Improve multiple_inherent_impl

Allow the lint to be ignored on the type definition, or any impl block

clippy_lints/src/inherent_impl.rs

@@ -57,18 +57,34 @@ impl<'tcx> LateLintPass<'tcx> for MultipleInherentImpl {
             ..
         }) = item.kind
         {
-            // Remember for each inherent implementation encountered its id and span,
-            // but filter out implementations that have generic params (type or lifetime)
-            // or are derived from a macro
-            if !in_macro(item.span) && generics.params.is_empty() {
+            // Remember the order inherent implementations are defined. Note `TyCtxt::crate_inherent_impls` does
+            // not have a defined order. TODO: Check if the generic parameters are the same.
+            if !in_macro(item.span)
+                && generics.params.is_empty()
+                && !is_allowed(cx, MULTIPLE_INHERENT_IMPL, item.hir_id())
+            {
                 self.impls.push((item.def_id, item.span));
             }
         }
     }
 
     fn check_crate_post(&mut self, cx: &LateContext<'tcx>, _: &'tcx Crate<'_>) {
         let mut type_map = FxHashMap::default();
-        for (ty, span) in self.impls.iter().map(|&(did, span)| (cx.tcx.type_of(did), span)) {
+        for (ty, span) in self
+            .impls
+            .iter()
+            .map(|&(did, span)| (cx.tcx.type_of(did), span))
+            .filter(|&(ty, _)| {
+                // Filter out any type which has `#[allow(clippy::multiple_inherent_impl)]`
+                ty.ty_adt_def().map_or(false, |adt| {
+                    !is_allowed(
+                        cx,
+                        MULTIPLE_INHERENT_IMPL,
+                        cx.tcx.hir().local_def_id_to_hir_id(adt.did.expect_local()),
+                    )
+                })
+            })
+        {
             match type_map.entry(ty) {
                 Entry::Vacant(e) => {
                     e.insert(span);

tests/ui/impl.rs

@@ -45,4 +45,23 @@ impl WithArgs<u64> {
     fn f3() {}
 }
 
+// Ok, the struct is allowed to have multiple impls.
+#[allow(clippy::multiple_inherent_impl)]
+struct Allowed;
+impl Allowed {}
+impl Allowed {}
+impl Allowed {}
+
+struct AllowedImpl;
+#[allow(clippy::multiple_inherent_impl)]
+impl AllowedImpl {}
+// Ok, the first block is skipped by this lint.
+impl AllowedImpl {}
+
+struct OneAllowedImpl;
+impl OneAllowedImpl {}
+#[allow(clippy::multiple_inherent_impl)]
+impl OneAllowedImpl {}
+impl OneAllowedImpl {} // Lint, only one of the three blocks is allowed.
+
 fn main() {}

tests/ui/impl.stderr

@@ -47,5 +47,17 @@ LL | |     fn f2() {}
 LL | | }
    | |_^
 
-error: aborting due to 3 previous errors
+error: multiple implementations of this structure
+  --> $DIR/impl.rs:65:1
+   |
+LL | impl OneAllowedImpl {} // Lint, only one of the three blocks is allowed.
+   | ^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: first implementation here
+  --> $DIR/impl.rs:62:1
+   |
+LL | impl OneAllowedImpl {}
+   | ^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 4 previous errors