1 files changed, 70 insertions, 0 deletions
diff --git a/src/tools/clippy/clippy_lints/src/needless_update.rs b/src/tools/clippy/clippy_lints/src/needless_update.rs
new file mode 100644
index 000000000..0bd29d177
--- /dev/null
+++ b/src/tools/clippy/clippy_lints/src/needless_update.rs
@@ -0,0 +1,70 @@
+use clippy_utils::diagnostics::span_lint;
+use rustc_hir::{Expr, ExprKind};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_middle::ty;
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+
+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.
+    ///
+    /// ### Example
+    /// ```rust
+    /// # struct Point {
+    /// #     x: i32,
+    /// #     y: i32,
+    /// #     z: i32,
+    /// # }
+    /// # let zero_point = Point { x: 0, y: 0, z: 0 };
+    /// Point {
+    ///     x: 1,
+    ///     y: 1,
+    ///     z: 1,
+    ///     ..zero_point
+    /// };
+    /// ```
+    ///
+    /// Use instead:
+    /// ```rust,ignore
+    /// // Missing field `z`
+    /// Point {
+    ///     x: 1,
+    ///     y: 1,
+    ///     ..zero_point
+    /// };
+    /// ```
+    #[clippy::version = "pre 1.29.0"]
+    pub NEEDLESS_UPDATE,
+    complexity,
+    "using `Foo { ..base }` when there are no missing fields"
+}
+
+declare_lint_pass!(NeedlessUpdate => [NEEDLESS_UPDATE]);
+
+impl<'tcx> LateLintPass<'tcx> for NeedlessUpdate {
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
+        if let ExprKind::Struct(_, fields, Some(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()
+                    && !def.variant(0_usize.into()).is_field_list_non_exhaustive()
+                {
+                    span_lint(
+                        cx,
+                        NEEDLESS_UPDATE,
+                        base.span,
+                        "struct update has no effect, all the fields in the struct have already been specified",
+                    );
+                }
+            }
+        }
+    }
+}