[Sema] Introduce utility service ValueDecl::getImportAccessFrom

Author: xymus

include/swift/AST/Decl.h

@@ -2783,6 +2783,16 @@ class ValueDecl : public Decl {
                         bool forConformance = false,
                         bool allowUsableFromInline = false) const;
 
+  using ImportAccessLevel = llvm::Optional<AttributedImport<ImportedModule>>;
+
+  /// Returns the import that may restrict the access to this decl
+  /// from \p useDC.
+  ///
+  /// If this decl and \p useDC are from the same module it returns
+  /// \c llvm::None. If there are many imports, it returns the most
+  /// permissive one.
+  ImportAccessLevel getImportAccessFrom(const DeclContext *useDC) const;
+
   /// Returns whether this declaration should be treated as \c open from
   /// \p useDC. This is very similar to #getFormalAccess, but takes
   /// \c \@testable into account.

lib/AST/Decl.cpp

@@ -4326,6 +4326,16 @@ bool ValueDecl::isAccessibleFrom(const DeclContext *useDC,
                      [&]() { return getFormalAccess(); });
 }
 
+ImportAccessLevel ValueDecl::getImportAccessFrom(const DeclContext *useDC) const {
+  ModuleDecl *Mod = getModuleContext();
+  if (useDC && useDC->getParentModule() != Mod) {
+    if (auto useSF = useDC->getParentSourceFile()) {
+      return useSF->getImportAccessLevel(Mod);
+    }
+  }
+  return llvm::None;
+}
+
 bool AbstractStorageDecl::isSetterAccessibleFrom(const DeclContext *DC,
                                                  bool forConformance) const {
   assert(isSettable(DC));