Vec::dedup optimization - panic gracefully

library/alloc/src/vec/mod.rs

@@ -1523,43 +1523,92 @@ pub fn dedup_by<F>(&mut self, mut same_bucket: F)
             return;
         }
 
-        let ptr = self.as_mut_ptr();
-        /* Offset of the element we want to check if it is duplicate */
-        let mut read: usize = 1;
-        /* Offset of the place where we want to place the non-duplicate
-         * when we find it. */
-        let mut write: usize = 1;
+        /* INVARIANT: vec.len() > read >= write > write-1 >= 0 */
+        struct FillGapOnDrop<'a, T, A: core::alloc::Allocator> {
+            /* Offset of the element we want to check if it is duplicate */
+            read: usize,
+
+            /* Offset of the place where we want to place the non-duplicate
+             * when we find it. */
+            write: usize,
+
+            /* The Vec that would need correction if `same_bucket` panicked */
+            vec: &'a mut Vec<T, A>,
+        }
+
+        impl<'a, T, A: core::alloc::Allocator> Drop for FillGapOnDrop<'a, T, A> {
+            fn drop(&mut self) {
+                /* This code gets executed either at the end of `dedup_by` or
+                 * when `same_bucket` panics */
+
+                /* SAFETY (if finishing successfully): self.read == len, so
+                 * no data is copied and length is set correctly */
+
+                /* SAFETY (if panicing): invariant guarantees that `read - write`
+                 * and `len - read` never overflow and that the copy is always
+                 * in-bounds. */
+                unsafe {
+                    let ptr = self.vec.as_mut_ptr();
+                    let len = self.vec.len();
+
+                    /* How many items were left when `same_bucket` paniced.
+                     * Basically vec[read..].len() */
+                    let items_left = len - self.read;
+
+                    /* Pointer to first item in vec[write..write+items_left] slice */
+                    let dropped_ptr = ptr.add(self.write);
+                    /* Pointer to first item in vec[read..] slice */
+                    let valid_ptr = ptr.add(self.read);
+
+                    /* Copy `vec[read..]` to `vec[write..write+items_left]`.
+                     * The slices can overlap, so `copy_nonoverlapping` cannot be used */
+                    ptr::copy(valid_ptr, dropped_ptr, items_left);
+
+                    /* How many items have been already dropped
+                     * Basically vec[read..write].len() */
+                    let dropped = self.read - self.write;
+
+                    self.vec.set_len(len - dropped);
+                }
+            }
+        }
+
+        let mut gap = FillGapOnDrop { read: 1, write: 1, vec: self };
+
+        let ptr = gap.vec.as_mut_ptr();
 
         /* Drop items while going through Vec, it should be more efficient than
          * doing slice partition_dedup + truncate */
 
-        /* INVARIANT: len > read >= write > write-1 >= 0
-         * SAFETY: Because of the invariant, read_ptr, prev_ptr and write_ptr
+        /* SAFETY: Because of the invariant, read_ptr, prev_ptr and write_ptr
          * are always in-bounds and read_ptr never aliases prev_ptr */
         unsafe {
-            while read < len {
-                let read_ptr = ptr.add(read);
-                let prev_ptr = ptr.add(write.wrapping_sub(1));
+            while gap.read < len {
+                let read_ptr = ptr.add(gap.read);
+                let prev_ptr = ptr.add(gap.write.wrapping_sub(1));
 
                 if same_bucket(&mut *read_ptr, &mut *prev_ptr) {
                     /* We have found duplicate, drop it in-place */
                     ptr::drop_in_place(read_ptr);
                 } else {
-                    let write_ptr = ptr.add(write);
+                    let write_ptr = ptr.add(gap.write);
 
                     /* Looks like doing just `copy` can be faster than
                      * conditional `copy_nonoverlapping` */
                     ptr::copy(read_ptr, write_ptr, 1);
 
                     /* We have filled that place, so go further */
-                    write += 1;
+                    gap.write += 1;
                 }
 
-                read += 1;
+                gap.read += 1;
             }
 
-            /* `write` items are inside vec, rest is already dropped */
-            self.set_len(write);
+            /* Technically we could let `gap` clean up with its Drop, but
+             * when `same_bucket` is guaranteed to not panic, this bloats a little
+             * the codegen, so we just do it manually */
+            gap.vec.set_len(gap.write);
+            mem::forget(gap);
         }
     }