1 files changed, 219 insertions, 0 deletions
diff --git a/src/librustc_ast/ptr.rs b/src/librustc_ast/ptr.rs
new file mode 100644
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+++ b/src/librustc_ast/ptr.rs
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+//! The AST pointer.
+//!
+//! Provides `P<T>`, a frozen owned smart pointer.
+//!
+//! # Motivations and benefits
+//!
+//! * **Identity**: sharing AST nodes is problematic for the various analysis
+//!   passes (e.g., one may be able to bypass the borrow checker with a shared
+//!   `ExprKind::AddrOf` node taking a mutable borrow).
+//!
+//! * **Immutability**: `P<T>` disallows mutating its inner `T`, unlike `Box<T>`
+//!   (unless it contains an `Unsafe` interior, but that may be denied later).
+//!   This mainly prevents mistakes, but can also enforces a kind of "purity".
+//!
+//! * **Efficiency**: folding can reuse allocation space for `P<T>` and `Vec<T>`,
+//!   the latter even when the input and output types differ (as it would be the
+//!   case with arenas or a GADT AST using type parameters to toggle features).
+//!
+//! * **Maintainability**: `P<T>` provides a fixed interface - `Deref`,
+//!   `and_then` and `map` - which can remain fully functional even if the
+//!   implementation changes (using a special thread-local heap, for example).
+//!   Moreover, a switch to, e.g., `P<'a, T>` would be easy and mostly automated.
+
+use std::fmt::{self, Debug, Display};
+use std::iter::FromIterator;
+use std::ops::{Deref, DerefMut};
+use std::{slice, vec};
+
+use rustc_serialize::{Decodable, Decoder, Encodable, Encoder};
+
+use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
+/// An owned smart pointer.
+pub struct P<T: ?Sized> {
+    ptr: Box<T>,
+}
+
+/// Construct a `P<T>` from a `T` value.
+#[allow(non_snake_case)]
+pub fn P<T: 'static>(value: T) -> P<T> {
+    P { ptr: box value }
+}
+
+impl<T: 'static> P<T> {
+    /// Move out of the pointer.
+    /// Intended for chaining transformations not covered by `map`.
+    pub fn and_then<U, F>(self, f: F) -> U
+    where
+        F: FnOnce(T) -> U,
+    {
+        f(*self.ptr)
+    }
+
+    /// Equivalent to `and_then(|x| x)`.
+    pub fn into_inner(self) -> T {
+        *self.ptr
+    }
+
+    /// Produce a new `P<T>` from `self` without reallocating.
+    pub fn map<F>(mut self, f: F) -> P<T>
+    where
+        F: FnOnce(T) -> T,
+    {
+        let x = f(*self.ptr);
+        *self.ptr = x;
+
+        self
+    }
+
+    /// Optionally produce a new `P<T>` from `self` without reallocating.
+    pub fn filter_map<F>(mut self, f: F) -> Option<P<T>>
+    where
+        F: FnOnce(T) -> Option<T>,
+    {
+        *self.ptr = f(*self.ptr)?;
+        Some(self)
+    }
+}
+
+impl<T: ?Sized> Deref for P<T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        &self.ptr
+    }
+}
+
+impl<T: ?Sized> DerefMut for P<T> {
+    fn deref_mut(&mut self) -> &mut T {
+        &mut self.ptr
+    }
+}
+
+impl<T: 'static + Clone> Clone for P<T> {
+    fn clone(&self) -> P<T> {
+        P((**self).clone())
+    }
+}
+
+impl<T: ?Sized + Debug> Debug for P<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        Debug::fmt(&self.ptr, f)
+    }
+}
+
+impl<T: Display> Display for P<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        Display::fmt(&**self, f)
+    }
+}
+
+impl<T> fmt::Pointer for P<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Pointer::fmt(&self.ptr, f)
+    }
+}
+
+impl<T: 'static + Decodable> Decodable for P<T> {
+    fn decode<D: Decoder>(d: &mut D) -> Result<P<T>, D::Error> {
+        Decodable::decode(d).map(P)
+    }
+}
+
+impl<T: Encodable> Encodable for P<T> {
+    fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
+        (**self).encode(s)
+    }
+}
+
+impl<T> P<[T]> {
+    pub const fn new() -> P<[T]> {
+        // HACK(eddyb) bypass the lack of a `const fn` to create an empty `Box<[T]>`
+        // (as trait methods, `default` in this case, can't be `const fn` yet).
+        P {
+            ptr: unsafe {
+                use std::ptr::NonNull;
+                std::mem::transmute(NonNull::<[T; 0]>::dangling() as NonNull<[T]>)
+            },
+        }
+    }
+
+    #[inline(never)]
+    pub fn from_vec(v: Vec<T>) -> P<[T]> {
+        P { ptr: v.into_boxed_slice() }
+    }
+
+    #[inline(never)]
+    pub fn into_vec(self) -> Vec<T> {
+        self.ptr.into_vec()
+    }
+}
+
+impl<T> Default for P<[T]> {
+    /// Creates an empty `P<[T]>`.
+    fn default() -> P<[T]> {
+        P::new()
+    }
+}
+
+impl<T: Clone> Clone for P<[T]> {
+    fn clone(&self) -> P<[T]> {
+        P::from_vec(self.to_vec())
+    }
+}
+
+impl<T> From<Vec<T>> for P<[T]> {
+    fn from(v: Vec<T>) -> Self {
+        P::from_vec(v)
+    }
+}
+
+impl<T> Into<Vec<T>> for P<[T]> {
+    fn into(self) -> Vec<T> {
+        self.into_vec()
+    }
+}
+
+impl<T> FromIterator<T> for P<[T]> {
+    fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> P<[T]> {
+        P::from_vec(iter.into_iter().collect())
+    }
+}
+
+impl<T> IntoIterator for P<[T]> {
+    type Item = T;
+    type IntoIter = vec::IntoIter<T>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.into_vec().into_iter()
+    }
+}
+
+impl<'a, T> IntoIterator for &'a P<[T]> {
+    type Item = &'a T;
+    type IntoIter = slice::Iter<'a, T>;
+    fn into_iter(self) -> Self::IntoIter {
+        self.ptr.into_iter()
+    }
+}
+
+impl<T: Encodable> Encodable for P<[T]> {
+    fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
+        Encodable::encode(&**self, s)
+    }
+}
+
+impl<T: Decodable> Decodable for P<[T]> {
+    fn decode<D: Decoder>(d: &mut D) -> Result<P<[T]>, D::Error> {
+        Ok(P::from_vec(Decodable::decode(d)?))
+    }
+}
+
+impl<CTX, T> HashStable<CTX> for P<T>
+where
+    T: ?Sized + HashStable<CTX>,
+{
+    fn hash_stable(&self, hcx: &mut CTX, hasher: &mut StableHasher) {
+        (**self).hash_stable(hcx, hasher);
+    }
+}