cross a bridge before one comes to it
cross a bridge before one comes to it {v. phr.}
To worry about future events or trouble before they happen. — Usually used in negative sentences, often as a proverb.
"Can I be a soldier when I grow up, Mother?" asked Johnny. "Don't cross that bridge until you come to it," said his mother.
Compare: BORROW TROUBLE.
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Source: A Dictionary of American Idioms