From 2fd94a4e01d9e8049df3651d0b29b3964cf235d8 Mon Sep 17 00:00:00 2001
From: ksaleem <ksaleem@digitalocean.com>
Date: Sun, 29 Jan 2023 13:48:06 -0500
Subject: [PATCH] prevents `len_without_is_empty` from yielding positive when
 `len` takes more than just `&self` in non-standard implementations.

changelog: Fix [`len_without_is_empty`] false positive when len has a
non-standard method signature

Fixes #9520
---
 clippy_lints/src/len_zero.rs     |  1 +
 tests/ui/len_without_is_empty.rs | 46 ++++++++++++++++++++++++++++++++
 2 files changed, 47 insertions(+)

diff --git a/clippy_lints/src/len_zero.rs b/clippy_lints/src/len_zero.rs
index 3c70c9cf19a..920ab7f0633 100644
--- a/clippy_lints/src/len_zero.rs
+++ b/clippy_lints/src/len_zero.rs
@@ -135,6 +135,7 @@ impl<'tcx> LateLintPass<'tcx> for LenZero {
             if item.ident.name == sym::len;
             if let ImplItemKind::Fn(sig, _) = &item.kind;
             if sig.decl.implicit_self.has_implicit_self();
+            if sig.decl.inputs.len() == 1;
             if cx.effective_visibilities.is_exported(item.owner_id.def_id);
             if matches!(sig.decl.output, FnRetTy::Return(_));
             if let Some(imp) = get_parent_as_impl(cx.tcx, item.hir_id());
diff --git a/tests/ui/len_without_is_empty.rs b/tests/ui/len_without_is_empty.rs
index 78397c2af34..b5dec6c46bd 100644
--- a/tests/ui/len_without_is_empty.rs
+++ b/tests/ui/len_without_is_empty.rs
@@ -282,4 +282,50 @@ impl AsyncLen {
     }
 }
 
+// issue #9520
+pub struct NonStandardLenAndIsEmptySignature;
+impl NonStandardLenAndIsEmptySignature {
+    // don't lint
+    pub fn len(&self, something: usize) -> usize {
+        something
+    }
+
+    pub fn is_empty(&self, something: usize) -> bool {
+        something == 0
+    }
+}
+
+// test case for #9520 with generics in the function signature
+pub trait TestResource {
+    type NonStandardSignatureWithGenerics: Copy;
+    fn lookup_content(&self, item: Self::NonStandardSignatureWithGenerics) -> Result<Option<&[u8]>, String>;
+}
+pub struct NonStandardSignatureWithGenerics(u32);
+impl NonStandardSignatureWithGenerics {
+    pub fn is_empty<T, U>(self, resource: &T) -> bool
+    where
+        T: TestResource<NonStandardSignatureWithGenerics = U>,
+        U: Copy + From<NonStandardSignatureWithGenerics>,
+    {
+        if let Ok(Some(content)) = resource.lookup_content(self.into()) {
+            content.is_empty()
+        } else {
+            true
+        }
+    }
+
+    // test case for #9520 with generics in the function signature
+    pub fn len<T, U>(self, resource: &T) -> usize
+    where
+        T: TestResource<NonStandardSignatureWithGenerics = U>,
+        U: Copy + From<NonStandardSignatureWithGenerics>,
+    {
+        if let Ok(Some(content)) = resource.lookup_content(self.into()) {
+            content.len()
+        } else {
+            0_usize
+        }
+    }
+}
+
 fn main() {}