Swap verification of debug impl on data type

This makes the lint check if T (instead of E) in `Result<T, E>` implements Debug.

Author: b-NC

clippy_lints/src/methods/err_expect.rs

@@ -22,8 +22,8 @@ pub(super) fn check(
         if meets_msrv(msrv, &msrvs::EXPECT_ERR);
 
         let result_type = cx.typeck_results().expr_ty(recv);
-        if let Some(error_type) = get_error_type(cx, result_type);
-        if has_debug_impl(error_type, cx);
+        if let Some(data_type) = get_data_type(cx, result_type);
+        if has_debug_impl(data_type, cx);
 
         then {
             span_lint_and_sugg(
@@ -39,10 +39,10 @@ pub(super) fn check(
     };
 }
 
-/// Given a `Result<T, E>` type, return its error type (`E`).
-fn get_error_type<'a>(cx: &LateContext<'_>, ty: Ty<'a>) -> Option<Ty<'a>> {
+/// Given a `Result<T, E>` type, return its data (`T`).
+fn get_data_type<'a>(cx: &LateContext<'_>, ty: Ty<'a>) -> Option<Ty<'a>> {
     match ty.kind() {
-        ty::Adt(_, substs) if is_type_diagnostic_item(cx, ty, sym::Result) => substs.types().nth(1),
+        ty::Adt(_, substs) if is_type_diagnostic_item(cx, ty, sym::Result) => substs.types().nth(0),
         _ => None,
     }
 }

tests/ui/err_expect.fixed

@@ -6,9 +6,9 @@ struct MyTypeNonDebug;
 struct MyTypeDebug;
 
 fn main() {
-    let test_debug: Result<u32, MyTypeDebug> = Ok(0);
+    let test_debug: Result<MyTypeDebug, u32> = Ok(MyTypeDebug);
     test_debug.expect_err("Testing debug type");
 
-    let test_non_debug: Result<u32, MyTypeNonDebug> = Ok(0);
+    let test_non_debug: Result<MyTypeNonDebug, u32> = Ok(MyTypeNonDebug);
     test_non_debug.err().expect("Testing non debug type");
 }

tests/ui/err_expect.rs

@@ -6,9 +6,9 @@ struct MyTypeNonDebug;
 struct MyTypeDebug;
 
 fn main() {
-    let test_debug: Result<u32, MyTypeDebug> = Ok(0);
+    let test_debug: Result<MyTypeDebug, u32> = Ok(MyTypeDebug);
     test_debug.err().expect("Testing debug type");
 
-    let test_non_debug: Result<u32, MyTypeNonDebug> = Ok(0);
+    let test_non_debug: Result<MyTypeNonDebug, u32> = Ok(MyTypeNonDebug);
     test_non_debug.err().expect("Testing non debug type");
 }