Trim suggestion parts to the subset that is purely additive

compiler/rustc_errors/src/emitter.rs

@@ -1976,9 +1976,11 @@ impl HumanEmitter {
             Some(Style::HeaderMsg),
         );
 
+        let other_suggestions = suggestions.len().saturating_sub(MAX_SUGGESTIONS);
+
         let mut row_num = 2;
         for (i, (complete, parts, highlights, _)) in
-            suggestions.iter().enumerate().take(MAX_SUGGESTIONS)
+            suggestions.into_iter().enumerate().take(MAX_SUGGESTIONS)
         {
             debug!(?complete, ?parts, ?highlights);
 
@@ -2168,7 +2170,7 @@ impl HumanEmitter {
                 self.draw_code_line(
                     &mut buffer,
                     &mut row_num,
-                    highlight_parts,
+                    &highlight_parts,
                     line_pos + line_start,
                     line,
                     show_code_change,
@@ -2214,7 +2216,12 @@ impl HumanEmitter {
             if let DisplaySuggestion::Diff | DisplaySuggestion::Underline | DisplaySuggestion::Add =
                 show_code_change
             {
-                for part in parts {
+                for mut part in parts {
+                    // If this is a replacement of, e.g. `"a"` into `"ab"`, adjust the
+                    // suggestion and snippet to look as if we just suggested to add
+                    // `"b"`, which is typically much easier for the user to understand.
+                    part.trim_trivial_replacements(sm);
+
                     let snippet = if let Ok(snippet) = sm.span_to_snippet(part.span) {
                         snippet
                     } else {
@@ -2377,9 +2384,12 @@ impl HumanEmitter {
                 row_num = row + 1;
             }
         }
-        if suggestions.len() > MAX_SUGGESTIONS {
-            let others = suggestions.len() - MAX_SUGGESTIONS;
-            let msg = format!("and {} other candidate{}", others, pluralize!(others));
+        if other_suggestions > 0 {
+            let msg = format!(
+                "and {} other candidate{}",
+                other_suggestions,
+                pluralize!(other_suggestions)
+            );
             buffer.puts(row_num, max_line_num_len + 3, &msg, Style::NoStyle);
         }
 

compiler/rustc_errors/src/lib.rs

@@ -246,6 +246,24 @@ impl SubstitutionPart {
         sm.span_to_snippet(self.span)
             .map_or(!self.span.is_empty(), |snippet| !snippet.trim().is_empty())
     }
+
+    /// Try to turn a replacement into an addition when the span that is being
+    /// overwritten matches either the prefix or suffix of the replacement.
+    fn trim_trivial_replacements(&mut self, sm: &SourceMap) {
+        if self.snippet.is_empty() {
+            return;
+        }
+        let Ok(snippet) = sm.span_to_snippet(self.span) else {
+            return;
+        };
+        if self.snippet.starts_with(&snippet) {
+            self.span = self.span.shrink_to_hi();
+            self.snippet = self.snippet[snippet.len()..].to_string();
+        } else if self.snippet.ends_with(&snippet) {
+            self.span = self.span.shrink_to_lo();
+            self.snippet = self.snippet[..self.snippet.len() - snippet.len()].to_string();
+        }
+    }
 }
 
 impl CodeSuggestion {