optimise simple cases of adding string literals

--HG--
extra : transplant_source : w%8C%DC%3BS%CE%B2%8AC%D2%14%D3%99%B9%E8Z%0F%983%AD

Cython/Compiler/Optimize.py

@@ -3442,6 +3442,31 @@ class ConstantFolding(Visitor.VisitorTransform, SkipDeclarations):
                                     constant_result = node.constant_result)
         return new_node
 
+    def visit_AddNode(self, node):
+        self._calculate_const(node)
+        if node.constant_result is ExprNodes.not_a_constant:
+            return node
+        if node.operand1.is_string_literal and node.operand2.is_string_literal:
+            # some people combine string literals with a '+'
+            str1, str2 = node.operand1, node.operand2
+            if isinstance(str1, ExprNodes.UnicodeNode) and isinstance(str2, ExprNodes.UnicodeNode):
+                bytes_value = None
+                if str1.bytes_value is not None and str2.bytes_value is not None:
+                    if str1.bytes_value.encoding == str2.bytes_value.encoding:
+                        bytes_value = BytesLiteral(str1.bytes_value + str2.bytes_value)
+                        bytes_value.encoding = str1.bytes_value.encoding
+                string_value = EncodedString(node.constant_result)
+                return ExprNodes.UnicodeNode(
+                    str1.pos, value=string_value, constant_result=node.constant_result, bytes_value=bytes_value)
+            elif isinstance(str1, ExprNodes.BytesNode) and isinstance(str2, ExprNodes.BytesNode):
+                if str1.value.encoding == str2.value.encoding:
+                    bytes_value = BytesLiteral(node.constant_result)
+                    bytes_value.encoding = str1.value.encoding
+                    return ExprNodes.BytesNode(str1.pos, value=bytes_value, constant_result=node.constant_result)
+            # all other combinations are rather complicated
+            # to get right in Py2/3: encodings, unicode escapes, ...
+        return self.visit_BinopNode(node)
+
     def visit_MulNode(self, node):
         self._calculate_const(node)
         if node.operand1.is_sequence_constructor:

tests/run/constant_folding.py

@@ -109,6 +109,27 @@ def binop_bool():
     return plus1, pmix1, minus1, and1, or1, ormix1, xor1, plus3, pmix3, minus3, and3, or3, ormix3, xor3
 
 
+@cython.test_fail_if_path_exists(
+    "//ReturnStatNode//AddNode",
+)
+@cython.test_assert_path_exists(
+    "//ListNode//AddNode",
+)
+def add_strings():
+    """
+    >>> u, b, rest = add_strings()
+    >>> u == 'abcdef' or u
+    True
+    >>> b == b'abcdef' or b
+    True
+    >>> rest
+    1
+    """
+    a = ["abc" + "def"]  # not currently optimised
+    # FIXME: test encodings and unicode escapes
+    return u"abc" + u"def", b"abc" + b"def", a[0] and 1
+
+
 @cython.test_fail_if_path_exists(
     "//SliceIndexNode",
 )