vendor/salsa/src/durability.rs

   1 /// Describes how likely a value is to change -- how "durable" it is.
   2 /// By default, inputs have `Durability::LOW` and interned values have
   3 /// `Durability::HIGH`. But inputs can be explicitly set with other
   4 /// durabilities.
   5 ///
   6 /// We use durabilities to optimize the work of "revalidating" a query
   7 /// after some input has changed. Ordinarily, in a new revision,
   8 /// queries have to trace all their inputs back to the base inputs to
   9 /// determine if any of those inputs have changed. But if we know that
  10 /// the only changes were to inputs of low durability (the common
  11 /// case), and we know that the query only used inputs of medium
  12 /// durability or higher, then we can skip that enumeration.
  13 ///
  14 /// Typically, one assigns low durabilites to inputs that the user is
  15 /// frequently editing. Medium or high durabilities are used for
  16 /// configuration, the source from library crates, or other things
  17 /// that are unlikely to be edited.
  18 #[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
  19 pub struct Durability(u8);
  20
  21 impl Durability {
  22     /// Low durability: things that change frequently.
  23     ///
  24     /// Example: part of the crate being edited
  25     pub const LOW: Durability = Durability(0);
  26
  27     /// Medium durability: things that change sometimes, but rarely.
  28     ///
  29     /// Example: a Cargo.toml file
  30     pub const MEDIUM: Durability = Durability(1);
  31
  32     /// High durability: things that are not expected to change under
  33     /// common usage.
  34     ///
  35     /// Example: the standard library or something from crates.io
  36     pub const HIGH: Durability = Durability(2);
  37
  38     /// The maximum possible durability; equivalent to HIGH but
  39     /// "conceptually" distinct (i.e., if we add more durability
  40     /// levels, this could change).
  41     pub(crate) const MAX: Durability = Self::HIGH;
  42
  43     /// Number of durability levels.
  44     pub(crate) const LEN: usize = 3;
  45
  46     pub(crate) fn index(self) -> usize {
  47         self.0 as usize
  48     }
  49 }