rustc_mir_dataflow

Module lattice

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Expand description

Traits used to represent lattices for use as the domain of a dataflow analysis.

§Overview

The most common lattice is a powerset of some set S, ordered by set inclusion. The Hasse diagram for the powerset of a set with two elements (X and Y) is shown below. Note that distinct elements at the same height in a Hasse diagram (e.g. {X} and {Y}) are incomparable, not equal.

     {X, Y}    <- top
      /  \
   {X}    {Y}
      \  /
       {}      <- bottom

The defining characteristic of a lattice—the one that differentiates it from a partially ordered set—is the existence of a unique least upper and greatest lower bound for every pair of elements. The lattice join operator (∨) returns the least upper bound, and the lattice meet operator (∧) returns the greatest lower bound. Types that implement one operator but not the other are known as semilattices. Dataflow analysis only uses the join operator and will work with any join-semilattice, but both should be specified when possible.

§PartialOrd

Given that it represents a partially ordered set, you may be surprised that JoinSemiLattice does not have PartialOrd as a supertrait. This is because most standard library types use lexicographic ordering instead of set inclusion for their PartialOrd impl. Since we do not actually need to compare lattice elements to run a dataflow analysis, there’s no need for a newtype wrapper with a custom PartialOrd impl. The only benefit would be the ability to check that the least upper (or greatest lower) bound returned by the lattice join (or meet) operator was in fact greater (or lower) than the inputs.

Enums§

  • FlatSet
    Extends a type T with top and bottom elements to make it a partially ordered set in which no value of T is comparable with any other.
  • MaybeReachable
    Extend a lattice with a bottom value to represent an unreachable execution.

Traits§