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.

1 files changed, 3 insertions, 3 deletions

diff --git a/compiler/rustc_index_macros/src/newtype.rs b/compiler/rustc_index_macros/src/newtype.rs
index 2a974fd2628..62de5c7c6c1 100644
--- a/compiler/rustc_index_macros/src/newtype.rs
+++ b/compiler/rustc_index_macros/src/newtype.rs
@@ -22,7 +22,7 @@ impl Parse for Newtype {
         let mut debug_format: Option<Lit> = None;
         let mut max = None;
         let mut consts = Vec::new();
-        let mut encodable = true;
+        let mut encodable = false;
         let mut ord = true;
         let mut gate_rustc_only = quote! {};
         let mut gate_rustc_only_cfg = quote! { all() };
@@ -34,8 +34,8 @@ impl Parse for Newtype {
                     gate_rustc_only_cfg = quote! { feature = "nightly" };
                     false
                 }
-                "custom_encodable" => {
-                    encodable = false;
+                "encodable" => {
+                    encodable = true;
                     false
                 }
                 "no_ord_impl" => {