fix [derive_partial_eq_without_eq] FP on trait projection

Author: J-ZhengLi

clippy_lints/src/derive.rs

@@ -451,8 +451,8 @@ fn check_partial_eq_without_eq<'tcx>(cx: &LateContext<'tcx>, span: Span, trait_r
         && let Some(def_id) = trait_ref.trait_def_id()
         && cx.tcx.is_diagnostic_item(sym::PartialEq, def_id)
         && !has_non_exhaustive_attr(cx.tcx, *adt)
+        && !ty_implements_eq_trait(cx.tcx, ty, eq_trait_def_id)
         && let param_env = param_env_for_derived_eq(cx.tcx, adt.did(), eq_trait_def_id)
-        && !implements_trait_with_env(cx.tcx, param_env, ty, eq_trait_def_id, None, &[])
         // If all of our fields implement `Eq`, we can implement `Eq` too
         && adt
             .all_fields()
@@ -471,6 +471,10 @@ fn check_partial_eq_without_eq<'tcx>(cx: &LateContext<'tcx>, span: Span, trait_r
     }
 }
 
+fn ty_implements_eq_trait<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>, eq_trait_id: DefId) -> bool {
+    tcx.non_blanket_impls_for_ty(eq_trait_id, ty).next().is_some()
+}
+
 /// Creates the `ParamEnv` used for the give type's derived `Eq` impl.
 fn param_env_for_derived_eq(tcx: TyCtxt<'_>, did: DefId, eq_trait_id: DefId) -> ParamEnv<'_> {
     // Initial map from generic index to param def.

tests/ui/derive_partial_eq_without_eq.fixed

@@ -153,4 +153,21 @@ pub enum MissingEqNonExhaustive3 {
     Bar,
 }
 
+mod issue_9413 {
+    pub trait Group {
+        type Element: Eq + PartialEq;
+    }
+
+    pub trait Suite {
+        type Group: Group;
+    }
+
+    #[derive(PartialEq, Eq)]
+    //~^ ERROR: you are deriving `PartialEq` and can implement `Eq`
+    pub struct Foo<C: Suite>(<C::Group as Group>::Element);
+
+    #[derive(PartialEq, Eq)]
+    pub struct Bar<C: Suite>(i32, <C::Group as Group>::Element);
+}
+
 fn main() {}

tests/ui/derive_partial_eq_without_eq.rs

@@ -153,4 +153,21 @@ pub enum MissingEqNonExhaustive3 {
     Bar,
 }
 
+mod issue_9413 {
+    pub trait Group {
+        type Element: Eq + PartialEq;
+    }
+
+    pub trait Suite {
+        type Group: Group;
+    }
+
+    #[derive(PartialEq)]
+    //~^ ERROR: you are deriving `PartialEq` and can implement `Eq`
+    pub struct Foo<C: Suite>(<C::Group as Group>::Element);
+
+    #[derive(PartialEq, Eq)]
+    pub struct Bar<C: Suite>(i32, <C::Group as Group>::Element);
+}
+
 fn main() {}

tests/ui/derive_partial_eq_without_eq.stderr

@@ -67,5 +67,11 @@ error: you are deriving `PartialEq` and can implement `Eq`
 LL |     #[derive(PartialEq)]
    |              ^^^^^^^^^ help: consider deriving `Eq` as well: `PartialEq, Eq`
 
-error: aborting due to 11 previous errors
+error: you are deriving `PartialEq` and can implement `Eq`
+  --> tests/ui/derive_partial_eq_without_eq.rs:165:14
+   |
+LL |     #[derive(PartialEq)]
+   |              ^^^^^^^^^ help: consider deriving `Eq` as well: `PartialEq, Eq`
+
+error: aborting due to 12 previous errors