This module defines the DepNode type which the compiler uses to represent
nodes in the dependency graph. A DepNode consists of a DepKind (which
specifies the kind of thing it represents, like a piece of HIR, MIR, etc)
and a Fingerprint, a 128 bit hash value the exact meaning of which
depends on the node’s DepKind. Together, the kind and the fingerprint
fully identify a dependency node, even across multiple compilation sessions.
In other words, the value of the fingerprint does not depend on anything
that is specific to a given compilation session, like an unpredictable
interning key (e.g., NodeId, DefId, Symbol) or the numeric value of a
pointer. The concept behind this could be compared to how git commit hashes
uniquely identify a given commit and has a few advantages:
A “work product” is an intermediate result that we save into the
incremental directory for later re-use. The primary example are
the object files that we save for each partition at code
generation time.