Intuitively, the first token of a rewrite rule determines if the rule
applies to a protocol requirement signature 'Self', or a generic
parameter in the top-level generic signature. A rewrite rule can never
take a type starting with the protocol 'Self' to a type starting with a
generic parameter, or vice versa.
Enforce this by defining the notion of a 'domain', which is the set of
protocols to which the first atom in a term applies to. This set can be
empty (if we have a generic parameter atom), or it may contain more than
one element (if we have an associated type atom for a merged associated
type).
Store the protocol's direct associated types separately from the inherited
associated types, since in a couple of places we only need the direct
associated types.
Also, factor out a new ProtocolGraph::compute() method that does all the
steps in the right order.
If a type parameter has a protocol conformance and a concrete type,
we want to map associated types of the conformance to their concrete
type witnesses.
This is implemented as a post-processing pass in the completion
procedure that runs after the equivalence class map has been built.
If we have an equivalence class T => { conforms_to: [ P ], concrete: Foo },
then for each associated type A of P, we generate a new rule:
T.[P:A].[concrete: Foo.A] => T.[P:A] (if Foo.A is concrete)
T.[P:A] => T.(Foo.A) (if Foo.A is abstract)
If this process introduced any new rules, we check for any new
overlaps by re-running Knuth-Bendix completion; this may in turn
introduce new concrete associated type overlaps, and so on.
The overall completion procedure now alternates between Knuth-Bendix
and rebuilding the equivalence class map; the rewrite system is
complete when neither step is able to introduce any new rules.
Protocol atoms map to the 'Self' parameter; GenericParam and Name
atoms map in the obvious way.
An associated type atom `[P1&P1&...&Pn:A]` has one or more protocols
P0...Pn and an identifier 'A'.
We map it back to a AssociatedTypeDecl as follows:
- For each protocol Pn, look for associated types A in Pn itself,
and all protocols that Pn refines.
- For each candidate associated type An in protocol Qn where
Pn refines Qn, get the associated type anchor An' defined in
protocol Qn', where Qn refines Qn'.
- Out of all the candidiate pairs (Qn', An'), pick the one where
the protocol Qn' is the lowest element according to the linear
order defined by TypeDecl::compare().
The associated type An' is then the canonical associated type
representative of the associated type atom `[P0&...&Pn:A]`.
If you have a same-type requirement like 'Self.Foo == Self' inside a
protocol P, we add a rewrite rule:
[P].Foo => [P]
Simplification turns this into
[P:Foo] => [P]
Previously, the order was backwards so we would end up with
[P] => [P:Foo]
Which would mess up the conformance information in the equivalence class map.
Also move a all headers other than RequirementMachine.h there, since
I don't expect they will be used outside of the rewrite system
implementation itself.
