test: snapshot for derive suggestion in diff files

compiler/rustc_errors/src/emitter.rs

@@ -1796,17 +1796,17 @@ impl EmitterWriter {
                 // telling users to make a change but not clarifying *where*.
                 let loc = sm.lookup_char_pos(parts[0].span.lo());
                 if loc.file.name != sm.span_to_filename(span) && loc.file.name.is_real() {
-                    buffer.puts(row_num - 1, 0, "--> ", Style::LineNumber);
+                    let arrow = "--> ";
-                    buffer.append(
+                    buffer.puts(row_num - 1, 0, arrow, Style::LineNumber);
-                        row_num - 1,
+                    let filename = sm.filename_for_diagnostics(&loc.file.name);
-                        &format!(
+                    let offset = sm.doctest_offset_line(&loc.file.name, loc.line);
-                            "{}:{}:{}",
+                    let message = format!("{}:{}:{}", filename, offset, loc.col.0 + 1);
-                            sm.filename_for_diagnostics(&loc.file.name),
+                    if row_num == 2 {
-                            sm.doctest_offset_line(&loc.file.name, loc.line),
+                        let col = usize::max(max_line_num_len + 1, arrow.len());
-                            loc.col.0 + 1,
+                        buffer.puts(1, col, &message, Style::LineAndColumn);
-                        ),
+                    } else {
-                        Style::LineAndColumn,
+                        buffer.append(row_num - 1, &message, Style::LineAndColumn);
-                    );
+                    }
                     for _ in 0..max_line_num_len {
                         buffer.prepend(row_num - 1, " ", Style::NoStyle);
                     }

tests/ui/modules/issue-107649.rs

@@ -0,0 +1,106 @@
+// compile-flags: -Z ui-testing=no
+#[path = "auxiliary/dummy_lib.rs"]
+mod lib;
+
+/// The function needs to be long enough to
+/// ensure `max_line_num_len` to be large enough
+/// for no-ui-testing
+fn main() {
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    ();
+    dbg!(lib::Dummy); //~ Error: `Dummy` doesn't implement `Debug`
+}

tests/ui/modules/issue-107649.stderr

@@ -0,0 +1,18 @@
+error[E0277]: `Dummy` doesn't implement `Debug`
+   --> $DIR/issue-107649.rs:105:5
+    |
+105 |     dbg!(lib::Dummy);
+    |     ^^^^^^^^^^^^^^^^ `Dummy` cannot be formatted using `{:?}`
+    |
+    = help: the trait `Debug` is not implemented for `Dummy`
+    = note: add `#[derive(Debug)]` to `Dummy` or manually `impl Debug for Dummy`
+    = note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `dbg` (in Nightly builds, run with -Z macro-backtrace for more info)
+help: consider annotating `Dummy` with `#[derive(Debug)]`
+   --> $DIR/auxiliary/dummy_lib.rs:2:1
+    |
+2   | #[derive(Debug)]
+    |
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.