hex-literal: ensure '/' as last byte causes error in hex!()

[thomascastleman]

Hello again,

I noticed that, because the way comment exclusion works, having a single forward slash / as the last byte given to hex!() causes no error to be given where there should be one. For instance, the following program compiles without error:

let _: [u8; 0] = hex!("/");

Note that this doesn't happen if there are bytes after the / (in which a case it errors).

My fix adds a check in the implementation of Iterator for ExcludingComments for if there are no more bytes and we're in the PotentialComment state. In this case it simply yields the previous byte (which was the foward slash) so that next_hex_val() will try to parse it as hex and panic with "invalid character". This has the nice property that it's consistent with the error you get currently if a single slash is encountered not at the end of the stream.

[newpavlov]

Good catch! But can't we simply panic on PotentialComment without yielding the previous byte?

[thomascastleman]

My thinking was to avoid replicating that same error message from next_hex_val, but if a panic right when we detect a final / is clearer, we can do that. How's this?

[newpavlov]

Ah, ExcludingComments yields a single /, so it results in an "invalid character" error. I would change it a bit, but for now I think we can merge this PR as is.