E0001: Unknown Name

You tried to use a name that hasn't been declared.

Example

fn main() {
    let x = y + 1;  // Error: `y` is not defined
}

How to fix

Make sure the variable is declared before you use it:

fn main() {
    let y = 10;
    let x = y + 1;  // OK
}

Or check for typos in the variable name.