Move salsa fork in-tree

crates/salsa/src/revision.rs

@@ -0,0 +1,70 @@
+use std::num::NonZeroU32;
+use std::sync::atomic::{AtomicU32, Ordering};
+
+/// Value of the initial revision, as a u32. We don't use 0
+/// because we want to use a `NonZeroU32`.
+const START: u32 = 1;
+
+/// A unique identifier for the current version of the database; each
+/// time an input is changed, the revision number is incremented.
+/// `Revision` is used internally to track which values may need to be
+/// recomputed, but is not something you should have to interact with
+/// directly as a user of salsa.
+#[derive(Copy, Clone, PartialEq, Eq, Hash, PartialOrd, Ord)]
+pub struct Revision {
+    generation: NonZeroU32,
+}
+
+impl Revision {
+    pub(crate) fn start() -> Self {
+        Self::from(START)
+    }
+
+    pub(crate) fn from(g: u32) -> Self {
+        Self {
+            generation: NonZeroU32::new(g).unwrap(),
+        }
+    }
+
+    pub(crate) fn next(self) -> Revision {
+        Self::from(self.generation.get() + 1)
+    }
+
+    fn as_u32(self) -> u32 {
+        self.generation.get()
+    }
+}
+
+impl std::fmt::Debug for Revision {
+    fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(fmt, "R{}", self.generation)
+    }
+}
+
+#[derive(Debug)]
+pub(crate) struct AtomicRevision {
+    data: AtomicU32,
+}
+
+impl AtomicRevision {
+    pub(crate) fn start() -> Self {
+        Self {
+            data: AtomicU32::new(START),
+        }
+    }
+
+    pub(crate) fn load(&self) -> Revision {
+        Revision::from(self.data.load(Ordering::SeqCst))
+    }
+
+    pub(crate) fn store(&self, r: Revision) {
+        self.data.store(r.as_u32(), Ordering::SeqCst);
+    }
+
+    /// Increment by 1, returning previous value.
+    pub(crate) fn fetch_then_increment(&self) -> Revision {
+        let v = self.data.fetch_add(1, Ordering::SeqCst);
+        assert!(v != u32::max_value(), "revision overflow");
+        Revision::from(v)
+    }
+}