Optimize enumerate() with start argument and (GH-4077)

Also fixes the control flow analysis for loop variable assignments in this case.
Closes https://github.com/cython/cython/issues/3787

Cython/Compiler/FlowControl.py

@@ -980,7 +980,7 @@ class ControlFlowAnalysis(CythonTransform):
                 if not entry or entry.is_builtin:
                     if function.name == 'reversed' and len(sequence.args) == 1:
                         sequence = sequence.args[0]
-                    elif function.name == 'enumerate' and len(sequence.args) == 1:
+                    elif function.name == 'enumerate' and len(sequence.args) in (1, 2):
                         if target.is_sequence_constructor and len(target.args) == 2:
                             iterator = sequence.args[0]
                             if iterator.is_name:
@@ -1016,7 +1016,11 @@ class ControlFlowAnalysis(CythonTransform):
             # Python strings, etc., while correctly falling back to an
             # object type when the base type cannot be handled.
 
-            self.mark_assignment(target, node.item)
+            self.mark_assignment(target, ExprNodes.IndexNode(
+                node.pos,
+                base=sequence,
+                index=ExprNodes.IntNode(target.pos, value='PY_SSIZE_T_MAX',
+                                        type=PyrexTypes.c_py_ssize_t_type)))
 
     def visit_AsyncForStatNode(self, node):
         return self.visit_ForInStatNode(node)

Cython/Compiler/TypeInference.py

@@ -111,7 +111,7 @@ class MarkParallelAssignments(EnvTransform):
                 if not entry or entry.is_builtin:
                     if function.name == 'reversed' and len(sequence.args) == 1:
                         sequence = sequence.args[0]
-                    elif function.name == 'enumerate' and len(sequence.args) == 1:
+                    elif function.name == 'enumerate' and len(sequence.args) in (1, 2):
                         if target.is_sequence_constructor and len(target.args) == 2:
                             iterator = sequence.args[0]
                             if iterator.is_name:

tests/run/for_in_string.pyx

+from __future__ import print_function
 cimport cython
+from cython cimport typeof
 
 bytes_abc = b'abc'
 bytes_ABC = b'ABC'
@@ -224,3 +226,27 @@ def for_pyucs4_in_enumerate_unicode(unicode s):
             return i
     else:
         return 'X'
+
+@cython.test_assert_path_exists("//ForFromStatNode")
+@cython.test_fail_if_path_exists("//ForInStatNode")
+def for_infered_type_in_enumerate_unicode(unicode s):
+    """
+    >>> for_infered_type_in_enumerate_unicode(unicode_abc)
+    Py_ssize_t Py_UCS4
+    'X'
+    >>> for_infered_type_in_enumerate_unicode(unicode_ABC)
+    Py_ssize_t Py_UCS4
+    2
+    >>> for_infered_type_in_enumerate_unicode(unicode_abc_null)
+    Py_ssize_t Py_UCS4
+    'X'
+    >>> for_infered_type_in_enumerate_unicode(unicode_ABC_null)
+    Py_ssize_t Py_UCS4
+    4
+    """
+    print(typeof(i), typeof(c))
+    for i, c in enumerate(s):
+        if c == u'C':
+            return i
+    else:
+        return 'X'