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:
The two words that make up the contraction "can't" are "cannot."
"who" and "will" make up the contraction "who'll."
It will.
"You" and "would" make up the contraction "you'd".
"you" and "would" make up the contraction "you'd."
You are.
It is
will + not
Does not.
"Weren't" is a contraction. It is composed of the words "were" and "not".
"Let us" combine to form the contraction "let's".
"Shan't" is a contraction of "shall not".