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* Create and cache flattened inheritance lists
The basic change here is to have a cached lookup that can map a `Type`,
or a `DeclRef` that might refer to a type or `extension`, to a list of
the *facets* that comprise it.
The notion of a *facet* here is similar to what the C++ standard calls
"sub-objects".
A declared type like a `struct` has:
* a facet for its own direct members
* one facet for each of its (transitive) base `struct` types
* one facet for each `interface` it conforms to
* one facet for each `extension` that applies to that type
The set of facets for a type is de-duplicated (so that "diamond"
inheritance patterns don't cause issues) and deterministically ordered,
using a variation of the C3 linearization algorithm.
The creation of a linearized list of facets should help the compiler
implementation in two key places:
* Testing if a type implements an interface (or inherits from a base
type) should now only take time linear in the number of (transitive)
bases of that type. We can simply scan the linearized facet list to
see if it contains a facet corresponding to the given base.
* Looking up the members of a type (or a value of a given type) should
be greatly simplified, since all of the members can be found in a
single linear scan of the facet list. In addition, those facets will
be ordered so that facets for "more derived" types will precede those
for "less derived" types, so that shadowing in the case of overrides
should be easier to implement.
This change only implements the first of these two improvements, since
there is already a *lot* of churn involved.
Notes and caveats:
* The handling of conjunction types (e.g., `IFoo & IBar`) complicates
the implementation, both because the simple approach to subtype
testing alluded to above is no longer complete, and also because
we need to be more careful about what forms of subtype witnesses
we construct, so that we can maintain the currently-required invariant
that two witnesses are only equal if they have matching structure.
* We don't implement the full/"proper" C3 algorithm here because it has
some failure cases that we'd still like to support. In particular if
we have both `IX : IA, IB` and `IY : IB, IA`, the C3 algorithm says it
is illegal to have `IZ : IX, IY` because the two bases it inherits
from disagree on the relative ordering of `IA` and `IB` in their
own linearizations. Handling such cases may make our implementation
less efficient, and it will also require testing of those corner
caes.
* When it comes time to revamp the implementation of lookup, we will
need to deal with the fact that a single linear list (seemingly)
cannot give us sufficient information to decide which of two members
of the same name should shadow the other, or if there is an ambiguity.
Or rather, it *can* give us that information if we are willing to
accept some very user-unfriendly behavior and simply say that
declarations earlier in the linearization always shadow later
declarations, even if the facets involved are not related by an
inheritance relationship of any kind.
* In order to remove one kind of vicious circularity from the approach,
the linearization that we are computing for `extension` declarations
will not be sufficient for lookups in the body of such an `extension`.
A future change may need to have support for creating and caching
two distinct linearizations for each `extension`: one that is to be
used when that `extension` is pulled into the linearization for a
type that it applies to, and another for when lookup will be performed
in the context of the `extension` itself.
* This change does *not* include the simple expedient of adding a direct
cache for subtype tests to the `SharedSemanticsContext`, although
adding such a cache would be a simple matter.
* This change introduces more deduplication for subtype witnesses,
which should enable more deduplication for other `Val`s (including
`Type`s), but it does not introduce any assumptions that equal
`Val`s or `Type`s must have identical pointer representations.
* Eventually we may find that, similar to the situation with `Type`s,
we will want to have a split between surface-level and canonicalized
versions of other `Val`s, including subtype witnesses.
* Fix clang error.
* remove debugging code.
---------
Co-authored-by: Yong He <yonghe@outlook.com>
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