Place generated impl block after the existing impl block (#17366)

Placing the generated impl block before the existing one means that the
attributes of the existing block will be applied to the newly generated
one. Generating the new impl block after the existing one is safer.

changelog: [`new_without_default`]: be more robust in the presence of
attributes on the impl block containing the `new()` implementation

Fixes #17361 (confirmed on original code)

Author: llogiq

clippy_lints/src/new_without_default.rs

@@ -179,7 +179,7 @@ impl<'tcx> LateLintPass<'tcx> for NewWithoutDefault {
                     impl_item.span,
                     format!("you should consider adding a `Default` implementation for `{self_type_snip}`"),
                     |diag| {
-                        diag.suggest_prepend_item(
+                        diag.suggest_append_item(
                             cx,
                             item.span,
                             "try adding this",

clippy_utils/src/sugg.rs

@@ -715,19 +715,19 @@ pub trait DiagExt<T: LintContext> {
         applicability: Applicability,
     );
 
-    /// Suggest to add an item before another.
+    /// Suggest to add an item after another.
     ///
     /// The item should not be indented (except for inner indentation).
     ///
     /// # Example
     ///
     /// ```rust,ignore
-    /// diag.suggest_prepend_item(cx, item,
+    /// diag.suggest_append_item(cx, item,
     /// "fn foo() {
     ///     bar();
     /// }");
     /// ```
-    fn suggest_prepend_item(&mut self, cx: &T, item: Span, msg: &str, new_item: &str, applicability: Applicability);
+    fn suggest_append_item(&mut self, cx: &T, item: Span, msg: &str, new_item: &str, applicability: Applicability);
 
     /// Suggest to completely remove an item.
     ///
@@ -759,24 +759,19 @@ impl<T: LintContext> DiagExt<T> for rustc_errors::Diag<'_, ()> {
         }
     }
 
-    fn suggest_prepend_item(&mut self, cx: &T, item: Span, msg: &str, new_item: &str, applicability: Applicability) {
+    fn suggest_append_item(&mut self, cx: &T, item: Span, msg: &str, new_item: &str, applicability: Applicability) {
         if let Some(indent) = indentation(cx, item) {
-            let span = item.with_hi(item.lo());
-
-            let mut first = true;
-            let new_item = new_item
-                .lines()
-                .map(|l| {
-                    if first {
-                        first = false;
-                        format!("{l}\n")
-                    } else {
-                        format!("{indent}{l}\n")
-                    }
-                })
-                .collect::<String>();
-
-            self.span_suggestion(span, msg.to_string(), format!("{new_item}\n{indent}"), applicability);
+            let span = item.shrink_to_hi();
+            let mut new_item_code = String::new();
+            for l in new_item.lines() {
+                writeln!(new_item_code, "{indent}{l}").unwrap();
+            }
+            self.span_suggestion(
+                span,
+                msg.to_string(),
+                format!("\n\n{}", new_item_code.strip_suffix('\n').unwrap()),
+                applicability,
+            );
         }
     }