rewrite function termination detection

fix dead code detection
fix detection of returns after valid branching returns
add tests

tests/functional/codegen/features/test_conditionals.py

@@ -7,7 +7,6 @@ def foo(i: bool) -> int128:
     else:
         assert 2 != 0
         return 7
-    return 11
     """
 
     c = get_contract_with_gas_estimation(conditional_return_code)

tests/functional/syntax/test_unbalanced_return.py

@@ -8,7 +8,7 @@
         """
 @external
 def foo() -> int128:
-    pass
+    pass  # missing return
     """,
         FunctionDeclarationException,
     ),
@@ -18,6 +18,7 @@ def foo() -> int128:
 def foo() -> int128:
     if False:
         return 123
+    # missing return
     """,
         FunctionDeclarationException,
     ),
@@ -27,19 +28,19 @@ def foo() -> int128:
 def test() -> int128:
     if 1 == 1 :
         return 1
-        if True:
+        if True:  # unreachable
             return 0
     else:
         assert msg.sender != msg.sender
     """,
-        FunctionDeclarationException,
+        StructureException,
     ),
     (
         """
 @internal
 def valid_address(sender: address) -> bool:
     selfdestruct(sender)
-    return True
+    return True  # unreachable
     """,
         StructureException,
     ),
@@ -48,7 +49,7 @@ def valid_address(sender: address) -> bool:
 @internal
 def valid_address(sender: address) -> bool:
     selfdestruct(sender)
-    a: address = sender
+    a: address = sender  # unreachable
     """,
         StructureException,
     ),
@@ -58,7 +59,7 @@ def valid_address(sender: address) -> bool:
 def valid_address(sender: address) -> bool:
     if sender == empty(address):
         selfdestruct(sender)
-        _sender: address = sender
+        _sender: address = sender  # unreachable
     else:
         return False
     """,
@@ -69,7 +70,7 @@ def valid_address(sender: address) -> bool:
 @internal
 def foo() -> bool:
     raw_revert(b"vyper")
-    return True
+    return True  # unreachable
     """,
         StructureException,
     ),
@@ -78,7 +79,7 @@ def foo() -> bool:
 @internal
 def foo() -> bool:
     raw_revert(b"vyper")
-    x: uint256 = 3
+    x: uint256 = 3  # unreachable
     """,
         StructureException,
     ),
@@ -88,12 +89,35 @@ def foo() -> bool:
 def foo(x: uint256) -> bool:
     if x == 2:
         raw_revert(b"vyper")
-        a: uint256 = 3
+        a: uint256 = 3  # unreachable
     else:
         return False
     """,
         StructureException,
     ),
+    (
+        """
+@internal
+def foo():
+    return
+    return  # unreachable
+    """,
+        StructureException,
+    ),
+    (
+        """
+@internal
+def foo() -> uint256:
+    if block.number % 2 == 0:
+        return 5
+    elif block.number % 3 == 0:
+        return 6
+    else:
+        return 10
+    return 0  # unreachable
+    """,
+        StructureException,
+    ),
 ]
 
 
@@ -154,7 +178,6 @@ def test() -> int128:
     else:
         x = keccak256(x)
         return 1
-    return 1
     """,
     """
 @external

vyper/semantics/analysis/local.py

@@ -69,35 +69,28 @@ def validate_functions(vy_module: vy_ast.Module) -> None:
     err_list.raise_if_not_empty()
 
 
-def check_for_terminus(node_list: list) -> bool:
-    terminus_nodes = []
-
-    # Check for invalid code after returns
-    last_node_pos = len(node_list) - 1
-    for idx, n in enumerate(node_list):
-        if n.is_terminus:
-            terminus_nodes.append(n)
-            if idx < last_node_pos:
-                # is not last statement in body.
-                raise StructureException(
-                    "Exit statement with succeeding code (that will not execute).",
-                    node_list[idx + 1],
-                )
+# finds the terminus node for a list of nodes.
+# raises an exception if any nodes are unreachable
+def find_terminating_node(node_list: list) -> Optional[vy_ast.VyperNode]:
+    ret = None
 
-    if len(terminus_nodes) > 1:
-        raise StructureException(
-            "Too many exit statements (return, raise or selfdestruct).", terminus_nodes[-1]
-        )
-    elif len(terminus_nodes) == 1:
-        return True
+    for node in node_list:
+        if ret is not None:
+            raise StructureException("Unreachable code!", node)
+        if node.is_terminus:
+            ret = node
 
-    for node in [i for i in node_list if isinstance(i, vy_ast.If)][::-1]:
-        if not node.orelse or not check_for_terminus(node.orelse):
-            continue
-        if not check_for_terminus(node.body):
-            continue
-        return True
-    return False
+        if isinstance(node, vy_ast.If):
+            body_terminates = find_terminating_node(node.body)
+
+            else_terminates = None
+            if node.orelse is not None:
+                else_terminates = find_terminating_node(node.orelse)
+
+            if body_terminates is not None and else_terminates is not None:
+                ret = else_terminates
+
+    return ret
 
 
 def _check_iterator_modification(
@@ -213,11 +206,13 @@ def analyze(self):
             self.visit(node)
 
         if self.func.return_type:
-            if not check_for_terminus(self.fn_node.body):
+            if not find_terminating_node(self.fn_node.body):
                 raise FunctionDeclarationException(
-                    f"Missing or unmatched return statements in function '{self.fn_node.name}'",
-                    self.fn_node,
+                    f"Missing return statement in function '{self.fn_node.name}'", self.fn_node
                 )
+        else:
+            # call find_terminator for its unreachable code detection side effect
+            find_terminating_node(self.fn_node.body)
 
         # visit default args
         assert self.func.n_keyword_args == len(self.fn_node.args.defaults)
@@ -519,7 +514,7 @@ def visit_Return(self, node):
                 raise FunctionDeclarationException("Return statement is missing a value", node)
             return
         elif self.func.return_type is None:
-            raise FunctionDeclarationException("Function does not return any values", node)
+            raise FunctionDeclarationException("Function should not return any values", node)
 
         if isinstance(values, vy_ast.Tuple):
             values = values.elements