1 files changed, 77 insertions, 0 deletions
diff --git a/compiler/rustc_ast_pretty/src/pp/ring.rs b/compiler/rustc_ast_pretty/src/pp/ring.rs
new file mode 100644
index 000000000..8187394fe
--- /dev/null
+++ b/compiler/rustc_ast_pretty/src/pp/ring.rs
@@ -0,0 +1,77 @@
+use std::collections::VecDeque;
+use std::ops::{Index, IndexMut};
+
+/// A view onto a finite range of an infinitely long sequence of T.
+///
+/// The Ts are indexed 0..infinity. A RingBuffer begins as a view of elements
+/// 0..0 (i.e. nothing). The user of the RingBuffer advances its left and right
+/// position independently, although only in the positive direction, and only
+/// with left <= right at all times.
+///
+/// Holding a RingBuffer whose view is elements left..right gives the ability to
+/// use Index and IndexMut to access elements i in the infinitely long queue for
+/// which left <= i < right.
+pub struct RingBuffer<T> {
+    data: VecDeque<T>,
+    // Abstract index of data[0] in the infinitely sized queue.
+    offset: usize,
+}
+
+impl<T> RingBuffer<T> {
+    pub fn new() -> Self {
+        RingBuffer { data: VecDeque::new(), offset: 0 }
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.data.is_empty()
+    }
+
+    pub fn push(&mut self, value: T) -> usize {
+        let index = self.offset + self.data.len();
+        self.data.push_back(value);
+        index
+    }
+
+    pub fn clear(&mut self) {
+        self.data.clear();
+    }
+
+    pub fn index_of_first(&self) -> usize {
+        self.offset
+    }
+
+    pub fn first(&self) -> Option<&T> {
+        self.data.front()
+    }
+
+    pub fn first_mut(&mut self) -> Option<&mut T> {
+        self.data.front_mut()
+    }
+
+    pub fn pop_first(&mut self) -> Option<T> {
+        let first = self.data.pop_front()?;
+        self.offset += 1;
+        Some(first)
+    }
+
+    pub fn last(&self) -> Option<&T> {
+        self.data.back()
+    }
+
+    pub fn last_mut(&mut self) -> Option<&mut T> {
+        self.data.back_mut()
+    }
+}
+
+impl<T> Index<usize> for RingBuffer<T> {
+    type Output = T;
+    fn index(&self, index: usize) -> &Self::Output {
+        &self.data[index.checked_sub(self.offset).unwrap()]
+    }
+}
+
+impl<T> IndexMut<usize> for RingBuffer<T> {
+    fn index_mut(&mut self, index: usize) -> &mut Self::Output {
+        &mut self.data[index.checked_sub(self.offset).unwrap()]
+    }
+}