From b5b2c9b8a4381a5b488aa647f78ad56abd1ad646 Mon Sep 17 00:00:00 2001
From: whitequark <cz@m-labs.hk>
Date: Sat, 3 Aug 2019 12:30:39 +0000
Subject: [PATCH] hdl.ir: warn if .elaborate() returns None.

Fixes #164.
---
 nmigen/hdl/ir.py           | 9 +++++++++
 nmigen/test/test_hdl_ir.py | 9 +++++++++
 2 files changed, 18 insertions(+)

diff --git a/nmigen/hdl/ir.py b/nmigen/hdl/ir.py
index 773bb8e..bb2cd0e 100644
--- a/nmigen/hdl/ir.py
+++ b/nmigen/hdl/ir.py
@@ -52,10 +52,12 @@ class DriverConflict(UserWarning):
 class Fragment:
     @staticmethod
     def get(obj, platform):
+        code = None
         while True:
             if isinstance(obj, Fragment):
                 return obj
             elif isinstance(obj, Elaboratable):
+                code = obj.elaborate.__code__
                 obj._Elaboratable__used = True
                 obj = obj.elaborate(platform)
             elif hasattr(obj, "elaborate"):
@@ -65,9 +67,16 @@ class Fragment:
                             .format(type(obj)),
                     category=RuntimeWarning,
                     stacklevel=2)
+                code = obj.elaborate.__code__
                 obj = obj.elaborate(platform)
             else:
                 raise AttributeError("Object '{!r}' cannot be elaborated".format(obj))
+            if obj is None and code is not None:
+                warnings.warn_explicit(
+                    message=".elaborate() returned None; missing return statement?",
+                    category=UserWarning,
+                    filename=code.co_filename,
+                    lineno=code.co_firstlineno)
 
     def __init__(self):
         self.ports = SignalDict()
diff --git a/nmigen/test/test_hdl_ir.py b/nmigen/test/test_hdl_ir.py
index b30238f..2ad5eb2 100644
--- a/nmigen/test/test_hdl_ir.py
+++ b/nmigen/test/test_hdl_ir.py
@@ -7,12 +7,21 @@ from ..hdl.mem import *
 from .tools import *
 
 
+class BadElaboratable(Elaboratable):
+    def elaborate(self, platform):
+        return
+
+
 class FragmentGetTestCase(FHDLTestCase):
     def test_get_wrong(self):
         with self.assertRaises(AttributeError,
                 msg="Object 'None' cannot be elaborated"):
             Fragment.get(None, platform=None)
 
+        with self.assertRaises(AttributeError,
+                msg="Object 'None' cannot be elaborated"):
+            Fragment.get(BadElaboratable(), platform=None)
+
 
 class FragmentGeneratedTestCase(FHDLTestCase):
     def test_find_subfragment(self):
-- 
2.30.2