Auto merge of #109786 - estebank:tweak-add-line-sugg, r=compiler-errors

Tweak output for 'add line' suggestion

Closes #108174

1 files changed, 49 insertions, 1 deletions

diff --git a/compiler/rustc_errors/src/emitter.rs b/compiler/rustc_errors/src/emitter.rs
index 81e8bcbf7cd..fe44799efdb 100644
--- a/compiler/rustc_errors/src/emitter.rs
+++ b/compiler/rustc_errors/src/emitter.rs
@@ -1832,6 +1832,12 @@ impl EmitterWriter {
             }
             let show_code_change = if has_deletion && !is_multiline {
                 DisplaySuggestion::Diff
+            } else if let [part] = &parts[..]
+                && part.snippet.ends_with('\n')
+                && part.snippet.trim() == complete.trim()
+            {
+                // We are adding a line(s) of code before code that was already there.
+                DisplaySuggestion::Add
             } else if (parts.len() != 1 || parts[0].snippet.trim() != complete.trim())
                 && !is_multiline
             {
@@ -1879,7 +1885,10 @@ impl EmitterWriter {
                 row_num += line_end - line_start;
             }
             let mut unhighlighted_lines = Vec::new();
+            let mut last_pos = 0;
+            let mut is_item_attribute = false;
             for (line_pos, (line, highlight_parts)) in lines.by_ref().zip(highlights).enumerate() {
+                last_pos = line_pos;
                 debug!(%line_pos, %line, ?highlight_parts);
 
                 // Remember lines that are not highlighted to hide them if needed
@@ -1887,6 +1896,12 @@ impl EmitterWriter {
                     unhighlighted_lines.push((line_pos, line));
                     continue;
                 }
+                if highlight_parts.len() == 1
+                    && line.trim().starts_with("#[")
+                    && line.trim().ends_with(']')
+                {
+                    is_item_attribute = true;
+                }
 
                 match unhighlighted_lines.len() {
                     0 => (),
@@ -1963,13 +1978,41 @@ impl EmitterWriter {
                     is_multiline,
                 )
             }
+            if let DisplaySuggestion::Add = show_code_change && is_item_attribute {
+                // The suggestion adds an entire line of code, ending on a newline, so we'll also
+                // print the *following* line, to provide context of what we're advicing people to
+                // do. Otherwise you would only see contextless code that can be confused for
+                // already existing code, despite the colors and UI elements.
+                // We special case `#[derive(_)]\n` and other attribute suggestions, because those
+                // are the ones where context is most useful.
+                let file_lines = sm
+                    .span_to_lines(span.primary_span().unwrap().shrink_to_hi())
+                    .expect("span_to_lines failed when emitting suggestion");
+                let line_num = sm.lookup_char_pos(parts[0].span.lo()).line;
+                if let Some(line) = file_lines.file.get_line(line_num - 1) {
+                    let line = normalize_whitespace(&line);
+                    self.draw_code_line(
+                        &mut buffer,
+                        &mut row_num,
+                        &[],
+                        line_num + last_pos + 1,
+                        &line,
+                        DisplaySuggestion::None,
+                        max_line_num_len,
+                        &file_lines,
+                        is_multiline,
+                    )
+                }
+            }
 
             // This offset and the ones below need to be signed to account for replacement code
             // that is shorter than the original code.
             let mut offsets: Vec<(usize, isize)> = Vec::new();
             // Only show an underline in the suggestions if the suggestion is not the
             // entirety of the code being shown and the displayed code is not multiline.
-            if let DisplaySuggestion::Diff | DisplaySuggestion::Underline = show_code_change {
+            if let DisplaySuggestion::Diff | DisplaySuggestion::Underline | DisplaySuggestion::Add =
+                show_code_change
+            {
                 draw_col_separator_no_space(&mut buffer, row_num, max_line_num_len + 1);
                 for part in parts {
                     let span_start_pos = sm.lookup_char_pos(part.span.lo()).col_display;
@@ -2247,6 +2290,10 @@ impl EmitterWriter {
                 }
             }
             buffer.append(*row_num, &normalize_whitespace(line_to_add), Style::NoStyle);
+        } else if let DisplaySuggestion::Add = show_code_change {
+            buffer.puts(*row_num, 0, &self.maybe_anonymized(line_num), Style::LineNumber);
+            buffer.puts(*row_num, max_line_num_len + 1, "+ ", Style::Addition);
+            buffer.append(*row_num, &normalize_whitespace(line_to_add), Style::NoStyle);
         } else {
             buffer.puts(*row_num, 0, &self.maybe_anonymized(line_num), Style::LineNumber);
             draw_col_separator(buffer, *row_num, max_line_num_len + 1);
@@ -2281,6 +2328,7 @@ enum DisplaySuggestion {
     Underline,
     Diff,
     None,
+    Add,
 }
 
 impl FileWithAnnotatedLines {