make needless_update ignore non_exhaustive structs

clippy_lints/src/needless_update.rs

@@ -8,6 +8,9 @@ declare_clippy_lint! {
  /// **What it does:** Checks for needlessly including a base struct on update
  /// when all fields are changed anyway.
  ///
+ /// This lint is not applied to structs marked with
+ /// [non_exhaustive](https://doc.rust-lang.org/reference/attributes/type_system.html).
+ ///
  /// **Why is this bad?** This will cost resources (because the base has to be
  /// somewhere), and make the code less readable.
  ///
@@ -49,7 +52,9 @@ impl<'tcx> LateLintPass<'tcx> for NeedlessUpdate {
  if let ExprKind::Struct(_, ref fields, Some(ref base)) = expr.kind {
  let ty = cx.typeck_results().expr_ty(expr);
  if let ty::Adt(def, _) = ty.kind() {
- if fields.len() == def.non_enum_variant().fields.len() {
+ if fields.len() == def.non_enum_variant().fields.len()
+ && !def.variants[0_usize.into()].is_field_list_non_exhaustive()
+ {
  span_lint(
  cx,
  NEEDLESS_UPDATE,

tests/ui/needless_update.rs

@@ -6,9 +6,20 @@ struct S {
  pub b: i32,
 }
 
+#[non_exhaustive]
+struct T {
+ pub x: i32,
+ pub y: i32,
+}
+
 fn main() {
  let base = S { a: 0, b: 0 };
  S { ..base }; // no error
  S { a: 1, ..base }; // no error
  S { a: 1, b: 1, ..base };
+
+ let base = T { x: 0, y: 0 };
+ T { ..base }; // no error
+ T { x: 1, ..base }; // no error
+ T { x: 1, y: 1, ..base }; // no error
 }

tests/ui/needless_update.stderr

@@ -1,5 +1,5 @@
 error: struct update has no effect, all the fields in the struct have already been specified
- --> $DIR/needless_update.rs:13:23
+ --> $DIR/needless_update.rs:19:23
  |
 LL | S { a: 1, b: 1, ..base };
  | ^^^^