Auto merge of #31400 - durka:civilized-deriving, r=alexcrichton

You can `#[derive(FromPrimitive)]`, but it [fails later in the compile](https://play.rust-lang.org/?gist=82cb8ad2fac49e3fe472&version=stable) due to hardcoding `std::num::FromPrimitive` which [was removed](eeb9488) (for some reason Github doesn't show `FromPrimitive` in the diff, but `git show` does).

Anyway, this PR removes the code. I didn't mark it as a breaking change, even though [this extremely contrived code using highly unstable features](https://play.rust-lang.org/?gist=1e1b1bbff962837b228a&version=nightly) is broken by it -- should I?

src/libsyntax_ext/deriving/mod.rs

@@ -63,7 +63,6 @@ pub mod decodable;
 pub mod hash;
 pub mod debug;
 pub mod default;
-pub mod primitive;
 
 #[path="cmp/partial_eq.rs"]
 pub mod partial_eq;
@@ -178,8 +177,6 @@ derive_traits! {
 
     "Default" => default::expand_deriving_default,
 
-    "FromPrimitive" => primitive::expand_deriving_from_primitive,
-
     "Send" => bounds::expand_deriving_unsafe_bound,
     "Sync" => bounds::expand_deriving_unsafe_bound,
     "Copy" => bounds::expand_deriving_copy,

src/test/compile-fail/deriving-primitive.rs

@@ -1,4 +1,4 @@
-// Copyright 2013 The Rust Project Developers. See the COPYRIGHT
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
 // file at the top-level directory of this distribution and at
 // http://rust-lang.org/COPYRIGHT.
 //
@@ -8,27 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-use std::num::FromPrimitive;
-use std::isize;
+#[derive(FromPrimitive)] //~ERROR `#[derive]` for custom traits is not stable
+enum Foo {}
 
-#[derive(FromPrimitive)]
-struct A { x: isize }
-//~^^ ERROR `FromPrimitive` cannot be derived for structs
-//~^^^ ERROR `FromPrimitive` cannot be derived for structs
+fn main() {}
 
-#[derive(FromPrimitive)]
-struct B(isize);
-//~^^ ERROR `FromPrimitive` cannot be derived for structs
-//~^^^ ERROR `FromPrimitive` cannot be derived for structs
-
-#[derive(FromPrimitive)]
-enum C { Foo(isize), Bar(usize) }
-//~^^ ERROR `FromPrimitive` cannot be derived for enums with non-unit variants
-//~^^^ ERROR `FromPrimitive` cannot be derived for enums with non-unit variants
-
-#[derive(FromPrimitive)]
-enum D { Baz { x: isize } }
-//~^^ ERROR `FromPrimitive` cannot be derived for enums with non-unit variants
-//~^^^ ERROR `FromPrimitive` cannot be derived for enums with non-unit variants
-
-pub fn main() {}