Auto merge of #142706 - fee1-dead-contrib:push-zsznlqyrzsqo, r=oli-obk

completely deduplicate `Visitor` and `MutVisitor`

r? oli-obk

This closes rust-lang/rust#127615.

### Discussion

> * Give every `MutVisitor::visit_*` method a corresponding `flat_map_*` method.

Not every AST node exists in a location where they can be mapped to multiple instances of themselves. Not every AST node exists in a location where they can be removed from existence (e.g. `filter_map_expr`). I don't think this is doable.

> * Give every `MutVisitor::visit_*` method a corresponding `Visitor` method and vice versa

The only three remaining method-level asymmetries after this PR are `visit_stmt` and `visit_nested_use_tree` (only on `Visitor`) and `visit_span` (only on `MutVisitor`).

`visit_stmt` doesn't seem applicable to `MutVisitor` because `walk_flat_map_stmt_kind` will ask `flat_map_item` / `filter_map_expr` to potentially turn a single `Stmt` to multiple based on what a visitor wants. So only using `flat_map_stmt` seems appropriate.

`visit_nested_use_tree` is used for `rustc_resolve` to track stuff. Not useful for `MutVisitor` for now.

`visit_span` is currently not used for `MutVisitor` already, it was just kept in case we want to revive rust-lang/rust#127241. cc `@cjgillot` maybe we could remove for now and re-insert later if we find a use-case? It does involve some extra effort to maintain.

* Remaining FIXMEs

`visit_lifetime` has an extra param for `Visitor` that's not in `MutVisitor`. This is again something only used by `rustc_resolve`. I think we can keep that symmetry for now.

1 files changed, 8 insertions, 5 deletions

diff --git a/compiler/rustc_resolve/src/late.rs b/compiler/rustc_resolve/src/late.rs
index 67dd87b0ae9..ac7bdda4195 100644
--- a/compiler/rustc_resolve/src/late.rs
+++ b/compiler/rustc_resolve/src/late.rs
@@ -1418,11 +1418,14 @@ impl<'ast, 'ra, 'tcx> Visitor<'ast> for LateResolutionVisitor<'_, 'ast, 'ra, 'tc
 
     fn visit_variant(&mut self, v: &'ast Variant) {
         self.resolve_doc_links(&v.attrs, MaybeExported::Ok(v.id));
-        visit::walk_variant(self, v)
-    }
-
-    fn visit_variant_discr(&mut self, discr: &'ast AnonConst) {
-        self.resolve_anon_const(discr, AnonConstKind::EnumDiscriminant);
+        self.visit_id(v.id);
+        walk_list!(self, visit_attribute, &v.attrs);
+        self.visit_vis(&v.vis);
+        self.visit_ident(&v.ident);
+        self.visit_variant_data(&v.data);
+        if let Some(discr) = &v.disr_expr {
+            self.resolve_anon_const(discr, AnonConstKind::EnumDiscriminant);
+        }
     }
 
     fn visit_field_def(&mut self, f: &'ast FieldDef) {