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, 4 insertions, 0 deletions

diff --git a/compiler/rustc_middle/src/mir/mod.rs b/compiler/rustc_middle/src/mir/mod.rs
index d4778cdccf3..4e6f5c3fb4e 100644
--- a/compiler/rustc_middle/src/mir/mod.rs
+++ b/compiler/rustc_middle/src/mir/mod.rs
@@ -736,6 +736,7 @@ impl SourceInfo {
 
 rustc_index::newtype_index! {
     #[derive(HashStable)]
+    #[encodable]
     #[debug_format = "_{}"]
     pub struct Local {
         const RETURN_PLACE = 0;
@@ -1171,6 +1172,7 @@ rustc_index::newtype_index! {
     /// [`CriticalCallEdges`]: ../../rustc_const_eval/transform/add_call_guards/enum.AddCallGuards.html#variant.CriticalCallEdges
     /// [guide-mir]: https://rustc-dev-guide.rust-lang.org/mir/
     #[derive(HashStable)]
+    #[encodable]
     #[debug_format = "bb{}"]
     pub struct BasicBlock {
         const START_BLOCK = 0;
@@ -1305,6 +1307,7 @@ impl<'tcx> BasicBlockData<'tcx> {
 
 rustc_index::newtype_index! {
     #[derive(HashStable)]
+    #[encodable]
     #[debug_format = "scope[{}]"]
     pub struct SourceScope {
         const OUTERMOST_SOURCE_SCOPE = 0;
@@ -1533,6 +1536,7 @@ impl UserTypeProjection {
 
 rustc_index::newtype_index! {
     #[derive(HashStable)]
+    #[encodable]
     #[debug_format = "promoted[{}]"]
     pub struct Promoted {}
 }