Auto merge of #13797 - bholley:atomic_refcell_map, r=SimonSapin

Implement AtomicRef{,Mut}::map

I was originally bundling this with #13777 but am splitting it out since that's been deprioritized.

r? @SimonSapin

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components/style/atomic_refcell.rs

@@ -2,7 +2,11 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
+#![allow(unsafe_code)]
+
+use owning_ref::{OwningRef, StableAddress};
 use parking_lot::{RwLock, RwLockReadGuard, RwLockWriteGuard};
+use std::ops::{Deref, DerefMut};
 
 /// Container type providing RefCell-like semantics for objects shared across
 /// threads.
@@ -20,17 +24,41 @@ use parking_lot::{RwLock, RwLockReadGuard, RwLockWriteGuard};
 /// RefCell, so we newtype it with some API sugar.
 pub struct AtomicRefCell<T>(RwLock<T>);
 
-pub type AtomicRef<'a, T> = RwLockReadGuard<'a, T>;
-pub type AtomicRefMut<'a, T> = RwLockWriteGuard<'a, T>;
+pub struct AtomicRef<'a, T: 'a>(RwLockReadGuard<'a, T>);
+unsafe impl<'a, T> StableAddress for AtomicRef<'a, T> {}
+
+impl<'a, T> Deref for AtomicRef<'a, T> {
+    type Target = T;
+    fn deref(&self) -> &T {
+        self.0.deref()
+    }
+}
+
+pub struct AtomicRefMut<'a, T: 'a>(RwLockWriteGuard<'a, T>);
+unsafe impl<'a, T> StableAddress for AtomicRefMut<'a, T> {}
+
+impl<'a, T> Deref for AtomicRefMut<'a, T> {
+    type Target = T;
+    fn deref(&self) -> &T {
+        self.0.deref()
+    }
+}
+
+impl<'a, T> DerefMut for AtomicRefMut<'a, T> {
+    fn deref_mut(&mut self) -> &mut T {
+        self.0.deref_mut()
+    }
+}
+
 impl<T> AtomicRefCell<T> {
     pub fn new(value: T) -> Self {
         AtomicRefCell(RwLock::new(value))
     }
     pub fn borrow(&self) -> AtomicRef<T> {
-        self.0.try_read().expect("already mutably borrowed")
+        AtomicRef(self.0.try_read().expect("already mutably borrowed"))
     }
     pub fn borrow_mut(&self) -> AtomicRefMut<T> {
-        self.0.try_write().expect("already borrowed")
+        AtomicRefMut(self.0.try_write().expect("already borrowed"))
     }
 }
 
@@ -39,3 +67,42 @@ impl<T: Default> Default for AtomicRefCell<T> {
         Self::new(T::default())
     }
 }
+
+/*
+ * Implement Ref{,Mut}::map()-like semantics for AtomicRef{,Mut}. We can't quite
+ * use AtomicRef{,Mut} as the mapped type, but we can use some trait tricks to
+ * allow us to pass MappedAtomicRef{,Mut} back into AtomicRef{,Mut}::map().
+ *
+ * Note: We cannot implement an AtomicRefMut::map() method until we have mutable
+ * OwningRef. See https://github.com/Kimundi/owning-ref-rs/pull/16
+ */
+pub type MappedAtomicRef<'a, T: 'a, U: 'a> = OwningRef<AtomicRef<'a, T>, U>;
+
+pub trait Map<'a, Base, Curr> {
+    fn map<New, F>(self, f: F) -> MappedAtomicRef<'a, Base, New>
+        where F: FnOnce(&Curr) -> &New;
+}
+
+impl<'a, Base> Map<'a, Base, Base> for AtomicRef<'a, Base> {
+    fn map<New, F>(self, f: F) -> MappedAtomicRef<'a, Base, New>
+        where F: FnOnce(&Base) -> &New
+    {
+        OwningRef::new(self).map(f)
+    }
+}
+
+impl<'a, Base, Curr> Map<'a, Base, Curr> for MappedAtomicRef<'a, Base, Curr> {
+    fn map<New, F>(self, f: F) -> MappedAtomicRef<'a, Base, New>
+        where F: FnOnce(&Curr) -> &New
+    {
+        self.map(f)
+    }
+}
+
+impl<'a, Base> AtomicRef<'a, Base> {
+    pub fn map<Curr, New, M, F>(orig: M, f: F) -> MappedAtomicRef<'a, Base, New>
+        where F: FnOnce(&Curr) -> &New, M: Map<'a, Base, Curr>
+    {
+        orig.map(f)
+    }
+}

tests/unit/style/atomic_refcell.rs

@@ -0,0 +1,41 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use style::atomic_refcell::{AtomicRef, AtomicRefCell};
+
+struct Foo {
+    u: u32,
+}
+
+struct Bar {
+    f: Foo,
+}
+
+#[test]
+fn map() {
+    let a = AtomicRefCell::new(Bar { f: Foo { u: 42 } });
+    let b = a.borrow();
+    assert_eq!(b.f.u, 42);
+    let c = AtomicRef::map(b, |x| &x.f);
+    assert_eq!(c.u, 42);
+    let d = AtomicRef::map(c, |x| &x.u);
+    assert_eq!(*d, 42);
+}
+
+/* FIXME(bholley): Enable once we have AtomicRefMut::map(), which is blocked on
+ * https://github.com/Kimundi/owning-ref-rs/pull/16
+#[test]
+fn map_mut() {
+    let a = AtomicRefCell::new(Bar { f: Foo { u: 42 } });
+    let mut b = a.borrow_mut();
+    assert_eq!(b.f.u, 42);
+    b.f.u = 43;
+    let mut c = AtomicRefMut::map(b, |x| &x.f);
+    assert_eq!(c.u, 43);
+    c.u = 44;
+    let mut d = AtomicRefMut::map(c, |x| &x.u);
+    assert_eq!(*d, 44);
+    *d. = 45;
+    assert_eq!(*d, 45);
+}*/

tests/unit/style/lib.rs

@@ -19,6 +19,7 @@ extern crate style_traits;
 extern crate url;
 extern crate util;
 
+mod atomic_refcell;
 mod attr;
 mod cache;
 mod logical_geometry;