From 6e12c54bcddb34b085497bf5ca0a6c02387992f0 Mon Sep 17 00:00:00 2001
From: Ada Alakbarova <ada.alakbarova@proton.me>
Date: Thu, 11 Sep 2025 20:09:54 +0200
Subject: [PATCH] fix(len_zero): don't eagerly call `GenericArgs::type_at`

Results in an ICE if the output type has no generic type params
---
 clippy_lints/src/len_zero.rs  |  6 +++---
 tests/ui/crashes/ice-15657.rs | 11 +++++++++++
 2 files changed, 14 insertions(+), 3 deletions(-)
 create mode 100644 tests/ui/crashes/ice-15657.rs

diff --git a/clippy_lints/src/len_zero.rs b/clippy_lints/src/len_zero.rs
index f44a5fdf715e..28a0fbc05115 100644
--- a/clippy_lints/src/len_zero.rs
+++ b/clippy_lints/src/len_zero.rs
@@ -371,9 +371,9 @@ fn parse_len_output<'tcx>(cx: &LateContext<'tcx>, sig: FnSig<'tcx>) -> Option<Le
 
     match *sig.output().kind() {
         ty::Int(_) | ty::Uint(_) => Some(LenOutput::Integral),
-        ty::Adt(adt, subs) if subs.type_at(0).is_integral() => match cx.tcx.get_diagnostic_name(adt.did()) {
-            Some(sym::Option) => Some(LenOutput::Option(adt.did())),
-            Some(sym::Result) => Some(LenOutput::Result(adt.did())),
+        ty::Adt(adt, subs) => match cx.tcx.get_diagnostic_name(adt.did()) {
+            Some(sym::Option) => subs.type_at(0).is_integral().then(|| LenOutput::Option(adt.did())),
+            Some(sym::Result) => subs.type_at(0).is_integral().then(|| LenOutput::Result(adt.did())),
             _ => None,
         },
         _ => None,
diff --git a/tests/ui/crashes/ice-15657.rs b/tests/ui/crashes/ice-15657.rs
new file mode 100644
index 000000000000..c6f6506cd8d0
--- /dev/null
+++ b/tests/ui/crashes/ice-15657.rs
@@ -0,0 +1,11 @@
+//@check-pass
+#![warn(clippy::len_zero)]
+
+pub struct S1;
+pub struct S2;
+
+impl S1 {
+    pub fn len(&self) -> S2 {
+        S2
+    }
+}