XO0153: unknown or malformed escape sequence

Escapes in Str and Bytes are \n \t \r \0 \\ \" \' \$ \xHH \u{...}. Anything else after a backslash is an error, as are \x without two hex digits, \u without braces, an invalid code point, and in Str a \xHH above \x7f (use \u{...} for non ASCII text). To write a backslash, double it, or use a raw string r"...", which has no escapes. See core 1.6.

Example

fn f() -> Str {
  "\q"
}

Fix

fn f() -> Str {
  r"\q"
}

Run xo explain XO0153 for this text in a terminal, or see section 10.6 of the specification.