Auto merge of rust-lang#73851 - matthewjasper:serialize-not-special, r=oli-obk

Remove most specialization use in serialization

Switching from specialization to min_specialization in the compiler made the unsoundness of how we used these traits pretty clear. This changes how the `Encodable` and `Decodable` traits work to be more friendly for types need a `TyCtxt` to deserialize.

The alternative design of having both `Encodable` and `TyEncodable` traits was considered, but doesn't really work because the following impls would conflict:

```
impl<E: Ecodable> TyEncodable for Encodable
impl<E: TyEcodable> TyEncodable for [E]
```

## How-to guide

- `Rustc(De|En)codable` is now spelled `Ty(De|En)coable` in `rustc_middle`, `Metadata(En|De)codable` in `rustc_metadata` where needed, and `(De|En)codable` everywhere else.
- Manual implementations of `(De|En)codable` shouldn't be much different.
- If you're adding a new interned type that needs to be en/decodable then the simplest thing way to handle this is:
    - Have the type be a wrapper around a reference to the interned data (i.e. do what `ty::Predicate` does, and not what all of the other interned types do)
    - Derive `Ty(En|De)codable` on the inner type
    - Implement `Encodable<impl TyEncoder>` by forwarding to the inner type.
    - Implement `Decodable<impl TyDecoder>` by decoding the inner type and then creating the wrapper around that (using the `tcx` from the decoder as needed).

cc @rust-lang/compiler for opinions on this change
r? @oli-obk

Author: bors

Cargo.lock

@@ -3334,6 +3334,7 @@ dependencies = [
  "rustc_hir",
  "rustc_incremental",
  "rustc_index",
+ "rustc_macros",
  "rustc_middle",
  "rustc_serialize",
  "rustc_session",
@@ -3364,6 +3365,7 @@ dependencies = [
  "rustc-rayon-core",
  "rustc_graphviz",
  "rustc_index",
+ "rustc_macros",
  "rustc_serialize",
  "smallvec 1.4.0",
  "stable_deref_trait",
@@ -3416,6 +3418,7 @@ dependencies = [
  "annotate-snippets 0.8.0",
  "atty",
  "rustc_data_structures",
+ "rustc_macros",
  "rustc_serialize",
  "rustc_span",
  "termcolor",
@@ -3437,6 +3440,7 @@ dependencies = [
  "rustc_errors",
  "rustc_feature",
  "rustc_lexer",
+ "rustc_macros",
  "rustc_parse",
  "rustc_serialize",
  "rustc_session",
@@ -3499,6 +3503,7 @@ dependencies = [
  "rustc_fs_util",
  "rustc_graphviz",
  "rustc_hir",
+ "rustc_macros",
  "rustc_middle",
  "rustc_serialize",
  "rustc_session",
@@ -3511,6 +3516,7 @@ name = "rustc_index"
 version = "0.0.0"
 dependencies = [
  "arrayvec 0.5.1",
+ "rustc_macros",
  "rustc_serialize",
 ]
 
@@ -3640,6 +3646,7 @@ dependencies = [
  "rustc_hir",
  "rustc_hir_pretty",
  "rustc_index",
+ "rustc_macros",
  "rustc_middle",
  "rustc_serialize",
  "rustc_session",
@@ -3815,6 +3822,7 @@ dependencies = [
  "rustc_data_structures",
  "rustc_errors",
  "rustc_index",
+ "rustc_macros",
  "rustc_serialize",
  "rustc_span",
  "smallvec 1.4.0",
@@ -3869,6 +3877,7 @@ name = "rustc_serialize"
 version = "0.0.0"
 dependencies = [
  "indexmap",
+ "rustc_macros",
  "smallvec 1.4.0",
 ]
 
@@ -3884,6 +3893,7 @@ dependencies = [
  "rustc_errors",
  "rustc_feature",
  "rustc_fs_util",
+ "rustc_macros",
  "rustc_serialize",
  "rustc_span",
  "rustc_target",

src/librustc_arena/lib.rs

@@ -611,11 +611,7 @@ macro_rules! which_arena_for_type {
 
 #[macro_export]
 macro_rules! declare_arena {
-    // This macro has to take the same input as
-    // `impl_arena_allocatable_decoders` which requires a second version of
-    // each type. We ignore that type until we can fix
-    // `impl_arena_allocatable_decoders`.
-    ([], [$($a:tt $name:ident: $ty:ty, $_gen_ty:ty;)*], $tcx:lifetime) => {
+    ([], [$($a:tt $name:ident: $ty:ty,)*], $tcx:lifetime) => {
         #[derive(Default)]
         pub struct Arena<$tcx> {
             pub dropless: $crate::DroplessArena,