Add 'isAsyncContext' to DeclContext

Querying the context for whether it is an async context is the easiest
way to detect if the context is async. Many decl contexts can be checked
at any point, but AbstractClosureExpr contexts must have been
typechecked. If this is called on an AbstractClosureExpr before the
types have been assigned, it may either crash or do bad things.

Author: etcwilde

include/swift/AST/DeclContext.h

@@ -296,6 +296,9 @@ class alignas(1 << DeclContextAlignInBits) DeclContext
   /// Returns the kind of context this is.
   DeclContextKind getContextKind() const;
 
+  /// Returns whether this context asynchronous
+  bool isAsyncContext() const;
+
   /// Returns whether this context has value semantics.
   bool hasValueSemantics() const;
 

lib/AST/DeclContext.cpp

@@ -1204,6 +1204,32 @@ bool DeclContext::isClassConstrainedProtocolExtension() const {
   return false;
 }
 
+bool DeclContext::isAsyncContext() const {
+  switch (getContextKind()) {
+  case DeclContextKind::Initializer:
+  case DeclContextKind::TopLevelCodeDecl:
+  case DeclContextKind::EnumElementDecl:
+  case DeclContextKind::ExtensionDecl:
+  case DeclContextKind::SerializedLocal:
+  case DeclContextKind::Module:
+  case DeclContextKind::FileUnit:
+  case DeclContextKind::GenericTypeDecl:
+    return false;
+  case DeclContextKind::AbstractClosureExpr:
+    return cast<AbstractClosureExpr>(this)->isBodyAsync();
+  case DeclContextKind::AbstractFunctionDecl: {
+    const AbstractFunctionDecl *function = cast<AbstractFunctionDecl>(this);
+    return function->hasAsync();
+  }
+  case DeclContextKind::SubscriptDecl: {
+    AccessorDecl *getter =
+        cast<SubscriptDecl>(this)->getAccessor(AccessorKind::Get);
+    return getter != nullptr && getter->hasAsync();
+  }
+  }
+  llvm_unreachable("Unhandled DeclContextKind switch");
+}
+
 SourceLoc swift::extractNearestSourceLoc(const DeclContext *dc) {
   switch (dc->getContextKind()) {
   case DeclContextKind::Module: