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

Revert "Add missing brace"

This reverts commit 85ad773.

Revert "Simplify base_expr"

This reverts commit 899aae4.

Revert "Warn write-only fields"

This reverts commit d3c69a4.

compiler/rustc_passes/src/dead.rs

@@ -338,12 +338,6 @@ 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 {