Commits on Sep 2, 2016

1. rustc: Implement custom derive (macros 1.1) …

   This commit is an implementation of [RFC 1681] which adds support to the
   compiler for first-class user-define custom `#[derive]` modes with a far more
   stable API than plugins have today.
   
   [RFC 1681]: https://github.com/rust-lang/rfcs/blob/master/text/1681-macros-1.1.md
   
   The main features added by this commit are:
   
   * A new `rustc-macro` crate-type. This crate type represents one which will
     provide custom `derive` implementations and perhaps eventually flower into the
     implementation of macros 2.0 as well.
   
   * A new `rustc_macro` crate in the standard distribution. This crate will
     provide the runtime interface between macro crates and the compiler. The API
     here is particularly conservative right now but has quite a bit of room to
     expand into any manner of APIs required by macro authors.
   
   * The ability to load new derive modes through the `#[macro_use]` annotations on
     other crates.
   
   All support added here is gated behind the `rustc_macro` feature gate, both for
   the library support (the `rustc_macro` crate) as well as the language features.
   
   There are a few minor differences from the implementation outlined in the RFC,
   such as the `rustc_macro` crate being available as a dylib and all symbols are
   `dlsym`'d directly instead of having a shim compiled. These should only affect
   the implementation, however, not the public interface.
   
   This commit also ended up touching a lot of code related to `#[derive]`, making
   a few notable changes:
   
   * Recognized derive attributes are no longer desugared to `derive_Foo`. Wasn't
     sure how to keep this behavior and *not* expose it to custom derive.
   
   * Derive attributes no longer have access to unstable features by default, they
     have to opt in on a granular level.
   
   * The `derive(Copy,Clone)` optimization is now done through another "obscure
     attribute" which is just intended to ferry along in the compiler that such an
     optimization is possible. The `derive(PartialEq,Eq)` optimization was also
     updated to do something similar.
   
   ---
   
   One part of this PR which needs to be improved before stabilizing are the errors
   and exact interfaces here. The error messages are relatively poor quality and
   there are surprising spects of this such as `#[derive(PartialEq, Eq, MyTrait)]`
   not working by default. The custom attributes added by the compiler end up
   becoming unstable again when going through a custom impl.
   
   Hopefully though this is enough to start allowing experimentation on crates.io!
   
   syntax-[breaking-change]

   

   alexcrichton committed Sep 2, 2016
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