Avoid type_complexity from impl Trait bounds

Author: jesco-absolut

clippy_lints/src/types/type_complexity.rs

@@ -42,6 +42,11 @@ impl<'tcx> Visitor<'tcx> for TypeComplexityVisitor {
     }
 
     fn visit_ty(&mut self, ty: &'tcx hir::Ty<'_, AmbigArg>) {
+        // Opaque types cannot be extracted into type aliases on stable. Ignore their
+        // bounds, but keep scoring surrounding type components that can be extracted.
+        if matches!(ty.kind, TyKind::OpaqueDef(..)) {
+            return;
+        }
         let (add_score, sub_nest) = match ty.kind {
             // &x and *x have only small overhead; don't mess with nesting level
             TyKind::Ptr(..) | TyKind::Ref(..) => (1, 0),

tests/ui/type_complexity.rs

@@ -87,3 +87,26 @@ struct D {
 }
 
 fn main() {}
+
+// Should not warn, because factoring `impl Trait` into a type alias is not stable (#17195).
+fn issue_17195<I, J>(
+    left: I,
+    right: J,
+) -> std::iter::Map<std::iter::Zip<I::IntoIter, J::IntoIter>, impl FnMut((I::Item, I::Item)) -> [I::Item; 2]>
+where
+    I: IntoIterator,
+    J: IntoIterator<Item = I::Item>,
+{
+    left.into_iter().zip(right).map(<[I::Item; 2]>::from)
+}
+
+// Complexity inside an opaque type cannot be factored into a type alias either.
+fn complex_opaque_bound() -> impl Fn(Vec<Vec<Box<(u32, u32, u32, u32)>>>) {
+    |_| {}
+}
+
+// The presence of an opaque type must not hide complexity in a sibling type that can be factored out.
+#[expect(clippy::type_complexity)]
+fn complex_after_opaque() -> (impl Iterator<Item = u32>, Vec<Vec<Box<(u32, u32, u32, u32)>>>) {
+    (std::iter::empty(), vec![])
+}