Introduce the DepGraph and DepTracking map abstractions,

along with a README explaining how they are to be used

1 files changed, 37 insertions, 0 deletions

diff --git a/src/librustc_data_structures/veccell/mod.rs b/src/librustc_data_structures/veccell/mod.rs
new file mode 100644
index 00000000000..1842d0a4958
--- /dev/null
+++ b/src/librustc_data_structures/veccell/mod.rs
@@ -0,0 +1,37 @@
+use std::cell::UnsafeCell;
+use std::mem;
+
+pub struct VecCell<T> {
+    data: UnsafeCell<Vec<T>>
+}
+
+impl<T> VecCell<T> {
+    pub fn with_capacity(capacity: usize) -> VecCell<T>{
+        VecCell { data: UnsafeCell::new(Vec::with_capacity(capacity)) }
+    }
+
+    #[inline]
+    pub fn push(&self, data: T) -> usize {
+        // The logic here, and in `swap` below, is that the `push`
+        // method on the vector will not recursively access this
+        // `VecCell`. Therefore, we can temporarily obtain mutable
+        // access, secure in the knowledge that even if aliases exist
+        // -- indeed, even if aliases are reachable from within the
+        // vector -- they will not be used for the duration of this
+        // particular fn call. (Note that we also are relying on the
+        // fact that `VecCell` is not `Sync`.)
+        unsafe {
+            let v = self.data.get();
+            (*v).push(data);
+            (*v).len()
+        }
+    }
+
+    pub fn swap(&self, mut data: Vec<T>) -> Vec<T> {
+        unsafe {
+            let v = self.data.get();
+            mem::swap(&mut *v, &mut data);
+        }
+        data
+    }
+}