What would 'std:;' do in c++?
std: its a label, usable as a target for goto.
As pointed by @Adam Rosenfield in a comment, it is a legal label name.
C++03 §6.1/1:
Labels have their own name space and do not interfere with other identifiers.
It's a label, followed by an empty statement, followed by the declaration of a string x.
Its a label which is followed by the string
(expression)std: (end of expression); (another expression)string x = y;