Auto merge of #142706 - fee1-dead-contrib:push-zsznlqyrzsqo, r=<try>

completely deduplicate `Visitor` and `MutVisitor`

r? oli-obk

This closes #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 #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.

Author: rust-bors[bot]

compiler/rustc_ast/src/ast_traits.rs

@@ -321,6 +321,13 @@ impl<Wrapped, Tag> AstNodeWrapper<Wrapped, Tag> {
     }
 }
 
+// FIXME: remove after `stmt_expr_attributes` is stabilized.
+impl<T, Tag> From<AstNodeWrapper<P<T>, Tag>> for AstNodeWrapper<T, Tag> {
+    fn from(value: AstNodeWrapper<P<T>, Tag>) -> Self {
+        AstNodeWrapper { wrapped: *value.wrapped, tag: value.tag }
+    }
+}
+
 impl<Wrapped: HasNodeId, Tag> HasNodeId for AstNodeWrapper<Wrapped, Tag> {
     fn node_id(&self) -> NodeId {
         self.wrapped.node_id()