I wrote out this whole analysis of why different existential types
might have the same logical content, and then I turned around and
immediately uniqued existential shapes purely by logical content
rather than the (generalized) formal type. Oh well. At least it's
not too late to make ABI changes like this.
We now store a reference to a mangling of the generalized formal
type directly in the shape. This type alone is sufficient to unique
the shape:
- By the nature of the generalization algorithm, every type parameter
in the generalization signature should be mentioned in the
generalized formal type in a deterministic order.
- By the nature of the generalization algorithm, every other
requirement in the generalization signature should be implied
by the positions in which generalization type parameters appear
(e.g. because the formal type is C<T> & P, where C constrains
its type parameter for well-formedness).
- The requirement signature and type expression are extracted from
the existential type.
As a result, we no longer rely on computing a unique hash at
compile time.
Storing this separately from the requirement signature potentially
allows runtimes with general shape support to work with future
extensions to existential types even if they cannot demangle the
generalized formal type.
Storing the generalized formal type also allows us to easily and
reliably extract the formal type of the existential. Otherwise,
it's quite a heroic endeavor to match requirements back up with
primary associated types. Doing so would also only allows us to
extract *some* matching formal type, not necessarily the *right*
formal type. So there's some good synergy here.
Creating a mangle-node tree is annoying, but it's much better
than trying to reproduce the mangling logic exactly.
Also, add support for mangling some existential types. The
specifier for parameterized protocol types has been future-proofed
against the coming change to include the associated type names
in the mangling.
The immediate use case is only concretely-constrained existential
types, which could use a much simpler representation, but I've
future-proofed the representation as much as I can; thus, the
requirement signature can have arbitrary parameters and
requirements, and the type can have an arbitrary type as the
sub-expression. The latter is also necessary for existential
metatypes.
The chief implementation complexity here is that we must be able
to agree on the identity of an existential type that might be
produced by substitution. Thus, for example, `any P<T>` when
`T == Int` must resolve to the same type metadata as
`any P<Int>`. To handle this, we identify the "shape" of the
existential type, consisting of those parts which cannot possibly
be the result of substitution, and then abstract the substitutable
"holes" as an application of a generalization signature. That
algorithm will come in a later patch; this patch just represents
it.
Uniquing existential shapes from the requirements would be quite
complex because of all the symbolic mangled names they use.
This is particularly true because it's not reasonable to require
translation units to agree about what portions they mangle vs.
reference symbolically. Instead, we expect the compiler to do
a cryptographic hash of a mangling of the shape, then use that
as the unique key identifying the shape.
This is just the core representation and runtime interface; other
parts of the runtime, such as dynamic casting and demangling
support, will come later.
