Replace custom_encodable with encodable.

By default, `newtype_index!` types get a default `Encodable`/`Decodable`
impl. You can opt out of this with `custom_encodable`. Opting out is the
opposite to how Rust normally works with autogenerated (derived) impls.

This commit inverts the behaviour, replacing `custom_encodable` with
`encodable` which opts into the default `Encodable`/`Decodable` impl.
Only 23 of the 59 `newtype_index!` occurrences need `encodable`.

Even better, there were eight crates with a dependency on
`rustc_serialize` just from unused default `Encodable`/`Decodable`
impls. This commit removes that dependency from those eight crates.

compiler/rustc_span/src/def_id.rs

@@ -13,7 +13,6 @@ pub type StableCrateIdMap =
     indexmap::IndexMap<StableCrateId, CrateNum, BuildHasherDefault<Unhasher>>;
 
 rustc_index::newtype_index! {
-    #[custom_encodable]
     #[debug_format = "crate{}"]
     pub struct CrateNum {}
 }
@@ -213,7 +212,6 @@ rustc_index::newtype_index! {
     /// A DefIndex is an index into the hir-map for a crate, identifying a
     /// particular definition. It should really be considered an interned
     /// shorthand for a particular DefPath.
-    #[custom_encodable] // (only encodable in metadata)
     #[debug_format = "DefIndex({})"]
     pub struct DefIndex {
         /// The crate root is always assigned index 0 by the AST Map code,
@@ -222,6 +220,7 @@ rustc_index::newtype_index! {
     }
 }
 
+// njn: I don't understand these
 impl<E: Encoder> Encodable<E> for DefIndex {
     default fn encode(&self, _: &mut E) {
         panic!("cannot encode `DefIndex` with `{}`", std::any::type_name::<E>());

compiler/rustc_span/src/hygiene.rs

@@ -60,7 +60,6 @@ pub struct SyntaxContextData {
 
 rustc_index::newtype_index! {
     /// A unique ID associated with a macro invocation and expansion.
-    #[custom_encodable]
     pub struct ExpnIndex {}
 }
 
@@ -80,7 +79,6 @@ impl fmt::Debug for ExpnId {
 
 rustc_index::newtype_index! {
     /// A unique ID associated with a macro invocation and expansion.
-    #[custom_encodable]
     #[no_ord_impl]
     #[debug_format = "expn{}"]
     pub struct LocalExpnId {}