library/compiler: add `PointeeSized` bounds

As core uses an extern type (`ptr::VTable`), the default `?Sized` to
`MetaSized` migration isn't sufficient, and some code that previously
accepted `VTable` needs relaxed to continue to accept extern types.

Similarly, the compiler uses many extern types in `rustc_codegen_llvm`
and in the `rustc_middle::ty::List` implementation (`OpaqueListContents`)
some bounds must be relaxed to continue to accept these types.

Unfortunately, due to the current inability to relax `Deref::Target`,
some of the bounds in the standard library are forced to be stricter than
they ideally would be.

2 files changed, 19 insertions, 2 deletions

diff --git a/compiler/rustc_serialize/src/lib.rs b/compiler/rustc_serialize/src/lib.rs
index 34be35e36ac..656ecfcab36 100644
--- a/compiler/rustc_serialize/src/lib.rs
+++ b/compiler/rustc_serialize/src/lib.rs
@@ -3,6 +3,7 @@
 // tidy-alphabetical-start
 #![allow(internal_features)]
 #![allow(rustc::internal)]
+#![cfg_attr(not(bootstrap), feature(sized_hierarchy))]
 #![cfg_attr(test, feature(test))]
 #![doc(
     html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/",
@@ -27,3 +28,19 @@ mod serialize;
 pub mod int_overflow;
 pub mod leb128;
 pub mod opaque;
+
+// This has nothing to do with `rustc_serialize` but it is convenient to define it in one place
+// for the rest of the compiler so that `cfg(bootstrap)` doesn't need to be littered throughout
+// the compiler wherever `PointeeSized` would be used. `rustc_serialize` happens to be the deepest
+// crate in the crate graph which uses `PointeeSized`.
+//
+// When bootstrap bumps, remove both the `cfg(not(bootstrap))` and `cfg(bootstrap)` lines below
+// and just import `std::marker::PointeeSized` whereever this item was used.
+
+#[cfg(not(bootstrap))]
+pub use std::marker::PointeeSized;
+
+#[cfg(bootstrap)]
+pub trait PointeeSized {}
+#[cfg(bootstrap)]
+impl<T: ?Sized> PointeeSized for T {}
diff --git a/compiler/rustc_serialize/src/serialize.rs b/compiler/rustc_serialize/src/serialize.rs
index 1eefd76f92b..8940d10696d 100644
--- a/compiler/rustc_serialize/src/serialize.rs
+++ b/compiler/rustc_serialize/src/serialize.rs
@@ -142,7 +142,7 @@ pub trait Decoder {
 ///   `rustc_metadata::rmeta::Lazy`.
 /// * `TyEncodable` should be used for types that are only serialized in crate
 ///   metadata or the incremental cache. This is most types in `rustc_middle`.
-pub trait Encodable<S: Encoder> {
+pub trait Encodable<S: Encoder>: crate::PointeeSized {
     fn encode(&self, s: &mut S);
 }
 
@@ -198,7 +198,7 @@ direct_serialize_impls! {
     char emit_char read_char
 }
 
-impl<S: Encoder, T: ?Sized> Encodable<S> for &T
+impl<S: Encoder, T: ?Sized + crate::PointeeSized> Encodable<S> for &T
 where
     T: Encodable<S>,
 {