lib.wiring.connect: diagnostic when no connection made.

If a connect() call results in no connections being made, and it's
because there were no outputs specified at all, issue an error.
Tests enumerate cases per
https://github.com/amaranth-lang/amaranth/pull/1153#issuecomment-1962810678.

Co-authored-by: Catherine <whitequark@whitequark.org>

amaranth/lib/wiring.py

@@ -1371,6 +1371,7 @@ def connect(m, *args, **kwargs):
     * For a given path, if any of the interface objects has an input port member corresponding
       to a constant value, then the rest of the interface objects must have output port members
       corresponding to the same constant value.
+    * When connecting multiple interface objects, at least one connection must be made.
 
     For example, if :py:`obj1` is being connected to :py:`obj2` and :py:`obj3`, and :py:`obj1.a.b`
     is an output, then :py:`obj2.a.b` and :py:`obj2.a.b` must exist and be inputs. If :py:`obj2.c`
@@ -1420,10 +1421,15 @@ def connect(m, *args, **kwargs):
                                   reasons_as_string)
         signatures[handle] = obj.signature
 
-    # Collate signatures and build connections.
+    # Connecting zero or one signatures is OK.
+    if len(signatures) <= 1:
+        return
+
+    # Collate signatures, build connections, track whether we see any input or output.
     flattens = {handle: iter(sorted(signature.members.flatten()))
                 for handle, signature in signatures.items()}
     connections = []
+    any_in, any_out = False, False
     # Each iteration of the outer loop is intended to connect several (usually a pair) members
     # to each other, e.g. an out member `[0].a` to an in member `[1].a`. However, because we
     # do not just check signatures for equality (in order to improve diagnostics), it is possible
@@ -1437,6 +1443,7 @@ def connect(m, *args, **kwargs):
         # implied in the flow of each port member, so the signature members are only classified
         # here to ensure they are not connected to port members.
         is_first = True
+        first_path = None
         sig_kind, out_kind, in_kind = [], [], []
         for handle, flattened_members in flattens.items():
             path_for_handle, member = next(flattened_members, (None, None))
@@ -1499,6 +1506,8 @@ def connect(m, *args, **kwargs):
             # There are no port members at this point; we're done with this path.
             continue
         # There are only port members after this point.
+        any_in = any_in or bool(in_kind)
+        any_out = any_out or bool(out_kind)
         is_first = True
         for (path, member) in in_kind + out_kind:
             member_shape = member.shape
@@ -1574,6 +1583,14 @@ def connect(m, *args, **kwargs):
                                        out_path=(*out_path, index), in_path=(*in_path, index))
             assert out_member.dimensions == in_member.dimensions
             connect_dimensions(out_member.dimensions, out_path=out_path, in_path=in_path)
+
+    # If no connections were made, and there were inputs but no outputs in the
+    # signatures, issue a diagnostic as this is most likely in error.
+    if len(connections) == 0 and any_in and not any_out:
+        raise ConnectionError(f"Only input to input connections have been made between several "
+                              f"interfaces; should one of them have been flipped?")
+
+
     # Now that we know all of the connections are legal, add them to the module. This is done
     # instead of returning them because adding them to a non-comb domain would subtly violate
     # assumptions that `connect()` is intended to provide.

tests/test_lib_wiring.py

@@ -980,6 +980,65 @@ class ConnectTestCase(unittest.TestCase):
             '(eq (sig q__a__0__0) (sig p__a__0__0))',
         ])
 
+    def test_connect_none(self):
+        # Connecting zero or more empty signatures is permitted as (a) it's not
+        # something you can write mistakenly out by hand, and (b) it permits
+        # generic code to expand to nothing without errors around edges.
+        m = Module()
+        connect(m)
+
+    def test_connect_empty(self):
+        m = Module()
+        connect(m, p=NS(signature=Signature({})))
+
+    def test_connect_empty_multi(self):
+        m = Module()
+        connect(m, p=NS(signature=Signature({})),
+                   q=NS(signature=Signature({})))
+
+    def test_connect_one(self):
+        # Connecting just one signature should be allowed for the same reasons
+        # as above. (It's possible to forget an argument, but that stands out.)
+        m = Module()
+        connect(m, p=NS(signature=Signature({"a": Out(1), "b": In(1)}),
+                        a=Signal(),
+                        b=Signal()))
+
+    def test_connect_one_in_only(self):
+        # As above, even if there's only inputs.
+        m = Module()
+        connect(m, p=NS(signature=Signature({"a": In(1)}),
+                        a=Signal()))
+
+    def test_connect_multi_in_only_fails(self):
+        # If we're only attempting to connect multiple inputs, we're not
+        # actually doing anything and it's most likely a mistake.
+        m = Module()
+        with self.assertRaisesRegex(ConnectionError,
+                r"^Only input to input connections have been made between several interfaces; "
+                r"should one of them have been flipped\?$"):
+            connect(m,
+                    p=NS(signature=Signature({"a": In(1), "b": In(8)}),
+                         a=Signal(),
+                         b=Signal(8)),
+                    q=NS(signature=Signature({"a": In(1), "b": In(8)}),
+                         a=Signal(),
+                         b=Signal(8)))
+
+    def test_connect_multi_some_in_pairs(self):
+        # Connecting matching inputs is an allowed no-op if there are also
+        # actual input-output connections to be made. See
+        # https://github.com/amaranth-lang/amaranth/pull/1153#issuecomment-1962810678
+        # for more discussion.
+        m = Module()
+        connect(m,
+                p=NS(signature=Signature({"a": In(1), "b": In(1)}),
+                     a=Signal(),
+                     b=Signal()),
+                q=NS(signature=Signature({"a": Out(1), "b": In(1)}),
+                     a=Signal(),
+                     b=Signal()))
+
 
 class ComponentTestCase(unittest.TestCase):
     def test_basic(self):