Parse lazy type alias syntax

[dtolnay]

Tracking issue: rust-lang/rust#112792

pub type ReadWrite<R, W> = Pair<R, W>
where
    R: std::io::Read,
    W: std::io::Write;

Ordinarily Rust does not accept where in this position. But with feature(lazy_type_alias), it does.

error: where clauses are not allowed after the type for type aliases
  --> src/main.rs:23:1
   |
23 | / where
24 | |     R: std::io::Read,
25 | |     W: std::io::Write;
   | |_____________________^
   |
   = note: see issue #112792 <https://github.com/rust-lang/rust/issues/112792> for more information
   = help: add `#![feature(lazy_type_alias)]` to the crate attributes to enable

[fmease]

NB: Since the stabilization of GATs, rustc syntactically accepts both where-clause locations (before, after, before & after).

I don't know how syn usually fares in these kinds of situations but technically speaking type T = () where; has been syntactically valid¹ for quite a while now.

Footnotes

1. according to rustc, not syn ↩