Auto merge of #64375 - kornelski:vecdrop, r=<try>

Guarantee vec.clear/truncate is O(1) for trivial types

For trivial types like `u8`, `vec.truncate()`/`vec.clear()` relies on the optimizer to remove the loop. This means more work in debug builds, and more work for the optimizer.

Avoiding this busywork is exactly what `mem::needs_drop::<T>()` is for.

src/liballoc/vec.rs

@@ -685,21 +685,25 @@ impl<T> Vec<T> {
     /// [`drain`]: #method.drain
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn truncate(&mut self, len: usize) {
-        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);
+        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);
+                // drop any extra elements
+                for _ in len..current_len {
+                    local_len.decrement_len(1);
+                    ptr = ptr.offset(-1);
+                    ptr::drop_in_place(ptr);
+                }
             }
+        } else if len < self.len {
+            self.len = len;
         }
     }