Jordan Rose 62f947d6ba [Serialization] Drop GenericEnvironmentID for GenericSignatureID (#26862) ...

A generic environment is always serialized as a GenericSignature with
a lazily-recreated environment, though sometimes it has to include
extra info specifically for generic environments used by SIL. The code
that was doing this claimed a bit for disambiguating between the two,
shrinking the permitted size of a compiled module from 2^31 bits to
2^30. (The code isn't just needlessly complicated; GenericEnvironments
used to be serialized with more information.)

Rather than have two representations for GenericEnvironmentID, this
commit just drops it altogether in favor of referencing
GenericSignatures directly.  This causes a negligible file size
shrinkage for swiftmodules in addition to eliminating the problematic
disambiguation bit.

For now, the Deserialization logic will continue to cache
GenericEnvironments that are used directly by Deserialization, but
really that should probably be done at the AST level. Then we can
simplify further to ModuleFile tracking a plain list of
GenericSignatures.

2019-08-28 09:38:19 -07:00

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