Look through `try`/`await` markers when looking for
out of place if/switch expressions, and customize
the effect checking diagnostic such that we error
that `try`/`await` are redundant on `if`/`switch`
expressions.
Introduce SingleValueStmtExpr, which allows the
embedding of a statement in an expression context.
This then allows us to parse and type-check `if`
and `switch` statements as expressions, gated
behind the `IfSwitchExpression` experimental
feature for now. In the future,
SingleValueStmtExpr could also be used for e.g
`do` expressions.
For now, only single expression branches are
supported for producing a value from an
`if`/`switch` expression, and each branch is
type-checked independently. A multi-statement
branch may only appear if it ends with a `throw`,
and it may not `break`, `continue`, or `return`.
The placement of `if`/`switch` expressions is also
currently limited by a syntactic use diagnostic.
Currently they're only allowed in bindings,
assignments, throws, and returns. But this could
be lifted in the future if desired.
Introduce discriminators into freestanding macro expansion expressions
and declarations. Compute these discriminators alongside closure and
local-declaration discriminators, checking them in the AST verifier.
With macro expansions, and really any code generation that produces
proper source locations, the parent source file of a declaration
context will be a generated source file rather than the source file
near the top of the declaration-context stack. Adjust the
implementation of `getParentSourceFile()` to return that innermost
source file by doing location-based lookup.
There are some cases of severely malformed code where we cannot compute
closure discriminators appropriate. Introduce a fallback to use the
unstable global numbering of discriminators in this case.
Rather than set closure discriminators in both the parser (for explicit
closures) and then later as part of contextualizing closures (for
autoclosures), do so via a request that sets all of the discriminators
for a given context.
Introduce `MacroExpansionExpr` and `MacroExpansionDecl` and plumb it through. Parse them in roughly the same way we parse `ObjectLiteralExpr`.
The syntax is gated under `-enable-experimental-feature Macros`.
Remove the preallocated closure discriminator from KeyPathExpr and go back
to expanding them using an AutoClosureExpr inside of a CaptureListExpr now
that that's supported. This allows the discriminator to be assigned during
type checking without disturbing the indexing of explicit closure literals.
Semantically, the capture list binding behavior doesn't require the scope
to be an explicit closure, and forming new ClosureExprs during type-checking
is difficult because they have to have preassigned discriminators, unlike
AutoClosureExprs which get discriminators introduced by Sema itself. Allowing
CaptureListExpr to hold an AutoClosureExpr makes it easier to synthesize
CaptureListExprs during type checking, to represent expressions with closure
semantics that evaluate some parts of the expression eagerly in the surrounding
context.
