fix(useless_conversion): stop adjustments when target type is reached

Author: keirsalterego

clippy_lints/src/useless_conversion.rs

@@ -456,13 +456,25 @@ fn has_eligible_receiver(cx: &LateContext<'_>, recv: &Expr<'_>, expr: &Expr<'_>)
 
 fn adjustments(cx: &LateContext<'_>, expr: &Expr<'_>) -> String {
     let mut prefix = String::new();
-    for adj in cx.typeck_results().expr_adjustments(expr) {
+
+    let adjustments = cx.typeck_results().expr_adjustments(expr);
+
+    let [.., last] = adjustments else { return prefix };
+    let target = last.target;
+
+    for adj in adjustments {
         match adj.kind {
             Adjust::Deref(_) => prefix = format!("*{prefix}"),
             Adjust::Borrow(AutoBorrow::Ref(AutoBorrowMutability::Mut { .. })) => prefix = format!("&mut {prefix}"),
             Adjust::Borrow(AutoBorrow::Ref(AutoBorrowMutability::Not)) => prefix = format!("&{prefix}"),
             _ => {},
         }
+
+        // Stop once we reach the final target type.
+        // This prevents over-adjusting (e.g. suggesting &**y instead of *y).
+        if adj.target == target {
+            break;
+        }
     }
     prefix
 }

tests/ui/useless_conversion.fixed

@@ -131,6 +131,16 @@ fn main() {
     dont_lint_into_iter_on_copy_iter();
     dont_lint_into_iter_on_static_copy_iter();
 
+    #[allow(clippy::into_iter_on_ref)]
+    {
+        // triggers the IntoIterator trait
+        fn consume(_: impl IntoIterator) {}
+
+        // Should suggest `*items` instead of `&**items`
+        let items = &&[1, 2, 3];
+        consume(items.into_iter());
+    }
+
     let _: String = "foo".into();
     let _: String = From::from("foo");
     let _ = String::from("foo");

tests/ui/useless_conversion.rs

@@ -131,6 +131,16 @@ fn main() {
     dont_lint_into_iter_on_copy_iter();
     dont_lint_into_iter_on_static_copy_iter();
 
+    #[allow(clippy::into_iter_on_ref)]
+    {
+        // triggers the IntoIterator trait
+        fn consume(_: impl IntoIterator) {}
+
+        // Should suggest `*items` instead of `&**items`
+        let items = &&[1, 2, 3];
+        consume(items.into_iter());
+    }
+
     let _: String = "foo".into();
     let _: String = From::from("foo");
     let _ = String::from("foo");