HasPlaceholder for None

Author: Centri3

clippy_lints/src/trivial_default_constructed_types.rs

@@ -35,14 +35,15 @@ impl<'tcx> LateLintPass<'tcx> for TrivialDefaultConstructedTypes {
             && let ExprKind::Call(call, _) = expr.kind
             && is_trait_item(cx, call, sym::Default)
         {
+            let mut app = Applicability::MachineApplicable;
             let ret_ty = cx
                 .typeck_results()
                 .expr_ty(call)
                 .fn_sig(cx.tcx)
                 .output()
                 .skip_binder()
                 .peel_refs();
-            if let Some(default) = default_value(cx, ret_ty) && !is_from_proc_macro(cx, expr) {
+            if let Some(default) = default_value(cx, ret_ty, &mut app) && !is_from_proc_macro(cx, expr) {
                 span_lint_and_sugg(
                     cx,
                     TRIVIAL_DEFAULT_CONSTRUCTED_TYPES,
@@ -55,7 +56,7 @@ impl<'tcx> LateLintPass<'tcx> for TrivialDefaultConstructedTypes {
             } else if let ty::Tuple(fields) = ret_ty.kind()
                 && fields.len() <= 3
                 && let Some(fields_default) = fields.iter()
-                    .map(|field| default_value(cx, field))
+                    .map(|field| default_value(cx, field, &mut app))
                     .collect::<Option<Vec<_>>>()
                 && !is_from_proc_macro(cx, expr)
             {
@@ -79,7 +80,7 @@ impl<'tcx> LateLintPass<'tcx> for TrivialDefaultConstructedTypes {
                     Applicability::MachineApplicable,
                 );
             } else if let ty::Array(ty, len) = ret_ty.kind()
-                && let Some(default) = default_value(cx, *ty)
+                && let Some(default) = default_value(cx, *ty, &mut app)
                 && !is_from_proc_macro(cx, expr)
             {
                 span_lint_and_sugg(
@@ -89,18 +90,21 @@ impl<'tcx> LateLintPass<'tcx> for TrivialDefaultConstructedTypes {
                     "constructing a trivial array using `default`",
                     "try",
                     format!("[{default}; {len}]"),
-                    Applicability::MachineApplicable,
+                    app,
                 );
             }
         }
     }
 }
 
 /// Gets the default value of `ty`.
-fn default_value(cx: &LateContext<'_>, ty: Ty<'_>) -> Option<Cow<'static, str>> {
+fn default_value(cx: &LateContext<'_>, ty: Ty<'_>, app: &mut Applicability) -> Option<Cow<'static, str>> {
     match ty.kind() {
-        ty::Adt(def, substs) if let [subst] = substs.as_slice() => {
-            is_lang_item_or_ctor(cx, def.did(), LangItem::Option).then(|| format!("None::<{subst}>").into())
+        ty::Adt(def, _) => {
+            *app = Applicability::HasPlaceholders;
+            // Checking if the generic argument is required would substantially increase the
+            // complexity of this lint, for now, just use a placeholder (`_`).
+            is_lang_item_or_ctor(cx, def.did(), LangItem::Option).then(|| "None::<_>".into())
         },
         ty::Bool => Some("false".into()),
         ty::Str => Some(r#""""#.into()),

tests/ui/trivial_default_constructed_types.fixed

@@ -8,7 +8,7 @@ extern crate proc_macros;
 
 fn main() {
     0u32;
-    let x: Option<u32> = None::<u32>;
+    let x: Option<u32> = None::<_>;
     let y: (usize,) = (0usize,);
     ();
     let x: [u32; 10] = [0u32; 10];

tests/ui/trivial_default_constructed_types.stderr

@@ -10,7 +10,7 @@ error: constructing a trivial type using `default`
   --> $DIR/trivial_default_constructed_types.rs:11:26
    |
 LL |     let x: Option<u32> = Option::default();
-   |                          ^^^^^^^^^^^^^^^^^ help: try: `None::<u32>`
+   |                          ^^^^^^^^^^^^^^^^^ help: try: `None::<_>`
 
 error: constructing a trivial tuple using `default`
   --> $DIR/trivial_default_constructed_types.rs:12:23