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Regression: FromIterator<TokenStream> is not implemented for TokenStream #49725

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SimonSapin opened this issue Apr 6, 2018 · 3 comments
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Regression: FromIterator<TokenStream> is not implemented for TokenStream #49725

SimonSapin opened this issue Apr 6, 2018 · 3 comments
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A-macros Area: All kinds of macros (custom derive, macro_rules!, proc macros, ..) regression-from-stable-to-nightly Performance or correctness regression from stable to nightly. T-libs-api Relevant to the library API team, which will review and decide on the PR/issue.

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@SimonSapin
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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.
@SimonSapin SimonSapin added A-macros Area: All kinds of macros (custom derive, macro_rules!, proc macros, ..) regression-from-stable-to-nightly Performance or correctness regression from stable to nightly. T-libs-api Relevant to the library API team, which will review and decide on the PR/issue. labels Apr 6, 2018
@SimonSapin SimonSapin mentioned this issue Apr 6, 2018
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@nox
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nox commented Apr 6, 2018 •
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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
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nox commented Apr 6, 2018

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

Oops. Indeed, that's a regression.

@alexcrichton alexcrichton mentioned this issue Apr 6, 2018
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@alexcrichton
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Egad sorry about that! I've posted #49734 to revert this change

alexcrichton added a commit to alexcrichton/rust that referenced this issue Apr 6, 2018
@alexcrichton
proc_macro: Generalize FromIterator impl
d985344 
While never intended to be stable we forgot that trait impls are insta-stable!
This construction of `FromIterator` wasn't our first choice of how to stabilize
the impl but our hands are tied at this point, so revert back to the original
definition of `FromIterator` before rust-lang#49597

Closes rust-lang#49725
kennytm added a commit to kennytm/rust that referenced this issue Apr 11, 2018
@kennytm
Rollup merge of rust-lang#49734 - alexcrichton:generalize-token-strea…
574c050 
…m, r=nikomatsakis

proc_macro: Generalize `FromIterator` impl

While never intended to be stable we forgot that trait impls are insta-stable!
This construction of `FromIterator` wasn't our first choice of how to stabilize
the impl but our hands are tied at this point, so revert back to the original
definition of `FromIterator` before rust-lang#49597

Closes rust-lang#49725
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A-macros Area: All kinds of macros (custom derive, macro_rules!, proc macros, ..) regression-from-stable-to-nightly Performance or correctness regression from stable to nightly. T-libs-api Relevant to the library API team, which will review and decide on the PR/issue.
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