Automatically treat nested modules as submodules

fixes PyO3#4286

newsfragments/4308.changed.md

@@ -0,0 +1 @@
+Nested declarative `#[pymodule]` are automatically treated as submodules (no `PyInit_` entrypoint is created)

pyo3-macros-backend/src/module.rs

@@ -80,7 +80,7 @@ impl PyModuleOptions {
     fn set_submodule(&mut self, submod: SubmoduleAttribute) -> Result<()> {
         ensure_spanned!(
             !self.is_submodule,
-            submod.span() => "`submodule` may only be specified once"
+            submod.span() => "`submodule` may only be specified once (it is implicitly always specified for nested modules)"
         );
 
         self.is_submodule = true;
@@ -116,7 +116,14 @@ pub fn pymodule_module_impl(
     } else {
         name.to_string()
     };
-    is_submodule = is_submodule || options.is_submodule;
+
+    is_submodule = match (is_submodule, options.is_submodule) {
+        (true, true) => {
+            bail_spanned!(module.span() => "`submodule` may only be specified once (it is implicitly always specified for nested modules)")
+        }
+        (false, false) => false,
+        (true, false) | (false, true) => true,
+    };
 
     let mut module_items = Vec::new();
     let mut module_items_cfg_attrs = Vec::new();
@@ -273,6 +280,7 @@ pub fn pymodule_module_impl(
                     )? {
                         set_module_attribute(&mut item_mod.attrs, &full_name);
                     }
+                    item_mod.attrs.push(parse_quote!(#[pyo3(submodule)]));
                 }
             }
             Item::ForeignMod(item) => {

tests/test_compile_error.rs

@@ -67,4 +67,5 @@ fn test_compile_errors() {
     t.compile_fail("tests/ui/abi3_weakref.rs");
     #[cfg(all(Py_LIMITED_API, not(Py_3_9)))]
     t.compile_fail("tests/ui/abi3_dict.rs");
+    t.compile_fail("tests/ui/duplicate_pymodule_submodule.rs");
 }

tests/test_declarative_module.rs

@@ -115,7 +115,7 @@ mod declarative_module {
         }
     }
 
-    #[pymodule(submodule)]
+    #[pymodule]
     #[pyo3(module = "custom_root")]
     mod inner_custom_root {
         use super::*;

tests/ui/duplicate_pymodule_submodule.rs

@@ -0,0 +1,7 @@
+#[pyo3::pymodule]
+mod mymodule {
+	#[pyo3::pymodule(submodule)]
+	mod submod {}
+}
+
+fn main() {}

tests/ui/duplicate_pymodule_submodule.stderr

@@ -0,0 +1,11 @@
+error: `submodule` may only be specified once (it is implicitly always specified for nested modules)
+ --> tests/ui/duplicate_pymodule_submodule.rs:4:2
+  |
+4 |     mod submod {}
+  |     ^^^
+
+error[E0433]: failed to resolve: use of undeclared crate or module `submod`
+ --> tests/ui/duplicate_pymodule_submodule.rs:4:6
+  |
+4 |     mod submod {}
+  |         ^^^^^^ use of undeclared crate or module `submod`