Add #[default] for newtype_index!

compiler/rustc_index_macros/src/lib.rs

@@ -26,6 +26,7 @@ mod newtype;
 ///
 /// Accepted attributes for customization:
 /// - `#[stable_hash]`: derives `StableHash`.
+/// - `#[default]`: derives `std::default::Default` returning `Self::ZERO`.
 /// - `#[encodable]`: derives `Encodable`/`Decodable`.
 /// - `#[orderable]`: derives `PartialOrd`/`Ord`, plus step-related methods.
 /// - `#[debug_format = "Foo({})"]`: derives `Debug` with particular output.

compiler/rustc_index_macros/src/newtype.rs

@@ -24,6 +24,7 @@ impl Parse for Newtype {
         let mut encodable = false;
         let mut ord = false;
         let mut stable_hash = false;
+        let mut default = false;
         let mut gate_rustc_only = quote! {};
         let mut gate_rustc_only_cfg = quote! { all() };
 
@@ -46,6 +47,10 @@ impl Parse for Newtype {
                     stable_hash = true;
                     false
                 }
+                "default" => {
+                    default = true;
+                    false
+                }
                 "max" => {
                     let Meta::NameValue(MetaNameValue { value: Expr::Lit(lit), .. }) = &attr.meta
                     else {
@@ -185,6 +190,19 @@ impl Parse for Newtype {
             quote! {}
         };
 
+        let default_impl = if default {
+            quote! {
+                impl std::default::Default for #name {
+                    #[inline]
+                    fn default() -> Self {
+                        Self::ZERO
+                    }
+                }
+            }
+        } else {
+            quote! {}
+        };
+
         let debug_impl = quote! {
             impl ::std::fmt::Debug for #name {
                 fn fmt(&self, fmt: &mut ::std::fmt::Formatter<'_>) -> ::std::fmt::Result {
@@ -323,6 +341,8 @@ impl Parse for Newtype {
 
             #stable_hash_impl
 
+            #default_impl
+
             impl From<#name> for u32 {
                 #[inline]
                 fn from(v: #name) -> u32 {

compiler/rustc_span/src/hygiene.rs

@@ -134,15 +134,10 @@ impl fmt::Debug for ExpnId {
 rustc_index::newtype_index! {
     /// A unique ID associated with a macro invocation and expansion.
     #[debug_format = "expn{}"]
+    #[default]
     pub struct LocalExpnId {}
 }
 
-impl Default for LocalExpnId {
-    fn default() -> LocalExpnId {
-        LocalExpnId::ROOT
-    }
-}
-
 // To ensure correctness of incremental compilation,
 // `LocalExpnId` must not implement `Ord` or `PartialOrd`.
 // See https://github.com/rust-lang/rust/issues/90317.

compiler/rustc_type_ir/src/lib.rs

@@ -375,6 +375,7 @@ rustc_index::newtype_index! {
     /// type -- an idealized representative of "types in general" that we
     /// use for checking generic functions.
     #[stable_hash]
+    #[default]
     #[encodable]
     #[orderable]
     #[debug_format = "U{}"]
@@ -422,12 +423,6 @@ impl UniverseIndex {
     }
 }
 
-impl Default for UniverseIndex {
-    fn default() -> Self {
-        Self::ROOT
-    }
-}
-
 rustc_index::newtype_index! {
     #[stable_hash]
     #[encodable]