1 files changed, 57 insertions, 0 deletions
diff --git a/src/tools/clippy/clippy_lints/src/partialeq_ne_impl.rs b/src/tools/clippy/clippy_lints/src/partialeq_ne_impl.rs
new file mode 100644
index 000000000..09ac514d0
--- /dev/null
+++ b/src/tools/clippy/clippy_lints/src/partialeq_ne_impl.rs
@@ -0,0 +1,57 @@
+use clippy_utils::diagnostics::span_lint_hir;
+use if_chain::if_chain;
+use rustc_hir::{Impl, Item, ItemKind};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+use rustc_span::sym;
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for manual re-implementations of `PartialEq::ne`.
+    ///
+    /// ### Why is this bad?
+    /// `PartialEq::ne` is required to always return the
+    /// negated result of `PartialEq::eq`, which is exactly what the default
+    /// implementation does. Therefore, there should never be any need to
+    /// re-implement it.
+    ///
+    /// ### Example
+    /// ```rust
+    /// struct Foo;
+    ///
+    /// impl PartialEq for Foo {
+    ///    fn eq(&self, other: &Foo) -> bool { true }
+    ///    fn ne(&self, other: &Foo) -> bool { !(self == other) }
+    /// }
+    /// ```
+    #[clippy::version = "pre 1.29.0"]
+    pub PARTIALEQ_NE_IMPL,
+    complexity,
+    "re-implementing `PartialEq::ne`"
+}
+
+declare_lint_pass!(PartialEqNeImpl => [PARTIALEQ_NE_IMPL]);
+
+impl<'tcx> LateLintPass<'tcx> for PartialEqNeImpl {
+    fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx Item<'_>) {
+        if_chain! {
+            if let ItemKind::Impl(Impl { of_trait: Some(ref trait_ref), items: impl_items, .. }) = item.kind;
+            if !cx.tcx.has_attr(item.def_id.to_def_id(), sym::automatically_derived);
+            if let Some(eq_trait) = cx.tcx.lang_items().eq_trait();
+            if trait_ref.path.res.def_id() == eq_trait;
+            then {
+                for impl_item in *impl_items {
+                    if impl_item.ident.name == sym::ne {
+                        span_lint_hir(
+                            cx,
+                            PARTIALEQ_NE_IMPL,
+                            impl_item.id.hir_id(),
+                            impl_item.span,
+                            "re-implementing `PartialEq::ne` is unnecessary",
+                        );
+                    }
+                }
+            }
+        };
+    }
+}