Add experimental feature flag EmbeddedExistentials

This flag can be used to gradually add the functionility that will allow use of
protocol values in embedded mode.

Author: aschwaighofer

SwiftCompilerSources/Sources/Optimizer/ModulePasses/EmbeddedSwiftDiagnostics.swift

@@ -88,7 +88,9 @@ private struct FunctionChecker {
          is InitExistentialAddrInst,
          is InitExistentialValueInst,
          is ExistentialMetatypeInst:
-      throw Diagnostic(.embedded_swift_existential_type, instruction.operands[0].value.type, at: instruction.location)
+         if !context.options.enableEmbeddedSwiftExistentials {
+             throw Diagnostic(.embedded_swift_existential_type, instruction.operands[0].value.type, at: instruction.location)
+         }
 
     case let aeb as AllocExistentialBoxInst:
       throw Diagnostic(.embedded_swift_existential_type, aeb.type, at: instruction.location)

SwiftCompilerSources/Sources/Optimizer/PassManager/Options.swift

@@ -44,6 +44,10 @@ struct Options {
     hasFeature(.Embedded)
   }
 
+  var enableEmbeddedSwiftExistentials: Bool {
+      hasFeature(.EmbeddedExistentials)
+  }
+
   var enableMergeableTraps: Bool {
     _bridged.enableMergeableTraps()
   }

include/swift/Basic/Features.def

@@ -558,6 +558,9 @@ EXPERIMENTAL_FEATURE(SwiftRuntimeAvailability, true)
 /// Allow use of `~Sendable`.
 SUPPRESSIBLE_EXPERIMENTAL_FEATURE(TildeSendable, false)
 
+/// Allow use of protocol typed values in Embedded mode (`Any` and friends)
+EXPERIMENTAL_FEATURE(EmbeddedExistentials, false)
+
 #undef EXPERIMENTAL_FEATURE_EXCLUDED_FROM_MODULE_INTERFACE
 #undef EXPERIMENTAL_FEATURE
 #undef UPCOMING_FEATURE

lib/AST/FeatureSet.cpp

@@ -128,6 +128,7 @@ UNINTERESTING_FEATURE(NonisolatedNonsendingByDefault)
 UNINTERESTING_FEATURE(KeyPathWithMethodMembers)
 UNINTERESTING_FEATURE(ImportMacroAliases)
 UNINTERESTING_FEATURE(NoExplicitNonIsolated)
+UNINTERESTING_FEATURE(EmbeddedExistentials)
 
 // TODO: Return true for inlinable function bodies with module selectors in them
 UNINTERESTING_FEATURE(ModuleSelector)

test/embedded/existential.swift

@@ -0,0 +1,23 @@
+// RUN: %target-swift-frontend -enable-experimental-feature EmbeddedExistentials -enable-experimental-feature Embedded -parse-as-library -wmo -emit-sil %s | %FileCheck %s
+
+// REQUIRES: swift_in_compiler
+// REQUIRES: optimized_stdlib
+// REQUIRES: swift_feature_Embedded
+// REQUIRES: swift_feature_EmbeddedExistentials
+
+class C {}
+
+// CHECK: sil @$e11existential4testyyF
+// CHECK: init_existential_addr
+// CHECK: } // end sil function '$e11existential4testyyF'
+
+func test() {
+  let any: any Any = C()
+}
+
+@main
+struct Main {
+  static func main() {
+    test()
+  }
+}