Allow //~| comments to preceed //~v comments.

[Jarcho]

Fixes an oversight from when //~v comments were added. Current implementation flips the order required which wouldn't be backwards compatible. Supporting stacking both above and below would be trivial if you'd prefer.

This should probably error when it's ambiguous which arrow to follow, though it currently binds above in such a case. e.g.

//~^ error
//~| error2 
//~v error3

[oli-obk]

Oh god lol. That does make sense, yea.

I think we should also error on

//~^ error
//~v error3

And require a newline between for readability

[Jarcho]

Do you want to allow them underneath?

//~vv error1
//~| error2

It's trivial either way.

[Jarcho]

While we're on breaking changes, how about inline matching:

//~ error1
//~| error2

//~| error1
//~ error2

//~| error1
//~ error2
//~| error3

The first is currently accepted. Should it be? What about the other two?

[Jarcho]

Current status:

Allowed:

//~| error1
//~v error2

Disallowed (not backwards compatible):

//~vv error1
//~| error2

//~ error1
//~| error2

Disallowed (backwards compatible):

//~| error1
//~ error2

//~^ error1
//~v error2

//~^ error1
//~| error2
//~| ...
//~v errorN

[oli-obk]

This lgtm, and the breaking change is acceptable. It was not intended to be supported

Please also add a test for

//~ error1
//~| error2

[Jarcho]

Done.

For chaining off //~ this has been supported for a while. It's probably unused, and it really isn't much better than using //~^ since it already takes multiple lines. Adding it back in would be easy though.

[oli-obk]

It's probably unused

I just saw it in the rustc ui tests last week 😆 but we should just eliminate them. They look weird