1 files changed, 72 insertions, 0 deletions
diff --git a/clippy_lints/src/default_constructed_unit_structs.rs b/clippy_lints/src/default_constructed_unit_structs.rs
new file mode 100644
index 00000000000..e529d81a7e9
--- /dev/null
+++ b/clippy_lints/src/default_constructed_unit_structs.rs
@@ -0,0 +1,72 @@
+use clippy_utils::{diagnostics::span_lint_and_sugg, is_from_proc_macro, match_def_path, paths};
+use hir::{def::Res, ExprKind};
+use rustc_errors::Applicability;
+use rustc_hir as hir;
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_middle::ty;
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Check for construction on unit struct using `default`.
+    ///
+    /// ### Why is this bad?
+    /// This adds code complexity and an unnecessary function call.
+    ///
+    /// ### Example
+    /// ```rust
+    /// # use std::marker::PhantomData;
+    /// #[derive(Default)]
+    /// struct S<T> {
+    ///     _marker: PhantomData<T>
+    /// }
+    ///
+    /// let _: S<i32> = S {
+    ///     _marker: PhantomData::default()
+    /// };
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// # use std::marker::PhantomData;
+    /// struct S<T> {
+    ///     _marker: PhantomData<T>
+    /// }
+    ///
+    /// let _: S<i32> = S {
+    ///     _marker: PhantomData
+    /// };
+    /// ```
+    #[clippy::version = "1.71.0"]
+    pub DEFAULT_CONSTRUCTED_UNIT_STRUCTS,
+    complexity,
+    "unit structs can be contructed without calling `default`"
+}
+declare_lint_pass!(DefaultConstructedUnitStructs => [DEFAULT_CONSTRUCTED_UNIT_STRUCTS]);
+
+impl LateLintPass<'_> for DefaultConstructedUnitStructs {
+    fn check_expr<'tcx>(&mut self, cx: &LateContext<'tcx>, expr: &'tcx hir::Expr<'tcx>) {
+        if_chain!(
+            // make sure we have a call to `Default::default`
+            if let hir::ExprKind::Call(fn_expr, &[]) = expr.kind;
+            if let ExprKind::Path(ref qpath@ hir::QPath::TypeRelative(_,_)) = fn_expr.kind;
+            if let Res::Def(_, def_id) = cx.qpath_res(qpath, fn_expr.hir_id);
+            if match_def_path(cx, def_id, &paths::DEFAULT_TRAIT_METHOD);
+            // make sure we have a struct with no fields (unit struct)
+            if let ty::Adt(def, ..) = cx.typeck_results().expr_ty(expr).kind();
+            if def.is_struct();
+            if let var @ ty::VariantDef { ctor: Some((hir::def::CtorKind::Const, _)), .. } = def.non_enum_variant();
+            if !var.is_field_list_non_exhaustive() && !is_from_proc_macro(cx, expr);
+            then {
+                span_lint_and_sugg(
+                    cx,
+                    DEFAULT_CONSTRUCTED_UNIT_STRUCTS,
+                    expr.span.with_lo(qpath.qself_span().hi()),
+                    "use of `default` to create a unit struct",
+                    "remove this call to `default`",
+                    String::new(),
+                    Applicability::MachineApplicable,
+                )
+            }
+        );
+    }
+}