1 files changed, 108 insertions, 0 deletions
diff --git a/compiler/rustc_data_structures/src/map_in_place.rs b/compiler/rustc_data_structures/src/map_in_place.rs
new file mode 100644
index 000000000..874de03d3
--- /dev/null
+++ b/compiler/rustc_data_structures/src/map_in_place.rs
@@ -0,0 +1,108 @@
+use smallvec::{Array, SmallVec};
+use std::ptr;
+
+pub trait MapInPlace<T>: Sized {
+    fn map_in_place<F>(&mut self, mut f: F)
+    where
+        F: FnMut(T) -> T,
+    {
+        self.flat_map_in_place(|e| Some(f(e)))
+    }
+
+    fn flat_map_in_place<F, I>(&mut self, f: F)
+    where
+        F: FnMut(T) -> I,
+        I: IntoIterator<Item = T>;
+}
+
+impl<T> MapInPlace<T> for Vec<T> {
+    fn flat_map_in_place<F, I>(&mut self, mut f: F)
+    where
+        F: FnMut(T) -> I,
+        I: IntoIterator<Item = T>,
+    {
+        let mut read_i = 0;
+        let mut write_i = 0;
+        unsafe {
+            let mut old_len = self.len();
+            self.set_len(0); // make sure we just leak elements in case of panic
+
+            while read_i < old_len {
+                // move the read_i'th item out of the vector and map it
+                // to an iterator
+                let e = ptr::read(self.as_ptr().add(read_i));
+                let iter = f(e).into_iter();
+                read_i += 1;
+
+                for e in iter {
+                    if write_i < read_i {
+                        ptr::write(self.as_mut_ptr().add(write_i), e);
+                        write_i += 1;
+                    } else {
+                        // If this is reached we ran out of space
+                        // in the middle of the vector.
+                        // However, the vector is in a valid state here,
+                        // so we just do a somewhat inefficient insert.
+                        self.set_len(old_len);
+                        self.insert(write_i, e);
+
+                        old_len = self.len();
+                        self.set_len(0);
+
+                        read_i += 1;
+                        write_i += 1;
+                    }
+                }
+            }
+
+            // write_i tracks the number of actually written new items.
+            self.set_len(write_i);
+        }
+    }
+}
+
+impl<T, A: Array<Item = T>> MapInPlace<T> for SmallVec<A> {
+    fn flat_map_in_place<F, I>(&mut self, mut f: F)
+    where
+        F: FnMut(T) -> I,
+        I: IntoIterator<Item = T>,
+    {
+        let mut read_i = 0;
+        let mut write_i = 0;
+        unsafe {
+            let mut old_len = self.len();
+            self.set_len(0); // make sure we just leak elements in case of panic
+
+            while read_i < old_len {
+                // move the read_i'th item out of the vector and map it
+                // to an iterator
+                let e = ptr::read(self.as_ptr().add(read_i));
+                let iter = f(e).into_iter();
+                read_i += 1;
+
+                for e in iter {
+                    if write_i < read_i {
+                        ptr::write(self.as_mut_ptr().add(write_i), e);
+                        write_i += 1;
+                    } else {
+                        // If this is reached we ran out of space
+                        // in the middle of the vector.
+                        // However, the vector is in a valid state here,
+                        // so we just do a somewhat inefficient insert.
+                        self.set_len(old_len);
+                        self.insert(write_i, e);
+
+                        old_len = self.len();
+                        self.set_len(0);
+
+                        read_i += 1;
+                        write_i += 1;
+                    }
+                }
+            }
+
+            // write_i tracks the number of actually written new items.
+            self.set_len(write_i);
+        }
+    }
+}