Regression: `FromIterator<TokenStream>` is not implemented for `TokenStream`

[SimonSapin]

Servo doesn’t build in today’s Nightly:

error[E0277]: the trait bound `proc_macro::TokenStream: std::iter::FromIterator<proc_macro::TokenStream>` is not satisfied
  --> components/dom_struct/lib.rs:26:53
   |
26 |     iter::once(attributes).chain(iter::once(input)).collect()
   |                                                     ^^^^^^^ a collection of type `proc_macro::TokenStream` cannot be built from an iterator over elements of type `proc_macro::TokenStream`
   |
   = help: the trait `std::iter::FromIterator<proc_macro::TokenStream>` is not implemented for `proc_macro::TokenStream`
error: aborting due to previous error

Since the TokenStream type is stable I think this is a breaking change.

CC @alexcrichton for #49597.

[nox]

The type is stable, but that impl is unstable.

Edit: just realised that's not the impl in question, just wanted to say that I don't think it's a breaking change with regard to stable.

[nox]

https://play.rust-lang.org/?gist=4af8713032317b911ee96efdc2d26887&version=stable

Oops. Indeed, that's a regression.

[alexcrichton]

Egad sorry about that! I've posted #49734 to revert this change