Fix new_without_default FP on private type with trait impl (#15782)

Closes rust-lang/rust-clippy#15778

changelog: [`new_without_default`] fix FP on private type with trait
impl

clippy_lints/src/new_without_default.rs

@@ -94,7 +94,7 @@ impl<'tcx> LateLintPass<'tcx> for NewWithoutDefault {
                             return;
                         }
                         if sig.decl.inputs.is_empty()
-                            && cx.effective_visibilities.is_reachable(impl_item.owner_id.def_id)
+                            && cx.effective_visibilities.is_exported(impl_item.owner_id.def_id)
                             && let self_ty = cx.tcx.type_of(item.owner_id).instantiate_identity()
                             && self_ty == return_ty(cx, impl_item.owner_id)
                             && let Some(default_trait_id) = cx.tcx.get_diagnostic_item(sym::Default)

tests/ui/new_without_default.fixed

@@ -322,3 +322,23 @@ where
         Self { _kv: None }
     }
 }
+
+mod issue15778 {
+    pub struct Foo(Vec<i32>);
+
+    impl Foo {
+        pub fn new() -> Self {
+            Self(Vec::new())
+        }
+    }
+
+    impl<'a> IntoIterator for &'a Foo {
+        type Item = &'a i32;
+
+        type IntoIter = std::slice::Iter<'a, i32>;
+
+        fn into_iter(self) -> Self::IntoIter {
+            self.0.as_slice().iter()
+        }
+    }
+}

tests/ui/new_without_default.rs

@@ -265,3 +265,23 @@ where
         Self { _kv: None }
     }
 }
+
+mod issue15778 {
+    pub struct Foo(Vec<i32>);
+
+    impl Foo {
+        pub fn new() -> Self {
+            Self(Vec::new())
+        }
+    }
+
+    impl<'a> IntoIterator for &'a Foo {
+        type Item = &'a i32;
+
+        type IntoIter = std::slice::Iter<'a, i32>;
+
+        fn into_iter(self) -> Self::IntoIter {
+            self.0.as_slice().iter()
+        }
+    }
+}