add more negative test cases, replace an unnecessary def path with diag item

Author: y21

clippy_lints/src/unconditional_recursion.rs

@@ -1,6 +1,5 @@
 use clippy_utils::diagnostics::span_lint_and_then;
-use clippy_utils::{expr_or_init, fn_def_id_with_node_args, get_trait_def_id, path_def_id};
-use itertools::Itertools;
+use clippy_utils::{expr_or_init, fn_def_id_with_node_args, path_def_id};
 use rustc_ast::BinOpKind;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_hir as hir;
@@ -311,10 +310,9 @@ where
 }
 
 impl UnconditionalRecursion {
-    #[allow(clippy::unnecessary_def_path)]
     fn init_default_impl_for_type_if_needed(&mut self, cx: &LateContext<'_>) {
         if self.default_impl_for_type.is_empty()
-            && let Some(default_trait_id) = get_trait_def_id(cx, &["core", "default", "Default"])
+            && let Some(default_trait_id) = cx.tcx.get_diagnostic_item(sym::Default)
         {
             let impls = cx.tcx.trait_impls_of(default_trait_id);
             for (ty, impl_def_ids) in impls.non_blanket_impls() {
@@ -409,7 +407,8 @@ fn check_from(cx: &LateContext<'_>, method_span: Span, method_def_id: LocalDefId
     // which calls back into our `From::from` again (`Into` is not specializable).
     // rustc's unconditional_recursion already catches calling `From::from` directly
     if let Some((fn_def_id, node_args)) = fn_def_id_with_node_args(cx, expr)
-        && let Some((s1, s2)) = node_args.iter().filter_map(ty::GenericArg::as_type).collect_tuple()
+        && let [s1, s2] = **node_args
+        && let (Some(s1), Some(s2)) = (s1.as_type(), s2.as_type())
         && let Some(trait_def_id) = cx.tcx.trait_of_item(fn_def_id)
         && cx.tcx.is_diagnostic_item(sym::Into, trait_def_id)
         && get_impl_trait_def_id(cx, method_def_id) == cx.tcx.get_diagnostic_item(sym::From)

tests/ui/unconditional_recursion.rs

@@ -372,4 +372,30 @@ impl<'a> From<BadFromTy2<'a>> for BadIntoTy2<'static> {
     }
 }
 
+// Different Into impl (<i16 as Into<i32>>), so no infinite recursion
+struct BadFromTy3;
+impl From<BadFromTy3> for i32 {
+    fn from(f: BadFromTy3) -> Self {
+        Into::into(1i16)
+    }
+}
+
+// A conditional return that ends the recursion
+struct BadFromTy4;
+impl From<BadFromTy4> for i32 {
+    fn from(f: BadFromTy4) -> Self {
+        if true {
+            return 42;
+        }
+        f.into()
+    }
+}
+
+// Types differ in refs, don't lint
+impl From<&BadFromTy4> for i32 {
+    fn from(f: &BadFromTy4) -> Self {
+        BadFromTy4.into()
+    }
+}
+
 fn main() {}