implement 'Re-implementing `PartialEq::ne`' lint

closes #86

CHANGELOG.md

@@ -319,6 +319,7 @@ All notable changes to this project will be documented in this file.
 [`out_of_bounds_indexing`]: https://github.com/Manishearth/rust-clippy/wiki#out_of_bounds_indexing
 [`overflow_check_conditional`]: https://github.com/Manishearth/rust-clippy/wiki#overflow_check_conditional
 [`panic_params`]: https://github.com/Manishearth/rust-clippy/wiki#panic_params
+[`partialeq_ne_impl`]: https://github.com/Manishearth/rust-clippy/wiki#partialeq_ne_impl
 [`precedence`]: https://github.com/Manishearth/rust-clippy/wiki#precedence
 [`print_stdout`]: https://github.com/Manishearth/rust-clippy/wiki#print_stdout
 [`print_with_newline`]: https://github.com/Manishearth/rust-clippy/wiki#print_with_newline

README.md

@@ -174,7 +174,7 @@ You can check out this great service at [clippy.bashy.io](https://clippy.bashy.i
 
 ## Lints
 
-There are 175 lints included in this crate:
+There are 176 lints included in this crate:
 
 name                                                                                                                   | default | triggers on
 -----------------------------------------------------------------------------------------------------------------------|---------|----------------------------------------------------------------------------------------------------------------------------------
@@ -293,6 +293,7 @@ name
 [out_of_bounds_indexing](https://github.com/Manishearth/rust-clippy/wiki#out_of_bounds_indexing)                       | deny    | out of bounds constant indexing
 [overflow_check_conditional](https://github.com/Manishearth/rust-clippy/wiki#overflow_check_conditional)               | warn    | overflow checks inspired by C which are likely to panic
 [panic_params](https://github.com/Manishearth/rust-clippy/wiki#panic_params)                                           | warn    | missing parameters in `panic!` calls
+[partialeq_ne_impl](https://github.com/Manishearth/rust-clippy/wiki#partialeq_ne_impl)                                 | warn    | re-implementing `PartialEq::ne`
 [precedence](https://github.com/Manishearth/rust-clippy/wiki#precedence)                                               | warn    | operations where precedence may be unclear
 [print_stdout](https://github.com/Manishearth/rust-clippy/wiki#print_stdout)                                           | allow   | printing on stdout
 [print_with_newline](https://github.com/Manishearth/rust-clippy/wiki#print_with_newline)                               | warn    | using `print!()` with a format string that ends in a newline

clippy_lints/src/lib.rs

@@ -111,6 +111,7 @@ pub mod ok_if_let;
 pub mod open_options;
 pub mod overflow_check_conditional;
 pub mod panic;
+pub mod partialeq_ne_impl;
 pub mod precedence;
 pub mod print;
 pub mod ptr;
@@ -259,6 +260,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry) {
     reg.register_late_lint_pass(box missing_doc::MissingDoc::new());
     reg.register_late_lint_pass(box ok_if_let::Pass);
     reg.register_late_lint_pass(box if_let_redundant_pattern_matching::Pass);
+    reg.register_late_lint_pass(box partialeq_ne_impl::Pass);
 
     reg.register_lint_group("clippy_restrictions", vec![
         arithmetic::FLOAT_ARITHMETIC,
@@ -415,6 +417,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry) {
         open_options::NONSENSICAL_OPEN_OPTIONS,
         overflow_check_conditional::OVERFLOW_CHECK_CONDITIONAL,
         panic::PANIC_PARAMS,
+        partialeq_ne_impl::PARTIALEQ_NE_IMPL,
         precedence::PRECEDENCE,
         print::PRINT_WITH_NEWLINE,
         ptr::CMP_NULL,

clippy_lints/src/partialeq_ne_impl.rs

@@ -0,0 +1,55 @@
+use rustc::lint::*;
+use rustc::hir::*;
+use utils::{is_automatically_derived, span_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.
+///
+/// **Known problems:** None.
+///
+/// **Example:**
+/// ```rust
+/// struct Foo;
+///
+/// impl PartialEq for Foo {
+///    fn eq(&self, other: &Foo) -> bool { ... }
+///    fn ne(&self, other: &Foo) -> bool { !(self == other) }
+/// }
+/// ```
+declare_lint! {
+    pub PARTIALEQ_NE_IMPL,
+    Warn,
+    "re-implementing `PartialEq::ne`"
+}
+
+#[derive(Clone, Copy)]
+pub struct Pass;
+
+impl LintPass for Pass {
+    fn get_lints(&self) -> LintArray {
+        lint_array!(PARTIALEQ_NE_IMPL)
+    }
+}
+
+impl LateLintPass for Pass {
+    fn check_item(&mut self, cx: &LateContext, item: &Item) {
+        if_let_chain! {[
+            let ItemImpl(_, _, _, Some(ref trait_ref), _, ref impl_items) = item.node,
+            !is_automatically_derived(&*item.attrs),
+            cx.tcx.expect_def(trait_ref.ref_id).def_id() == cx.tcx.lang_items.eq_trait().unwrap(),
+        ], {
+            for impl_item in impl_items {
+                if &*impl_item.name.as_str() == "ne" {
+                    span_lint(cx,
+                              PARTIALEQ_NE_IMPL,
+                              impl_item.span,
+                              "re-implementing `PartialEq::ne` is unnecessary")
+                }
+            }
+        }};
+    }
+}

tests/compile-fail/partialeq_ne_impl.rs

@@ -0,0 +1,15 @@
+#![feature(plugin)]
+#![plugin(clippy)]
+
+#![deny(warnings)]
+#![allow(dead_code)]
+
+struct Foo;
+
+impl PartialEq for Foo {
+    fn eq(&self, _: &Foo) -> bool { true }
+    fn ne(&self, _: &Foo) -> bool { false }
+    //~^ ERROR re-implementing `PartialEq::ne` is unnecessary
+}
+
+fn main() {}