Prevent #if in @abi in module interfaces

When a language feature is used inside an `@abi` attribute, we should behave as though it was used on its counterpart. This was already half-implemented—we ensured the counterpart would use the feature—but we didn’t make the ABI decl aware that the counterpart was its parent for feature detection purposes. As a result, we would print `#if` inside the `@abi` attribute, which isn’t valid.

Author: beccadax

lib/AST/FeatureSet.cpp

@@ -709,8 +709,12 @@ FeatureSet swift::getUniqueFeaturesUsed(Decl *decl) {
   // Remove all the features used by all enclosing declarations.
   Decl *enclosingDecl = decl;
   while (!features.empty()) {
+    // If we were in an @abi attribute, collect from the API counterpart.
+    auto abiRole = ABIRoleInfo(enclosingDecl);
+    if (!abiRole.providesAPI() && abiRole.getCounterpart())
+      enclosingDecl = abiRole.getCounterpart();
     // Find the next outermost enclosing declaration.
-    if (auto accessor = dyn_cast<AccessorDecl>(enclosingDecl))
+    else if (auto accessor = dyn_cast<AccessorDecl>(enclosingDecl))
       enclosingDecl = accessor->getStorage();
     else
       enclosingDecl = enclosingDecl->getDeclContext()->getAsDecl();

test/ModuleInterface/attrs.swift

@@ -85,6 +85,20 @@ public struct MutatingTest {
 @abi(func abiSpiFunc())
 @_spi(spiGroup) public func abiSpiFunc() {}
 
+// We should print feature guards outside, but not inside, an @abi attribute.
+@abi(func sendingABI() -> sending Any?)
+public func sendingABI() -> Any? { nil }
+// CHECK: #if {{.*}} && $ABIAttribute
+// CHECK: @abi(func sendingABI() -> sending Any?)
+// CHECK: public func sendingABI() -> Any?
+// CHECK: #elseif {{.*}} && $SendingArgsAndResults
+// CHECK: @_silgen_name("$s5attrs10sendingABIypSgyF")
+// CHECK: public func sendingABI() -> Any?
+// CHECK: #else
+// CHECK: @_silgen_name("$s5attrs10sendingABIypSgyF")
+// CHECK: public func sendingABI() -> Any?
+// CHECK: #endif
+
 @concurrent
 public func testExecutionConcurrent() async {}
 // CHECK: @concurrent public func testExecutionConcurrent() async