From e1afe4b06d1359678934387a9639e3fa4bfc46be Mon Sep 17 00:00:00 2001
From: Catherine <whitequark@whitequark.org>
Date: Wed, 5 Mar 2025 00:25:30 +0000
Subject: [PATCH] hdl.ir: improve error raised when returning None from
 `elaborate`.

Fixes #1569.
---
 amaranth/hdl/_ir.py  | 14 +++++---------
 tests/test_hdl_ir.py | 10 ++++------
 2 files changed, 9 insertions(+), 15 deletions(-)

diff --git a/amaranth/hdl/_ir.py b/amaranth/hdl/_ir.py
index 0bdbb97e6..1370bdea4 100644
--- a/amaranth/hdl/_ir.py
+++ b/amaranth/hdl/_ir.py
@@ -49,8 +49,8 @@ class DomainRequirementFailed(Exception):
 class Fragment:
     @staticmethod
     def get(obj, platform):
-        code = None
         origins = []
+        returned_by = ""
         while True:
             if isinstance(obj, Fragment):
                 if hasattr(obj, "origins"):
@@ -58,19 +58,15 @@ def get(obj, platform):
                 return obj
             elif isinstance(obj, Elaboratable):
                 code = obj.elaborate.__code__
+                returned_by = f", returned by {code.co_filename}:{code.co_firstlineno}"
                 UnusedElaboratable._MustUse__silence = False
                 obj._MustUse__used = True
                 new_obj = obj.elaborate(platform)
             else:
-                raise TypeError(f"Object {obj!r} is not an 'Elaboratable' nor 'Fragment'")
+                raise TypeError(
+                    f"Object {obj!r} is not an 'Elaboratable' nor 'Fragment'{returned_by}")
             if new_obj is obj:
-                raise RecursionError(f"Object {obj!r} elaborates to itself")
-            if new_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)
+                raise RecursionError(f"Object {obj!r} elaborates to itself{returned_by}")
             origins.append(obj)
             obj = new_obj
 
diff --git a/tests/test_hdl_ir.py b/tests/test_hdl_ir.py
index bbfc86ce3..c017f38e9 100644
--- a/tests/test_hdl_ir.py
+++ b/tests/test_hdl_ir.py
@@ -31,15 +31,13 @@ def test_get_wrong_none(self):
                 r"^Object None is not an 'Elaboratable' nor 'Fragment'$"):
             Fragment.get(None, platform=None)
 
-        with self.assertWarnsRegex(UserWarning,
-                r"^\.elaborate\(\) returned None; missing return statement\?$"):
-            with self.assertRaisesRegex(TypeError,
-                    r"^Object None is not an 'Elaboratable' nor 'Fragment'$"):
-                Fragment.get(ElaboratesToNone(), platform=None)
+        with self.assertRaisesRegex(TypeError,
+                r"^Object None is not an 'Elaboratable' nor 'Fragment', returned by .+?:19$"):
+            Fragment.get(ElaboratesToNone(), platform=None)
 
     def test_get_wrong_self(self):
         with self.assertRaisesRegex(RecursionError,
-                r"^Object <.+?ElaboratesToSelf.+?> elaborates to itself$"):
+                r"^Object <.+?ElaboratesToSelf.+?> elaborates to itself, returned by .+?:24$"):
             Fragment.get(ElaboratesToSelf(), platform=None)