serialize: Fully deprecate the library

This commit completes the deprecation story for the in-tree serialization
library. The compiler will now emit a warning whenever it encounters
`deriving(Encodable)` or `deriving(Decodable)`, and the library itself is now
marked `#[unstable]` for when feature staging is enabled.

All users of serialization can migrate to the `rustc-serialize` crate on
crates.io which provides the exact same interface as the libserialize library
in-tree. The new deriving modes are named `RustcEncodable` and `RustcDecodable`
and require `extern crate "rustc-serialize" as rustc_serialize` at the crate
root in order to expand correctly.

To migrate all crates, add the following to your `Cargo.toml`:

    [dependencies]
    rustc-serialize = "0.1.1"

And then add the following to your crate root:

    extern crate "rustc-serialize" as rustc_serialize;

Finally, rename `Encodable` and `Decodable` deriving modes to `RustcEncodable`
and `RustcDecodable`.

[breaking-change]

1 files changed, 2 insertions, 0 deletions

diff --git a/src/libsyntax/lib.rs b/src/libsyntax/lib.rs
index 5f62c74ef07..d5093c5055c 100644
--- a/src/libsyntax/lib.rs
+++ b/src/libsyntax/lib.rs
@@ -34,6 +34,8 @@ extern crate serialize;
 extern crate term;
 extern crate libc;
 
+extern crate "serialize" as rustc_serialize; // used by deriving
+
 pub mod util {
     pub mod interner;
     #[cfg(test)]