[Distributed] Correct distributed accessor mangling for protocol calls

This is a crucial fix without which we can crash on some distributed
protocol declarations with @resolvable. We cannot "just" use a String to
represent the "fake base" of the thunks, and must instead find the
$Target macro generated type and use it as the base of the thunk's
mangling.

Calls are made in such way that record for the protocol requirement:
`$s4main28GreeterDefinedSystemProtocolP5greetSSyYaKFTEHF` points at
`$$s4main29$GreeterDefinedSystemProtocolC5greetSSyYaKFTE` which makes a
dispatch through the _apropriate_ witness table.
And the record for the $witness named e.g.
`$s4main29$GreeterDefinedSystemProtocolC5greetSSyYaKFTEHF` points to
`$s4main28GreeterDefinedSystemProtocolPAA11Distributed01_F9ActorStubRzrlE5greetSSyYaKFTE`
which is an extension method: `distributed thunk (extension in main):main.GreeterDefinedSystemProtocol< where A: Distributed._DistributedActorStub>.greet() async throws -> Swift.String`,
this very specific design allows us to call the "right method" on the
recieving end of a remote call where we do not know the recipient type.

Author: ktoso

lib/AST/ASTMangler.cpp

@@ -4452,12 +4452,32 @@ void ASTMangler::appendDistributedThunk(
     return nullptr;
   };
 
-  if (auto *P = referenceInProtocolContextOrRequirement()) {
-    appendContext(P->getDeclContext(), base,
-                  thunk->getAlternateModuleName());
-    appendIdentifier(Twine("$", P->getNameStr()).str());
-    appendOperator("C"); // necessary for roundtrip, though we don't use it
+  // Determine if we should mangle with a $Target substitute decl context,
+  // this matters for @Resolvable calls / protocol calls where the caller
+  // does not know the type of the recipient distributed actor, and we use the
+  // $Target type as substitute to then generically invoke it on the type of the
+  // recipient, whichever 'protocol Type'-conforming type it will be.
+  NominalTypeDecl *stubActorDecl = nullptr;
+  if (auto P = referenceInProtocolContextOrRequirement()) {
+    auto &C = thunk->getASTContext();
+    auto M = thunk->getModuleContext();
+
+    SmallVector<ValueDecl *, 1> stubClassLookupResults;
+    C.lookupInModule(M, ("$" + P->getNameStr()).str(), stubClassLookupResults);
+
+    if (stubClassLookupResults.size() > 0) {
+      stubActorDecl =
+          dyn_cast_or_null<NominalTypeDecl>(stubClassLookupResults.front());
+    }
+  }
+
+  if (stubActorDecl) {
+    // Effectively mangle the thunk as if it was declared on the $StubTarget
+    // type, rather than on a `protocol Target`.
+    appendContext(stubActorDecl, base, thunk->getAlternateModuleName());
   } else {
+    // There's no need to replace the context, this is a normal concrete type
+    // remote call identifier.
     appendContextOf(thunk, base);
   }
 

test/Distributed/Runtime/distributed_actor_func_calls_remoteCall_extension.swift

@@ -40,7 +40,9 @@ func test() async throws {
   let ref = try KappaProtocolImpl.resolve(id: local.id, using: system)
 
   let reply = try await ref.echo(name: "Caplin")
-  // CHECK: >> remoteCall: on:main.KappaProtocolImpl, target:main.$KappaProtocol.echo(name:), invocation:FakeInvocationEncoder(genericSubs: [main.KappaProtocolImpl], arguments: ["Caplin"], returnType: Optional(Swift.String), errorType: nil), throwing:Swift.Never, returning:Swift.String
+  // CHECK: >> remoteCall: on:main.KappaProtocolImpl, target:echo(name:), invocation:FakeInvocationEncoder(genericSubs: [main.KappaProtocolImpl], arguments: ["Caplin"], returnType: Optional(Swift.String), errorType: nil), throwing:Swift.Never, returning:Swift.String
+  // CHECK: > execute distributed target: echo(name:), identifier: $s4main13KappaProtocolPAAE4echo4nameS2S_tYaKFTE
+  // CHECK: > decode generic subs: [main.KappaProtocolImpl]
 
   // CHECK: << remoteCall return: Echo: Caplin
   print("reply: \(reply)")

test/Distributed/Runtime/distributed_actor_func_calls_remoteCall_through_generic.swift

@@ -1,6 +1,6 @@
 // RUN: %empty-directory(%t)
 // RUN: %target-swift-frontend-emit-module -emit-module-path %t/FakeDistributedActorSystems.swiftmodule -module-name FakeDistributedActorSystems -target %target-swift-5.7-abi-triple %S/../Inputs/FakeDistributedActorSystems.swift
-// RUN: %target-build-swift -module-name main -target %target-swift-5.7-abi-triple -j2 -parse-as-library -I %t %s %S/../Inputs/FakeDistributedActorSystems.swift -o %t/a.out
+// RUN: %target-build-swift -module-name main -target %target-swift-5.7-abi-triple -j2 -parse-as-library -plugin-path %swift-plugin-dir -I %t %s %S/../Inputs/FakeDistributedActorSystems.swift -o %t/a.out
 // RUN: %target-codesign %t/a.out
 // RUN: %target-run %t/a.out | %FileCheck %s
 

test/Distributed/Runtime/distributed_actor_localSystem_distributedProtocol.swift

@@ -64,7 +64,7 @@ func test_distributedVariable<DA: WorkerProtocol>(actor: DA) async throws -> Str
     let v1 = try await test_distributedVariable(actor: actor)
     print("v1 = \(v1)") // CHECK: v1 = implemented variable
 
-    let v = try await actor.distributedVariable
-    print("v = \(v)") // CHECK: v = implemented variable
+    let v2 = try await actor.distributedVariable
+    print("v2 = \(v2)") // CHECK: v2 = implemented variable
   }
 }