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_middle/src/middle/region.rs

@@ -148,6 +148,7 @@ rustc_index::newtype_index! {
     /// * The subscope with `first_statement_index == 1` is scope of `c`,
     ///   and thus does not include EXPR_2, but covers the `...`.
     #[derive(HashStable)]
+    #[encodable]
     pub struct FirstStatementIndex {}
 }
 

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 {}

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 {}
 }

compiler/rustc_middle/src/mir/query.rs

@@ -132,6 +132,7 @@ pub struct UnsafetyCheckResult {
 
 rustc_index::newtype_index! {
     #[derive(HashStable)]
+    #[encodable]
     #[debug_format = "_{}"]
     pub struct CoroutineSavedLocal {}
 }

compiler/rustc_middle/src/ty/sty.rs

@@ -1612,6 +1612,7 @@ impl fmt::Debug for EarlyParamRegion {
 rustc_index::newtype_index! {
     /// A **region** (lifetime) **v**ariable **ID**.
     #[derive(HashStable)]
+    #[encodable]
     #[debug_format = "'?{}"]
     pub struct RegionVid {}
 }

compiler/rustc_middle/src/ty/typeck_results.rs

@@ -578,6 +578,7 @@ impl<'a, V> LocalTableInContextMut<'a, V> {
 
 rustc_index::newtype_index! {
     #[derive(HashStable)]
+    #[encodable]
     #[debug_format = "UserType({})"]
     pub struct UserTypeAnnotationIndex {
         const START_INDEX = 0;