Implement two more special cases in fstring joining to reduce the need for...

Implement two more special cases in fstring joining to reduce the need for runtime character size detection.

Cython/Compiler/ExprNodes.py

@@ -3037,12 +3037,27 @@ class JoinedStrNode(ExprNode):
             is_ascii = False
             if isinstance(node, UnicodeNode):
                 try:
+                    # most strings will be ASCII or at least Latin-1
                     node.value.encode('iso8859-1')
                     max_char_value = '255'
                     node.value.encode('us-ascii')
                     is_ascii = True
                 except UnicodeEncodeError:
-                    pass
+                    if max_char_value != '255':
+                        # not ISO8859-1 => check BMP limit
+                        max_char = max(map(ord, node.value))
+                        if max_char < 0xD800:
+                            # BMP-only, no surrogate pairs used
+                            max_char_value = '65535'
+                            ulength = str(len(node.value))
+                        elif max_char >= 65536:
+                            # cleary outside of BMP, and not on a 16-bit Unicode system
+                            max_char_value = '1114111'
+                            ulength = str(len(node.value))
+                        else:
+                            # not really worth implementing a check for surrogate pairs here
+                            # drawback: C code can differ when generating on Py2 with 2-byte Unicode
+                            pass
                 else:
                     ulength = str(len(node.value))
             elif isinstance(node, FormattedValueNode) and node.value.type.is_numeric: