Function replace_errors_with_variables 

pub fn replace_errors_with_variables<'db, T>(
    interner: DbInterner<'db>,
    t: &T,
) -> Canonical<'db, T>
where
    T: TypeFoldable<DbInterner<'db>> + Clone,

‘Canonicalizes’ the t by replacing any errors with new variables. Also ensures there are no unbound variables or inference variables anywhere in the t.