Well, that's the problem with ain't, it's not a contraction of anything specific. That's why is still slang, or as some dictionaries say, informal. It's used as a substitute for many phrases, such as am not, are not, is not, has not, have not, etc.
All of these terms have a correct contraction:
It will.
You are.
It is
"Weren't" is a contraction. It is composed of the words "were" and "not".
Won't is a contraction of will not.
'I' and 'HAVE'
"Shan't" is a contraction of "shall not".
They and are make up they're
The two words that make up the contraction let's are let and us.
Haven't is the contraction of have not.
contraction made from the two words you will: you'll
Would've is would have