Implement AST visitors using a derive macro.

compiler/rustc_macros/src/visitable.rs

@@ -0,0 +1,82 @@
+use quote::quote;
+use synstructure::BindingInfo;
+
+pub(super) fn visitable_derive(mut s: synstructure::Structure<'_>) -> proc_macro2::TokenStream {
+    if let syn::Data::Union(_) = s.ast().data {
+        panic!("cannot derive on union")
+    }
+
+    let has_attr = |bind: &BindingInfo<'_>, name| {
+        let mut found = false;
+        bind.ast().attrs.iter().for_each(|attr| {
+            if !attr.path().is_ident("visitable") {
+                return;
+            }
+            let _ = attr.parse_nested_meta(|nested| {
+                if nested.path.is_ident(name) {
+                    found = true;
+                }
+                Ok(())
+            });
+        });
+        found
+    };
+
+    let get_attr = |bind: &BindingInfo<'_>, name: &str| {
+        let mut content = None;
+        bind.ast().attrs.iter().for_each(|attr| {
+            if !attr.path().is_ident("visitable") {
+                return;
+            }
+            let _ = attr.parse_nested_meta(|nested| {
+                if nested.path.is_ident(name) {
+                    let value = nested.value()?;
+                    let value = value.parse()?;
+                    content = Some(value);
+                }
+                Ok(())
+            });
+        });
+        content
+    };
+
+    s.add_bounds(synstructure::AddBounds::Generics);
+    s.bind_with(|_| synstructure::BindStyle::Ref);
+    let ref_visit = s.each(|bind| {
+        let extra = get_attr(bind, "extra").unwrap_or(quote! {});
+        if has_attr(bind, "ignore") {
+            quote! {}
+        } else {
+            quote! { rustc_ast_ir::try_visit!(crate::visit::Visitable::visit(#bind, __visitor, (#extra))) }
+        }
+    });
+
+    s.bind_with(|_| synstructure::BindStyle::RefMut);
+    let mut_visit = s.each(|bind| {
+        let extra = get_attr(bind, "extra").unwrap_or(quote! {});
+        if has_attr(bind, "ignore") {
+            quote! {}
+        } else {
+            quote! { crate::mut_visit::MutVisitable::visit_mut(#bind, __visitor, (#extra)) }
+        }
+    });
+
+    s.gen_impl(quote! {
+        gen impl<'__ast, __V> crate::visit::Walkable<'__ast, __V> for @Self
+            where __V: crate::visit::Visitor<'__ast>,
+        {
+            fn walk_ref(&'__ast self, __visitor: &mut __V) -> __V::Result {
+                match *self { #ref_visit }
+                <__V::Result as rustc_ast_ir::visit::VisitorResult>::output()
+            }
+        }
+
+        gen impl<__V> crate::mut_visit::MutWalkable<__V> for @Self
+            where __V: crate::mut_visit::MutVisitor,
+        {
+            fn walk_mut(&mut self, __visitor: &mut __V) {
+                match *self { #mut_visit }
+            }
+        }
+    })
+}