reject empty interfaces

tests/functional/syntax/modules/test_exports.py

@@ -466,3 +466,19 @@ def __init__():
     with pytest.raises(InterfaceViolation) as e:
         compile_code(main, input_bundle=input_bundle)
     assert e.value._message == "requested `lib1.ifoo` but `lib1` does not implement `lib1.ifoo`!"
+
+
+def test_export_empty_interface(make_input_bundle):
+    lib1 = """
+def an_internal_function():
+    pass
+    """
+    main = """
+import lib1
+
+exports: lib1.__interface__
+    """
+    input_bundle = make_input_bundle({"lib1.vy": lib1})
+    with pytest.raises(StructureException) as e:
+        compile_code(main, input_bundle=input_bundle)
+    assert e.value._message == "lib1 has no external functions!"

vyper/semantics/analysis/base.py

@@ -96,6 +96,7 @@ class AnalysisResult:
 class ModuleInfo(AnalysisResult):
     module_t: "ModuleT"
     alias: str
+    # import_node: vy_ast._ImportStmt # maybe could be useful
     ownership: ModuleOwnership = ModuleOwnership.NO_OWNERSHIP
     ownership_decl: Optional[vy_ast.VyperNode] = None
 

vyper/semantics/analysis/module.py

@@ -528,6 +528,12 @@ def visit_ExportsDecl(self, node):
                     for fn in interface_t.functions.values()
                     if fn.is_external
                 ]
+
+                if len(funcs) == 0:
+                    path = module_info.module_node.path
+                    msg = f"{module_info.alias} (located at `{path}`) has no external functions!"
+                    raise StructureException(msg, item)
+
             else:
                 raise StructureException(
                     f"not a function or interface: `{info.typ}`", info.typ.decl_node, item