T>>,
-            hole: Option<usize>,
-        }
-
-        impl<T> Drop for HoleVec<T> {
-            fn drop(&mut self) {
-                unsafe {
-                    for (index, slot) in self.vec.iter_mut().enumerate() {
-                        if self.hole != Some(index) {
-                            mem::ManuallyDrop::drop(slot);
-                        }
-                    }
-                }
-            }
-        }
-
-        unsafe {
-            let (ptr, length, capacity) = self.into_raw_parts();
-            let vec = Vec::from_raw_parts(ptr.cast(), length, capacity);
-            let mut hole_vec = HoleVec { vec, hole: None };
-
-            for (index, slot) in hole_vec.vec.iter_mut().enumerate() {
-                hole_vec.hole = Some(index);
-                let original = mem::ManuallyDrop::take(slot);
-                let mapped = f(original)?;
-                *slot = mem::ManuallyDrop::new(mapped);
-                hole_vec.hole = None;
-            }
-
-            mem::forget(hole_vec);
-            Ok(Vec::from_raw_parts(ptr, length, capacity))
-        }
+        self.into_iter().map(f).collect()
     }
 }