Add ExtractIf for SmallVec

Author: GnomedDev

compiler/rustc_data_structures/src/lib.rs

@@ -66,6 +66,7 @@ pub mod packed;
 pub mod profiling;
 pub mod sharded;
 pub mod small_c_str;
+pub mod smallvec;
 pub mod snapshot_map;
 pub mod sorted_map;
 pub mod sso;

compiler/rustc_data_structures/src/smallvec.rs

@@ -0,0 +1,101 @@
+//! A version of [`std::vec::ExtractIf`] for `SmallVec`.
+//!
+//! Remove once https://github.com/servo/rust-smallvec/issues/360 is solved.
+
+use core::slice;
+use std::ptr;
+
+use smallvec::{Array, SmallVec};
+
+/// An iterator for [`SmallVec`] which uses a closure to determine if an element should be removed.
+#[must_use = "iterators are lazy and do nothing unless consumed"]
+pub struct ExtractIf<'a, A, F>
+where
+    A: Array,
+{
+    vec: &'a mut SmallVec<A>,
+    /// The index of the item that will be inspected by the next call to `next`.
+    idx: usize,
+    /// The number of items that have been drained (removed) thus far.
+    del: usize,
+    /// The original length of `vec` prior to draining.
+    old_len: usize,
+    /// The filter test predicate.
+    pred: F,
+}
+
+impl<'a, A, F> ExtractIf<'a, A, F>
+where
+    A: Array,
+{
+    pub fn new(vec: &'a mut SmallVec<A>, filter: F) -> Self {
+        let old_len = vec.len();
+
+        // Guard against us getting leaked (leak amplification)
+        unsafe {
+            vec.set_len(0);
+        }
+
+        ExtractIf { vec, idx: 0, del: 0, old_len, pred: filter }
+    }
+}
+
+impl<A, F> Iterator for ExtractIf<'_, A, F>
+where
+    A: Array,
+    F: FnMut(&mut A::Item) -> bool,
+{
+    type Item = A::Item;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        unsafe {
+            while self.idx < self.old_len {
+                let i = self.idx;
+                let v = slice::from_raw_parts_mut(self.vec.as_mut_ptr(), self.old_len);
+                let drained = (self.pred)(&mut v[i]);
+                // Update the index *after* the predicate is called. If the index
+                // is updated prior and the predicate panics, the element at this
+                // index would be leaked.
+                self.idx += 1;
+                if drained {
+                    self.del += 1;
+                    return Some(ptr::read(&v[i]));
+                } else if self.del > 0 {
+                    let del = self.del;
+                    let src: *const A::Item = &v[i];
+                    let dst: *mut A::Item = &mut v[i - del];
+                    ptr::copy_nonoverlapping(src, dst, 1);
+                }
+            }
+            None
+        }
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (0, Some(self.old_len - self.idx))
+    }
+}
+
+impl<A, F> Drop for ExtractIf<'_, A, F>
+where
+    A: Array,
+{
+    fn drop(&mut self) {
+        unsafe {
+            if self.idx < self.old_len && self.del > 0 {
+                // This is a pretty messed up state, and there isn't really an
+                // obviously right thing to do. We don't want to keep trying
+                // to execute `pred`, so we just backshift all the unprocessed
+                // elements and tell the vec that they still exist. The backshift
+                // is required to prevent a double-drop of the last successfully
+                // drained item prior to a panic in the predicate.
+                let ptr = self.vec.as_mut_ptr();
+                let src = ptr.add(self.idx);
+                let dst = src.sub(self.del);
+                let tail_len = self.old_len - self.idx;
+                src.copy_to(dst, tail_len);
+            }
+            self.vec.set_len(self.old_len - self.del);
+        }
+    }
+}