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.

2 files changed, 4 insertions, 4 deletions

diff --git a/compiler/rustc_builtin_macros/src/cfg_eval.rs b/compiler/rustc_builtin_macros/src/cfg_eval.rs
index fe44350863c..ec3b87467a9 100644
--- a/compiler/rustc_builtin_macros/src/cfg_eval.rs
+++ b/compiler/rustc_builtin_macros/src/cfg_eval.rs
@@ -161,7 +161,7 @@ impl MutVisitor for CfgEval<'_> {
     }
 
     #[instrument(level = "trace", skip(self))]
-    fn visit_method_receiver_expr(&mut self, expr: &mut P<ast::Expr>) {
+    fn visit_method_receiver_expr(&mut self, expr: &mut ast::Expr) {
         self.0.configure_expr(expr, true);
         mut_visit::walk_expr(self, expr);
     }
diff --git a/compiler/rustc_builtin_macros/src/test_harness.rs b/compiler/rustc_builtin_macros/src/test_harness.rs
index 77aafd3469a..111c85d49eb 100644
--- a/compiler/rustc_builtin_macros/src/test_harness.rs
+++ b/compiler/rustc_builtin_macros/src/test_harness.rs
@@ -6,7 +6,7 @@ use rustc_ast as ast;
 use rustc_ast::entry::EntryPointType;
 use rustc_ast::mut_visit::*;
 use rustc_ast::ptr::P;
-use rustc_ast::visit::{Visitor, walk_item};
+use rustc_ast::visit::Visitor;
 use rustc_ast::{ModKind, attr};
 use rustc_errors::DiagCtxtHandle;
 use rustc_expand::base::{ExtCtxt, ResolverExpand};
@@ -146,11 +146,11 @@ impl<'a> MutVisitor for TestHarnessGenerator<'a> {
         ) = item.kind
         {
             let prev_tests = mem::take(&mut self.tests);
-            walk_item_kind(&mut item.kind, item.span, item.id, &mut item.vis, (), self);
+            ast::mut_visit::walk_item(self, item);
             self.add_test_cases(item.id, span, prev_tests);
         } else {
             // But in those cases, we emit a lint to warn the user of these missing tests.
-            walk_item(&mut InnerItemLinter { sess: self.cx.ext_cx.sess }, &item);
+            ast::visit::walk_item(&mut InnerItemLinter { sess: self.cx.ext_cx.sess }, &item);
         }
     }
 }