Make the semantics of Vec::truncate(N) consistent with slices.

src/liballoc/vec.rs

@@ -685,25 +685,20 @@ impl<T> Vec<T> {
     /// [`drain`]: #method.drain
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn truncate(&mut self, len: usize) {
-        if mem::needs_drop::<T>() {
-            let current_len = self.len;
-            unsafe {
-                let mut ptr = self.as_mut_ptr().add(self.len);
-                // Set the final length at the end, keeping in mind that
-                // dropping an element might panic. Works around a missed
-                // optimization, as seen in the following issue:
-                // https://github.com/rust-lang/rust/issues/51802
-                let mut local_len = SetLenOnDrop::new(&mut self.len);
-
-                // drop any extra elements
-                for _ in len..current_len {
-                    local_len.decrement_len(1);
-                    ptr = ptr.offset(-1);
-                    ptr::drop_in_place(ptr);
-                }
+        // This is safe because:
+        //
+        // * the slice passed to `drop_in_place` is valid; the `len > self.len`
+        //   case avoids creating an invalid slice, and
+        // * the `len` of the vector is shrunk before calling `drop_in_place`,
+        //   such that no value will be dropped twice in case `drop_in_place`
+        //   were to panic once (if it panics twice, the program aborts).
+        unsafe {
+            if len > self.len {
+                return;
             }
-        } else if len <= self.len {
+            let s = self.get_unchecked_mut(len..) as *mut _;
             self.len = len;
+            ptr::drop_in_place(s);
         }
     }
 
@@ -1590,11 +1585,6 @@ impl<'a> SetLenOnDrop<'a> {
     fn increment_len(&mut self, increment: usize) {
         self.local_len += increment;
     }
-
-    #[inline]
-    fn decrement_len(&mut self, decrement: usize) {
-        self.local_len -= decrement;
-    }
 }
 
 impl Drop for SetLenOnDrop<'_> {