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

Author: khayyamsaleem

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());

tests/ui/len_without_is_empty.rs

@@ -282,4 +282,17 @@ 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
+    }
+}
+
 fn main() {}