Remove visitor

Author: Kobzol

compiler/rustc_builtin_macros/src/deriving/from.rs

@@ -1,13 +1,13 @@
 use rustc_ast as ast;
-use rustc_ast::{FieldDef, Item, ItemKind, VariantData};
+use rustc_ast::{ItemKind, VariantData};
 use rustc_expand::base::{Annotatable, ExtCtxt};
-use rustc_span::{Ident, Span, kw, sym};
+use rustc_span::{kw, sym, Ident, Span};
 use thin_vec::thin_vec;
 
 use crate::deriving::generic::ty::{Bounds, Path, PathKind, Ty};
 use crate::deriving::generic::{
-    BlockOrExpr, FieldlessVariantsStrategy, MethodDef, SubstructureFields, TraitDef,
-    combine_substructure,
+    combine_substructure, BlockOrExpr, FieldlessVariantsStrategy, MethodDef, SubstructureFields,
+    TraitDef,
 };
 use crate::deriving::pathvec_std;
 use crate::errors;
@@ -22,12 +22,23 @@ pub(crate) fn expand_deriving_from(
     push: &mut dyn FnMut(Annotatable),
     is_const: bool,
 ) {
-    let mut visitor = ExtractNonSingleFieldStruct { cx, field: None };
-    item.visit_with(&mut visitor);
+    let field = match &item {
+        Annotatable::Item(item) => match &item.kind {
+            ItemKind::Struct(_, _, data) => match data.fields() {
+                [field] => Some(field.clone()),
+                _ => None,
+            },
+            _ => None,
+        },
+        _ => None,
+    };
 
     // Make sure that the derive is only invoked on single-field [tuple] structs.
     // From this point below, we know that there is exactly one field.
-    let Some(field) = visitor.field else { return };
+    let Some(field) = field else {
+        cx.dcx().emit_err(errors::DeriveFromWrongTarget { span });
+        return;
+    };
 
     let path = Path::new_(
         pathvec_std!(convert::From),
@@ -98,23 +109,3 @@ pub(crate) fn expand_deriving_from(
 
     from_trait_def.expand(cx, mitem, item, push);
 }
-
-struct ExtractNonSingleFieldStruct<'a, 'b> {
-    cx: &'a ExtCtxt<'b>,
-    field: Option<FieldDef>,
-}
-
-impl<'a, 'b> rustc_ast::visit::Visitor<'a> for ExtractNonSingleFieldStruct<'a, 'b> {
-    fn visit_item(&mut self, item: &'a Item) -> Self::Result {
-        match &item.kind {
-            ItemKind::Struct(_, _, data) => match data.fields() {
-                [field] => self.field = Some(field.clone()),
-                _ => {}
-            },
-            _ => {}
-        };
-        if self.field.is_none() {
-            self.cx.dcx().emit_err(errors::DeriveFromWrongTarget { span: item.span });
-        }
-    }
-}