Failure to set direction results in an unhelpful failure from a seemingly infallible method

[rsheeter]

harfrust/src/hb/ot_shape_plan.rs

Line 50 in d47a254

assert_ne!(direction, Direction::Invalid);

failing is spectacularly unhelpful. An infallible looking API can fail, and when it does what prints offers no guidance wrt what to do. Ideally we'd avoid being in a bad state but if we need to allow for it then lets use Result?

[behdad]

Fixed in #214

[rsheeter]

An infallible looking API can fail, and when it does what prints offers no guidance wrt what to do

It's nice that it prints now but it's still a fallible api that looks infallible, no? Why not structure the APIs such that this isn't a reachable state?