just add "to_owned()" and problem solved. This function doesn't protect nothing, it's a bullshit. I love Rust, but author is far from the theme he is trying to describe. And theme is dangerous enough.
let _rows = sql_query("SELECT * FROM users WHERE username=?", &[username]);
The statement is static, but the [username] part is not, it's just a variable that can have whatever username you want.