Warn write-only fields

compiler/rustc_passes/src/dead.rs

@@ -37,6 +37,19 @@ fn should_explore(tcx: TyCtxt<'_>, hir_id: hir::HirId) -> bool {
     )
 }
 
+fn base_expr<'a>(expr: &'a hir::Expr<'a>) -> &'a hir::Expr<'a> {
+    let mut current = expr;
+    loop {
+        match current.kind {
+            hir::ExprKind::Field(base, ..) => {
+                current = base;
+            }
+            _ => break,
+        }
+    }
+    current
+}
+
 struct MarkSymbolVisitor<'tcx> {
     worklist: Vec<hir::HirId>,
     tcx: TyCtxt<'tcx>,
@@ -263,6 +276,12 @@ 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, ..) => {
+                // Ignore write to field
+                self.visit_expr(base_expr(left));
+                self.visit_expr(right);
+                return;
+            }
             hir::ExprKind::Field(ref lhs, ..) => {
                 self.handle_field_access(&lhs, expr.hir_id);
             }