Tweak the declare_builtin_index macro (#11534)

This allows for specifying custom meta attributes (additionaly derives, docs,
etc). This functionality is not yet used now, but will be in future commits.

Also makes the macro syntax more similar to a regular struct definition, which
allows for easier `grep`-ability.

Author: fitzgen

crates/environ/src/builtin.rs

@@ -255,8 +255,11 @@ macro_rules! foreach_builtin_function {
 /// `ComponentBuiltinFunctionIndex` using the iterator macro, e.g.
 /// `foreach_builtin_function`, as the way to generate accessor methods.
 macro_rules! declare_builtin_index {
-    ($index_name:ident, $iter:ident) => {
-        /// An index type for builtin functions.
+    (
+        $(#[$attr:meta])*
+        pub struct $index_name:ident : $for_each_builtin:ident ;
+    ) => {
+        $(#[$attr])*
         #[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
         pub struct $index_name(u32);
 
@@ -272,7 +275,7 @@ macro_rules! declare_builtin_index {
                 self.0
             }
 
-            $iter!(declare_builtin_index_constructors);
+            $for_each_builtin!(declare_builtin_index_constructors);
         }
     };
 }
@@ -340,7 +343,10 @@ macro_rules! declare_builtin_index_constructors {
 }
 
 // Define `struct BuiltinFunctionIndex`
-declare_builtin_index!(BuiltinFunctionIndex, foreach_builtin_function);
+declare_builtin_index! {
+    /// An index type for builtin functions.
+    pub struct BuiltinFunctionIndex : foreach_builtin_function;
+}
 
 /// Return value of [`BuiltinFunctionIndex::trap_sentinel`].
 pub enum TrapSentinel {

crates/environ/src/component.rs

@@ -205,7 +205,7 @@ macro_rules! foreach_builtin_component_function {
 }
 
 // Define `struct ComponentBuiltinFunctionIndex`
-declare_builtin_index!(
-    ComponentBuiltinFunctionIndex,
-    foreach_builtin_component_function
-);
+declare_builtin_index! {
+    /// An index type for component builtin functions.
+    pub struct ComponentBuiltinFunctionIndex: foreach_builtin_component_function;
+}