From 26c0f97579763363310bc32cf6d8ef4cd6f5588f Mon Sep 17 00:00:00 2001
From: y21 <30553356+y21@users.noreply.github.com>
Date: Sat, 2 Sep 2023 21:55:44 +0200
Subject: [PATCH] [`len_without_is_empty`]: follow type alias

---
 clippy_lints/src/len_zero.rs         |  8 ++++++++
 tests/ui/len_without_is_empty.rs     | 23 +++++++++++++++++++++++
 tests/ui/len_without_is_empty.stderr |  8 +++++++-
 3 files changed, 38 insertions(+), 1 deletion(-)

diff --git a/clippy_lints/src/len_zero.rs b/clippy_lints/src/len_zero.rs
index deba232bdd2..c06b35ca0da 100644
--- a/clippy_lints/src/len_zero.rs
+++ b/clippy_lints/src/len_zero.rs
@@ -424,6 +424,14 @@ fn check_for_is_empty(
     item_name: Symbol,
     item_kind: &str,
 ) {
+    // Implementor may be a type alias, in which case we need to get the `DefId` of the aliased type to
+    // find the correct inherent impls.
+    let impl_ty = if let Some(adt) = cx.tcx.type_of(impl_ty).skip_binder().ty_adt_def() {
+        adt.did()
+    } else {
+        return;
+    };
+
     let is_empty = Symbol::intern("is_empty");
     let is_empty = cx
         .tcx
diff --git a/tests/ui/len_without_is_empty.rs b/tests/ui/len_without_is_empty.rs
index ac6c3e06365..d623601110e 100644
--- a/tests/ui/len_without_is_empty.rs
+++ b/tests/ui/len_without_is_empty.rs
@@ -436,4 +436,27 @@ impl DifferingErrors {
     }
 }
 
+// Issue #11165
+pub struct Aliased1;
+pub type Alias1 = Aliased1;
+
+impl Alias1 {
+    pub fn len(&self) -> usize {
+        todo!()
+    }
+
+    pub fn is_empty(&self) -> bool {
+        todo!()
+    }
+}
+
+pub struct Aliased2;
+pub type Alias2 = Aliased2;
+impl Alias2 {
+    pub fn len(&self) -> usize {
+        //~^ ERROR: type `Alias2` has a public `len` method, but no `is_empty` method
+        todo!()
+    }
+}
+
 fn main() {}
diff --git a/tests/ui/len_without_is_empty.stderr b/tests/ui/len_without_is_empty.stderr
index 1042ea2e1b3..50a8c8086b7 100644
--- a/tests/ui/len_without_is_empty.stderr
+++ b/tests/ui/len_without_is_empty.stderr
@@ -139,5 +139,11 @@ error: struct `AsyncResultLenWithoutIsEmpty` has a public `len` method, but no `
 LL |     pub async fn len(&self) -> Result<usize, ()> {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 15 previous errors
+error: type `Alias2` has a public `len` method, but no `is_empty` method
+  --> $DIR/len_without_is_empty.rs:456:5
+   |
+LL |     pub fn len(&self) -> usize {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 16 previous errors