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, 2 insertions, 0 deletions

diff --git a/compiler/rustc_middle/src/mir/coverage.rs b/compiler/rustc_middle/src/mir/coverage.rs
index 08d377a8695..ccd2cd76ad8 100644
--- a/compiler/rustc_middle/src/mir/coverage.rs
+++ b/compiler/rustc_middle/src/mir/coverage.rs
@@ -17,6 +17,7 @@ rustc_index::newtype_index! {
     /// Note that LLVM handles counter IDs as `uint32_t`, so there is no need
     /// to use a larger representation on the Rust side.
     #[derive(HashStable)]
+    #[encodable]
     #[max = 0xFFFF_FFFF]
     #[debug_format = "CounterId({})"]
     pub struct CounterId {}
@@ -37,6 +38,7 @@ rustc_index::newtype_index! {
     /// Note that LLVM handles expression IDs as `uint32_t`, so there is no need
     /// to use a larger representation on the Rust side.
     #[derive(HashStable)]
+    #[encodable]
     #[max = 0xFFFF_FFFF]
     #[debug_format = "ExpressionId({})"]
     pub struct ExpressionId {}