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, 2 deletions

diff --git a/tests/ui-fulldeps/rustc-dev-remap.only-remap.stderr b/tests/ui-fulldeps/rustc-dev-remap.only-remap.stderr
index f54b6803b34..0c969b9c6d8 100644
--- a/tests/ui-fulldeps/rustc-dev-remap.only-remap.stderr
+++ b/tests/ui-fulldeps/rustc-dev-remap.only-remap.stderr
@@ -9,6 +9,7 @@ LL |     type Result = NotAValidResultType;
              ControlFlow<T>
 note: required by a bound in `rustc_ast::visit::Visitor::Result`
   --> /rustc-dev/xyz/compiler/rustc_ast/src/visit.rs:LL:COL
+   = note: this error originates in the macro `common_visitor_and_walkers` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui-fulldeps/rustc-dev-remap.remap-unremap.stderr b/tests/ui-fulldeps/rustc-dev-remap.remap-unremap.stderr
index 438c23458e2..6ac8c3046f6 100644
--- a/tests/ui-fulldeps/rustc-dev-remap.remap-unremap.stderr
+++ b/tests/ui-fulldeps/rustc-dev-remap.remap-unremap.stderr
@@ -10,8 +10,9 @@ LL |     type Result = NotAValidResultType;
 note: required by a bound in `rustc_ast::visit::Visitor::Result`
   --> $COMPILER_DIR_REAL/rustc_ast/src/visit.rs:LL:COL
    |
-LL |     type Result: VisitorResult = ();
-   |                  ^^^^^^^^^^^^^ required by this bound in `Visitor::Result`
+LL | common_visitor_and_walkers!(Visitor<'a>);
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `Visitor::Result`
+   = note: this error originates in the macro `common_visitor_and_walkers` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to 1 previous error