Handle emplacement oddity?

[ehuss]

The following fails to parse in libsyntax due to the <- being a token, it requires spaces to parse properly.

if x<-1 {}

The lyg grammar parses it in a sane way.

It's never been clear to me whether or not this is a bug in rustc. We'll need to figure out our policy for documenting bugs.

[eddyb]

This is a bug in rustc, <- should decompose into < and -. cc @petrochenkov @varkor

[Centril]

@eddyb I don't think it's a bug; see rust-lang/rust#60803.

[eddyb]

Ah, I misremembered the decision there. I think the approach taken is a hack, and relies on the obsolete (pre-proc_macro) token model in libsyntax - whenever we get rid of that, it will implicitly switch to also parsing like a < -b (at least we have tests, but it's still a bit scary).

In that case, I propose we treat it as if it were supported, i.e. Expr |= Expr "<-" Expr, which will cause an ambiguity with the a < -b parse, that we would need to resolve anyway if it were suported.
Only then can we start restricting it away (and not risk the implicit fallback to a < -b).