rust-clippy/clippy_lints/src/partialeq_ne_impl.rs

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<'a, 'tcx> LateLintPass<'a, 'tcx> for Pass {
    fn check_item(&mut self, cx: &LateContext<'a, 'tcx>, item: &'tcx Item) {
        if_chain! {
            if let ItemImpl(_, _, _, _, Some(ref trait_ref), _, ref impl_items) = item.node;
            if !is_automatically_derived(&*item.attrs);
            if let Some(eq_trait) = cx.tcx.lang_items().eq_trait();
            if trait_ref.path.def.def_id() == eq_trait;
            then {
                for impl_item in impl_items {
                    if impl_item.name == "ne" {
                        span_lint(cx,
                                  PARTIALEQ_NE_IMPL,
                                  impl_item.span,
                                  "re-implementing `PartialEq::ne` is unnecessary")
                    }
                }
            }
        };
    }
}