We are leaving this as an open part of the design space. In the mean time if
people need a +0 parameter, they can use __shared with sending.
rdar://129116182
TLDR: This makes it so that we always can parse sending/transferring but changes
the semantic language effects to be keyed on RegionBasedIsolation instead.
----
The key thing that makes this all work is that I changed all of the "special"
semantic changes originally triggered on *ArgsAndResults to now be triggered
based on RegionBasedIsolation being enabled. This makes a lot of sense since we
want these semantic changes specifically to be combined with the checkers that
RegionBasedIsolation turns on. As a result, even though this causes these two
features to always be enabled, we just parse it but we do not use it for
anything semantically.
rdar://128961672
A few things:
1. Internally except for in the parser and the clang importer, we only represent
'sending'. This means that it will be easy to remove 'transferring' once enough
time has passed.
2. I included a warning that suggested to the user to change 'transferring' ->
'sending'.
3. I duplicated the parsing diagnostics for 'sending' so both will still get
different sets of diagnostics for parsing issues... but anywhere below parsing,
I have just changed 'transferring' to 'sending' since transferring isn't
represented at those lower levels.
4. Since SendingArgsAndResults is always enabled when TransferringArgsAndResults
is enabled (NOTE not vis-a-versa), we know that we can always parse sending. So
we import "transferring" as "sending". This means that even if one marks a
function with "transferring", the compiler will guard it behind a
SendingArgsAndResults -D flag and in the imported header print out sending.
rdar://128216574
Instead it is a bit on ParamDecl and SILParameterInfo. I preserve the consuming
behavior by making it so that the type checker changes the ParamSpecifier to
ImplicitlyCopyableConsuming if we have a default param specifier and
transferring is set. NOTE: The user can never write ImplicitlyCopyableConsuming.
NOTE: I had to expand the amount of flags that can be stored in ParamDecl so I
stole bits from TypeRepr and added some logic for packing option bits into
TyRepr and DefaultValue.
rdar://121324715
