From 58a06b6a9986149937d7efa97eba83df207a1c7c Mon Sep 17 00:00:00 2001
From: Nicholas Nethercote <n.nethercote@gmail.com>
Date: Mon, 6 May 2024 13:51:15 +1000
Subject: [PATCH] Remove `enum_from_u32`.

It's a macro that just creates an enum with a `from_u32` method. It has
two arms. One is unused and the other has a single use.

This commit inlines that single use and removes the whole macro. This
increases readability because we don't have two different macros
interacting (`enum_from_u32` and `language_item_table`).
---
 compiler/rustc_data_structures/src/lib.rs    |  1 -
 compiler/rustc_data_structures/src/macros.rs | 37 --------------------
 compiler/rustc_hir/src/lang_items.rs         | 32 ++++++++++-------
 3 files changed, 19 insertions(+), 51 deletions(-)
 delete mode 100644 compiler/rustc_data_structures/src/macros.rs

diff --git a/compiler/rustc_data_structures/src/lib.rs b/compiler/rustc_data_structures/src/lib.rs
index 407ee0453e5..85b5a3cdb7c 100644
--- a/compiler/rustc_data_structures/src/lib.rs
+++ b/compiler/rustc_data_structures/src/lib.rs
@@ -62,7 +62,6 @@ pub mod fx;
 pub mod graph;
 pub mod intern;
 pub mod jobserver;
-pub mod macros;
 pub mod marker;
 pub mod memmap;
 pub mod obligation_forest;
diff --git a/compiler/rustc_data_structures/src/macros.rs b/compiler/rustc_data_structures/src/macros.rs
deleted file mode 100644
index e05491f6ff6..00000000000
--- a/compiler/rustc_data_structures/src/macros.rs
+++ /dev/null
@@ -1,37 +0,0 @@
-#[macro_export]
-macro_rules! enum_from_u32 {
-    ($(#[$attr:meta])* pub enum $name:ident {
-        $($(#[$var_attr:meta])* $variant:ident = $e:expr,)*
-    }) => {
-        $(#[$attr])*
-        pub enum $name {
-            $($(#[$var_attr])* $variant = $e),*
-        }
-
-        impl $name {
-            pub fn from_u32(u: u32) -> Option<$name> {
-                $(if u == $name::$variant as u32 {
-                    return Some($name::$variant)
-                })*
-                None
-            }
-        }
-    };
-    ($(#[$attr:meta])* pub enum $name:ident {
-        $($(#[$var_attr:meta])* $variant:ident,)*
-    }) => {
-        $(#[$attr])*
-        pub enum $name {
-            $($(#[$var_attr])* $variant,)*
-        }
-
-        impl $name {
-            pub fn from_u32(u: u32) -> Option<$name> {
-                $(if u == $name::$variant as u32 {
-                    return Some($name::$variant)
-                })*
-                None
-            }
-        }
-    }
-}
diff --git a/compiler/rustc_hir/src/lang_items.rs b/compiler/rustc_hir/src/lang_items.rs
index e870a04127a..c4be67cdd88 100644
--- a/compiler/rustc_hir/src/lang_items.rs
+++ b/compiler/rustc_hir/src/lang_items.rs
@@ -55,21 +55,27 @@ macro_rules! language_item_table {
     (
         $( $(#[$attr:meta])* $variant:ident, $module:ident :: $name:ident, $method:ident, $target:expr, $generics:expr; )*
     ) => {
-
-        rustc_data_structures::enum_from_u32! {
-            /// A representation of all the valid lang items in Rust.
-            #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash, Encodable, Decodable)]
-            pub enum LangItem {
-                $(
-                    #[doc = concat!("The `", stringify!($name), "` lang item.")]
-                    ///
-                    $(#[$attr])*
-                    $variant,
-                )*
-            }
+        /// A representation of all the valid lang items in Rust.
+        #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash, Encodable, Decodable)]
+        pub enum LangItem {
+            $(
+                #[doc = concat!("The `", stringify!($name), "` lang item.")]
+                $(#[$attr])*
+                $variant,
+            )*
         }
 
         impl LangItem {
+            fn from_u32(u: u32) -> Option<LangItem> {
+                // This implementation is clumsy, but makes no assumptions
+                // about how discriminant tags are allocated within the
+                // range `0 .. std::mem::variant_count::<LangItem>()`.
+                $(if u == LangItem::$variant as u32 {
+                    return Some(LangItem::$variant)
+                })*
+                None
+            }
+
             /// Returns the `name` symbol in `#[lang = "$name"]`.
             /// For example, [`LangItem::PartialEq`]`.name()`
             /// would result in [`sym::eq`] since it is `#[lang = "eq"]`.
@@ -147,7 +153,7 @@ language_item_table! {
     Clone,                   sym::clone,               clone_trait,                Target::Trait,          GenericRequirement::None;
     Sync,                    sym::sync,                sync_trait,                 Target::Trait,          GenericRequirement::Exact(0);
     DiscriminantKind,        sym::discriminant_kind,   discriminant_kind_trait,    Target::Trait,          GenericRequirement::None;
-    /// The associated item of the [`DiscriminantKind`] trait.
+    /// The associated item of the `DiscriminantKind` trait.
     Discriminant,            sym::discriminant_type,   discriminant_type,          Target::AssocTy,        GenericRequirement::None;
 
     PointeeTrait,            sym::pointee_trait,       pointee_trait,              Target::Trait,          GenericRequirement::None;