Fix bad suggestion for generics in new_without_default lint

Author: ThibsG

clippy_lints/src/new_without_default.rs

@@ -1,5 +1,6 @@
 use clippy_utils::diagnostics::span_lint_hir_and_then;
 use clippy_utils::paths;
+use clippy_utils::source::snippet;
 use clippy_utils::sugg::DiagnosticBuilderExt;
 use clippy_utils::{get_trait_def_id, return_ty};
 use if_chain::if_chain;
@@ -62,7 +63,10 @@ impl<'tcx> LateLintPass<'tcx> for NewWithoutDefault {
     #[allow(clippy::too_many_lines)]
     fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx hir::Item<'_>) {
         if let hir::ItemKind::Impl(hir::Impl {
-            of_trait: None, items, ..
+            of_trait: None,
+            ref generics,
+            items,
+            ..
         }) = item.kind
         {
             for assoc_item in items {
@@ -126,6 +130,7 @@ impl<'tcx> LateLintPass<'tcx> for NewWithoutDefault {
                                         }
                                     }
 
+                                    let generics_sugg = snippet(cx, generics.span, "");
                                     span_lint_hir_and_then(
                                         cx,
                                         NEW_WITHOUT_DEFAULT,
@@ -140,7 +145,7 @@ impl<'tcx> LateLintPass<'tcx> for NewWithoutDefault {
                                                 cx,
                                                 item.span,
                                                 "try this",
-                                                &create_new_without_default_suggest_msg(self_ty),
+                                                &create_new_without_default_suggest_msg(self_ty, &generics_sugg),
                                                 Applicability::MaybeIncorrect,
                                             );
                                         },
@@ -155,12 +160,12 @@ impl<'tcx> LateLintPass<'tcx> for NewWithoutDefault {
     }
 }
 
-fn create_new_without_default_suggest_msg(ty: Ty<'_>) -> String {
+fn create_new_without_default_suggest_msg(ty: Ty<'_>, generics_sugg: &str) -> String {
     #[rustfmt::skip]
     format!(
-"impl Default for {} {{
+"impl{} Default for {} {{
     fn default() -> Self {{
         Self::new()
     }}
-}}", ty)
+}}", generics_sugg, ty)
 }

tests/ui/new_without_default.rs

@@ -159,4 +159,19 @@ impl NewNotEqualToDerive {
     }
 }
 
+// see #6933
+pub struct FooGenerics<T>(std::marker::PhantomData<T>);
+impl<T> FooGenerics<T> {
+    pub fn new() -> Self {
+        Self(Default::default())
+    }
+}
+
+pub struct BarGenerics<T>(std::marker::PhantomData<T>);
+impl<T: Copy> BarGenerics<T> {
+    pub fn new() -> Self {
+        Self(Default::default())
+    }
+}
+
 fn main() {}

tests/ui/new_without_default.stderr

@@ -43,7 +43,7 @@ LL | |     }
    |
 help: try this
    |
-LL | impl Default for LtKo<'c> {
+LL | impl<'c> Default for LtKo<'c> {
 LL |     fn default() -> Self {
 LL |         Self::new()
 LL |     }
@@ -67,5 +67,39 @@ LL |     }
 LL | }
    |
 
-error: aborting due to 4 previous errors
+error: you should consider adding a `Default` implementation for `FooGenerics<T>`
+  --> $DIR/new_without_default.rs:165:5
+   |
+LL | /     pub fn new() -> Self {
+LL | |         Self(Default::default())
+LL | |     }
+   | |_____^
+   |
+help: try this
+   |
+LL | impl<T> Default for FooGenerics<T> {
+LL |     fn default() -> Self {
+LL |         Self::new()
+LL |     }
+LL | }
+   |
+
+error: you should consider adding a `Default` implementation for `BarGenerics<T>`
+  --> $DIR/new_without_default.rs:172:5
+   |
+LL | /     pub fn new() -> Self {
+LL | |         Self(Default::default())
+LL | |     }
+   | |_____^
+   |
+help: try this
+   |
+LL | impl<T: Copy> Default for BarGenerics<T> {
+LL |     fn default() -> Self {
+LL |         Self::new()
+LL |     }
+LL | }
+   |
+
+error: aborting due to 6 previous errors