rustc_public: Don't impl Serialize for ThreadLocalIndex.

It meant that `DefId(0, ThreadLocalIndex)` would go to json as `[0,
null]`, instead of just `0` (like before this PR).

Now it's back to going to `0`, so this PR won't effect JSON consumers.

This can't be achieved with just `#[serde(skip)]`:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=b4aafe4ff5312c4ddedd5e17ee361d5a

Author: aDotInTheVoid

compiler/rustc_public/src/abi.rs

@@ -109,7 +109,7 @@ impl LayoutShape {
     }
 }
 
-#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, Serialize)]
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
 pub struct Layout(usize, ThreadLocalIndex);
 index_impl!(Layout);
 

compiler/rustc_public/src/crate_def.rs

@@ -1,13 +1,11 @@
 //! Module that define a common trait for things that represent a crate definition,
 //! such as, a function, a trait, an enum, and any other definitions.
 
-use serde::Serialize;
-
 use crate::ty::{GenericArgs, Span, Ty, index_impl};
 use crate::{AssocItems, Crate, Symbol, ThreadLocalIndex, with};
 
 /// A unique identification number for each item accessible for the current compilation unit.
-#[derive(Clone, Copy, PartialEq, Eq, Hash, Serialize)]
+#[derive(Clone, Copy, PartialEq, Eq, Hash)]
 pub struct DefId(pub(crate) usize, ThreadLocalIndex);
 index_impl!(DefId);
 

compiler/rustc_public/src/lib.rs

@@ -37,7 +37,10 @@ pub use crate::crate_def::{CrateDef, CrateDefItems, CrateDefType, DefId};
 pub use crate::error::*;
 use crate::mir::mono::StaticDef;
 use crate::mir::{Body, Mutability};
-use crate::ty::{AssocItem, FnDef, ForeignModuleDef, ImplDef, ProvenanceMap, Span, TraitDef, Ty};
+use crate::ty::{
+    AssocItem, FnDef, ForeignModuleDef, ImplDef, ProvenanceMap, Span, TraitDef, Ty,
+    serialize_index_impl,
+};
 use crate::unstable::Stable;
 
 pub mod abi;
@@ -57,8 +60,9 @@ pub mod visitor;
 pub type Symbol = String;
 
 /// The number that identifies a crate.
-#[derive(Clone, Copy, Serialize, PartialEq, Eq, Debug)]
+#[derive(Clone, Copy, PartialEq, Eq, Debug)]
 pub struct CrateNum(pub(crate) usize, ThreadLocalIndex);
+serialize_index_impl!(CrateNum);
 
 impl Debug for DefId {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
@@ -294,7 +298,6 @@ impl rustc_public_bridge::bridge::Allocation<compiler_interface::BridgeTys>
 }
 
 #[derive(Clone, Copy, Hash, PartialEq, Eq, Default)]
-#[derive(serde::Serialize)] // TODO: Don't.
 /// Marker type for indexes into [`TLV`].
 ///
 /// Makes things `!Send`/`!Sync`, so users don't move `rustc_public`` types to

compiler/rustc_public/src/mir/alloc.rs

@@ -47,7 +47,7 @@ impl GlobalAlloc {
 }
 
 /// A unique identification number for each provenance
-#[derive(Clone, Copy, PartialEq, Eq, Debug, Hash, Serialize)]
+#[derive(Clone, Copy, PartialEq, Eq, Debug, Hash)]
 pub struct AllocId(usize, ThreadLocalIndex);
 index_impl!(AllocId);
 

compiler/rustc_public/src/mir/mono.rs

@@ -241,7 +241,7 @@ impl From<StaticDef> for CrateItem {
     }
 }
 
-#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, Serialize)]
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
 pub struct InstanceDef(usize, ThreadLocalIndex);
 index_impl!(InstanceDef);
 

compiler/rustc_public/src/rustc_internal/mod.rs

@@ -53,7 +53,7 @@ where
 }
 
 pub fn crate_num(item: &crate::Crate) -> CrateNum {
-    CrateNum::from_u32(item.id.0)
+    item.id.0.into()
 }
 
 // A thread local variable that stores a pointer to the tables mapping between TyCtxt

compiler/rustc_public/src/ty.rs

@@ -13,7 +13,7 @@ use crate::mir::mono::StaticDef;
 use crate::target::MachineInfo;
 use crate::{Filename, IndexedVal, Opaque, ThreadLocalIndex};
 
-#[derive(Copy, Clone, Eq, PartialEq, Hash, Serialize)]
+#[derive(Copy, Clone, Eq, PartialEq, Hash)]
 pub struct Ty(usize, ThreadLocalIndex);
 
 impl Debug for Ty {
@@ -151,7 +151,7 @@ pub enum TyConstKind {
     ZSTValue(Ty),
 }
 
-#[derive(Copy, Clone, Debug, Eq, PartialEq, Hash, Serialize)]
+#[derive(Copy, Clone, Debug, Eq, PartialEq, Hash)]
 pub struct TyConstId(usize, ThreadLocalIndex);
 
 /// Represents a constant in MIR
@@ -212,7 +212,7 @@ impl MirConst {
     }
 }
 
-#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, Serialize)]
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
 pub struct MirConstId(usize, ThreadLocalIndex);
 
 type Ident = Opaque;
@@ -255,7 +255,7 @@ pub struct Placeholder<T> {
     pub bound: T,
 }
 
-#[derive(Clone, Copy, PartialEq, Eq, Hash, Serialize)]
+#[derive(Clone, Copy, PartialEq, Eq, Hash)]
 pub struct Span(usize, ThreadLocalIndex);
 
 impl Debug for Span {
@@ -1566,9 +1566,23 @@ macro_rules! index_impl {
                 self.0
             }
         }
+        $crate::ty::serialize_index_impl!($name);
+    };
+}
+macro_rules! serialize_index_impl {
+    ($name:ident) => {
+        impl ::serde::Serialize for $name {
+            fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+            where
+                S: ::serde::Serializer,
+            {
+                let n: usize = self.0; // Make sure we're serializing an int.
+                ::serde::Serialize::serialize(&n, serializer)
+            }
+        }
     };
 }
-pub(crate) use index_impl;
+pub(crate) use {index_impl, serialize_index_impl};
 
 index_impl!(TyConstId);
 index_impl!(MirConstId);
@@ -1588,7 +1602,7 @@ index_impl!(Span);
 /// `a` is in the variant with the `VariantIdx` of `0`,
 /// `c` is in the variant with the `VariantIdx` of `1`, and
 /// `g` is in the variant with the `VariantIdx` of `0`.
-#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, Serialize)]
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
 pub struct VariantIdx(usize, ThreadLocalIndex);
 
 index_impl!(VariantIdx);