RFC: Boil Down Externs

Dearest Reviewer,

I hope all is well. I have been enjoying rust immensely. Recently I was
working on converting an old ruby project to rust. I was adding my 20ish
crate to my project and after scrolling down I thought, "Do I need all the
`extern crate` lines?".

I set out by making a macro, my first, which was easy and fun. I posted
my macro to reddit wondering if I had missed something. After many
great comments someone suggested an rfc with a wink
face[1](https://www.reddit.com/r/rust/comments/5q7ssv/long_list_of_externs/).

I figured why not. I wrote up this start and then went over to
[internals.rust-lang.org](https://internals.rust-lang.org/t/pre-rfc-boil-down-externs/4676).
Here I received advice on syntax and missing cases to my rfc. Excellent
feedback again!

I hope its considered to allow one line of `extern crate` to accept many
crates. I understand completely if it is not.

Thanks again
Becker

Author: sbeckeriv

text/0000-boil-down-externs.md

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+- Feature Name: boil_down_externs
+- Start Date: 2017-01-26
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+
+This RFC proposes to change the current `extern crate` syntax to allow
+for multiple crates to be listed.
+
+# Motivation
+[motivation]: #motivation
+
+This proposes will improve the ergonomics of the Rust language for
+projects that have many dependencies.
+
+# Detailed design
+[design]: #detailed-design
+
+Implementation would take the current `extern crate` but allow a list of
+crates instead of a single crate. A single create would just be a single
+list item.
+
+Example:
+```
+extern crate {
+   rocket, rocket_contrib, serde_json as json, chrono,
+   dotenv, postgres, r2d2,  r2d2_diesel, tera as template,
+   serde_derive, toml, glob
+};
+```
+
+```
+pub extern crate {rocket, rocket_contrib};
+```
+
+Meta items before the `extern` ie. `#[macro_use]` would be applied to
+all crates listed.
+
+Example:
+```
+#[macro_use]
+extern crate {diesel, diesel_codegen, lazy_static, serde_derive};
+```
+
+Duplicate externs would error like it does currently.
+
+# How We Teach This
+[how-we-teach-this]: #how-we-teach-this
+
+No new names or terminology needed to teach this. Examples should be
+updated to include the list syntax.
+
+Examples in both books, The Rust Programming Language and Rust by
+Example, should be updated where more then one crate is used. A small
+section should be added to http://rustbyexample.com/crates.html
+
+rustfmt should have an opinion how to format the list of crates as well.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+1. Adds multiple ways to do things to a language.
+2. Diffs can mask changes in lists.
+3. Would be replaced buy future improvements to crate ecosystem
+4. Differing opinions on `extern crate` and it's relationship to `mod`
+
+# Alternatives
+[alternatives]: #alternatives
+
+A crate could be release with a macro. Like the example but one that supports meta values.
+
+```
+macro_rules! externs {
+    ( $( $x:ident ),* ) => {
+        $(
+            extern crate $x;
+        )*
+    };
+}
+externs![rocket, rocket_contrib, serde_json]
+```
+
+Supporting meta values could be an issue with the macro.
+Recursion could also be an issue. [Details discussed
+here](https://botbot.me/mozilla/rust-internals/2017-01-29/?msg=80105364&page=1)
+
+# Unresolved questions
+