A protocol conformance can be ill-formed due to isolation mismatches
between witnesses and requirements, or with associated conformances.
Previously, such failures would be emitted as a number of separate
errors (downgraded to warnings in Swift 5), one for each witness and
potentially an extra for associated conformances. The rest was a
potential flood of diagnostics that was hard to sort through.
Collect all of the isolation-related problems for a given conformance
together and produce a single error (downgraded to a warning when
appropriate) that describes the overall issue. That error will have up
to three notes suggesting specific courses of action:
* Isolating the conformance (when the experimental feature is enabled)
* Marking the witnesses as 'nonisolated' where needed
*
The diagnostic also has notes to point out the witnesses/associated
conformances that have isolation problems. There is a new educational
note that also describes these options.
We give the same treatment to missing 'distributed' on witnesses to a
distributed protocol.
Rather than emitting isolation-related diagnostics for conformances, such
as witnesses that have incompatible isolation or associated conformances
that are differently isolated, capture all of those diagnostics in a
single side table and diagnose them all together.
This refactoring doesn't change the way we actually diagnose the
issue. That comes next.
Switch over to split caching for the conformance isolation request,
which optimizes for the common case where the conformance is
nonisolated. Also put the explicit global actor TypeExpr* in an
ASTContext side table, so we don't take a pointer's worth of storage
in every conformance.
For that side table, introduce a new "ASTContext::GlobalCache" that's
there only for side tables, so we don't have to go add get/set
operations to ASTContext and recompile the world every time we want to
add a side table like this.
Thanks, Slava!
