Auto merge of rust-lang#86212 - pnkfelix:mainline-targetted-revert-81473-warn-write-only-fields, r=simulacrum

Revert PR 81473 to resolve (on mainline) issues 81626 and 81658.

This is a nightly-targetted variant of PR rust-lang#83171

The intent is to just address issue rust-lang#81658 on all release channels, rather that keep repeatedly reverting PR rust-lang#83171 on beta.

However, our intent is *also* to reland PR rust-lang#83171 after we have addressed issue rust-lang#81658 , most likely by coupling the re-landing of PR rust-lang#83171 with an enhancement like PR rust-lang#83004

Author: bors

compiler/rustc_passes/src/dead.rs

@@ -134,6 +134,7 @@ impl<'tcx> MarkSymbolVisitor<'tcx> {
         }
     }
 
+    #[allow(dead_code)] // FIXME(81658): should be used + lint reinstated after #83171 relands.
     fn handle_assign(&mut self, expr: &'tcx hir::Expr<'tcx>) {
         if self
             .typeck_results()
@@ -150,6 +151,7 @@ impl<'tcx> MarkSymbolVisitor<'tcx> {
         }
     }
 
+    #[allow(dead_code)] // FIXME(81658): should be used + lint reinstated after #83171 relands.
     fn check_for_self_assign(&mut self, assign: &'tcx hir::Expr<'tcx>) {
         fn check_for_self_assign_helper(
             tcx: TyCtxt<'tcx>,
@@ -338,12 +340,6 @@ impl<'tcx> Visitor<'tcx> for MarkSymbolVisitor<'tcx> {
             hir::ExprKind::MethodCall(..) => {
                 self.lookup_and_handle_method(expr.hir_id);
             }
-            hir::ExprKind::Assign(ref left, ref right, ..) => {
-                self.handle_assign(left);
-                self.check_for_self_assign(expr);
-                self.visit_expr(right);
-                return;
-            }
             hir::ExprKind::Field(ref lhs, ..) => {
                 self.handle_field_access(&lhs, expr.hir_id);
             }

src/test/ui/borrowck/borrowck-assign-to-subfield.rs

@@ -1,6 +1,5 @@
 // run-pass
 // pretty-expanded FIXME #23616
-#![allow(dead_code)]
 
 pub fn main() {
     struct A {

src/test/ui/lint/dead-code/self-assign.rs

@@ -1,6 +1,8 @@
 // Test that dead code warnings are issued for superfluous assignments of
 // fields or variables to themselves (issue #75356).
 
+// ignore-test FIXME(81658, 83171)
+
 // check-pass
 #![allow(unused_assignments)]
 #![warn(dead_code)]