Move salsa fork in-tree

crates/salsa/src/durability.rs

@@ -0,0 +1,49 @@
+/// Describes how likely a value is to change -- how "durable" it is.
+/// By default, inputs have `Durability::LOW` and interned values have
+/// `Durability::HIGH`. But inputs can be explicitly set with other
+/// durabilities.
+///
+/// We use durabilities to optimize the work of "revalidating" a query
+/// after some input has changed. Ordinarily, in a new revision,
+/// queries have to trace all their inputs back to the base inputs to
+/// determine if any of those inputs have changed. But if we know that
+/// the only changes were to inputs of low durability (the common
+/// case), and we know that the query only used inputs of medium
+/// durability or higher, then we can skip that enumeration.
+///
+/// Typically, one assigns low durabilites to inputs that the user is
+/// frequently editing. Medium or high durabilities are used for
+/// configuration, the source from library crates, or other things
+/// that are unlikely to be edited.
+#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
+pub struct Durability(u8);
+
+impl Durability {
+    /// Low durability: things that change frequently.
+    ///
+    /// Example: part of the crate being edited
+    pub const LOW: Durability = Durability(0);
+
+    /// Medium durability: things that change sometimes, but rarely.
+    ///
+    /// Example: a Cargo.toml file
+    pub const MEDIUM: Durability = Durability(1);
+
+    /// High durability: things that are not expected to change under
+    /// common usage.
+    ///
+    /// Example: the standard library or something from crates.io
+    pub const HIGH: Durability = Durability(2);
+
+    /// The maximum possible durability; equivalent to HIGH but
+    /// "conceptually" distinct (i.e., if we add more durability
+    /// levels, this could change).
+    pub(crate) const MAX: Durability = Self::HIGH;
+
+    /// Number of durability levels.
+    pub(crate) const LEN: usize = 3;
+
+    pub(crate) fn index(self) -> usize {
+        self.0 as usize
+    }
+}