swift234

Represent all type variables within the constraint graph without simplification.

Previously, the constraint graph only represented type variables that
were both unbound and were the representatives within their respective
equivalence classes. To achieve this, each constraint was fully
simplified when it was added to the graph, which is a fairly expensive
process. This representation made certain operations---merging two type
variables, replacing a type variable with a fixed type, etc---both
hard to implement and hard to reverse, forcing us to rebuild the
constraint graph each time.

Now, add all type variables to the graph (including those with fixed
type bindings and non-representatives) and add constraints without
simplification. Separately track the equivalence classes of each type
variable (in the representative's node) and adjacencies due to type
variables showing up in the fixed type bindings of other type
variables. Although not yet implemented, the merging and type variable
replacement operations are far easier to implement (and more
efficient) with this representation, and are also easier to undo,
making this a step toward creating and updating a single consistent,
global constraint graph rather than re-creating a constraint graph
during each solver step.

Performance-wise, this is a 4% regression when type-checking the
standard library. I expect to make that up easily once we switch to a
single constraint graph.




Swift SVN r10897