Trim suggestion parts to the subset that is purely additive

Author: compiler-errors

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 {

tests/ui/associated-types/defaults-suitability.current.stderr

@@ -135,7 +135,7 @@ LL |     type Baz = T;
 help: consider further restricting type parameter `T` with trait `Clone`
    |
 LL |     Self::Baz: Clone, T: std::clone::Clone
-   |                     ~~~~~~~~~~~~~~~~~~~~~~
+   |                       ++++++++++++++++++++
 
 error: aborting due to 8 previous errors
 

tests/ui/associated-types/defaults-suitability.next.stderr

@@ -135,7 +135,7 @@ LL |     type Baz = T;
 help: consider further restricting type parameter `T` with trait `Clone`
    |
 LL |     Self::Baz: Clone, T: std::clone::Clone
-   |                     ~~~~~~~~~~~~~~~~~~~~~~
+   |                       ++++++++++++++++++++
 
 error: aborting due to 8 previous errors
 

tests/ui/associated-types/issue-38821.stderr

@@ -14,7 +14,7 @@ LL | impl<T: NotNull> IntoNullable for T {
 help: consider extending the `where` clause, but there might be an alternative better way to express this requirement
    |
 LL |     Expr: Expression<SqlType=<Col::SqlType as IntoNullable>::Nullable>, <Col as Expression>::SqlType: NotNull
-   |                                                                       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+   |                                                                         +++++++++++++++++++++++++++++++++++++
 
 error[E0277]: the trait bound `<Col as Expression>::SqlType: NotNull` is not satisfied
   --> $DIR/issue-38821.rs:40:1
@@ -38,7 +38,7 @@ LL | impl<T: NotNull> IntoNullable for T {
 help: consider extending the `where` clause, but there might be an alternative better way to express this requirement
    |
 LL |     Expr: Expression<SqlType=<Col::SqlType as IntoNullable>::Nullable>, <Col as Expression>::SqlType: NotNull
-   |                                                                       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+   |                                                                         +++++++++++++++++++++++++++++++++++++
 
 error[E0277]: the trait bound `<Col as Expression>::SqlType: NotNull` is not satisfied
   --> $DIR/issue-38821.rs:23:10

tests/ui/associated-types/issue-54108.current.stderr

@@ -13,7 +13,7 @@ LL |     type Size: Add<Output = Self::Size>;
 help: consider further restricting the associated type
    |
 LL |     T: SubEncoder, <T as SubEncoder>::ActualSize: Add
-   |                  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+   |                    ++++++++++++++++++++++++++++++++++
 
 error: aborting due to 1 previous error
 

tests/ui/associated-types/issue-54108.next.stderr

@@ -13,7 +13,7 @@ LL |     type Size: Add<Output = Self::Size>;
 help: consider further restricting the associated type
    |
 LL |     T: SubEncoder, <T as SubEncoder>::ActualSize: Add
-   |                  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+   |                    ++++++++++++++++++++++++++++++++++
 
 error: aborting due to 1 previous error
 

tests/ui/attributes/rustc_confusables.stderr

@@ -36,7 +36,7 @@ LL |     x.inser();
 help: there is a method `insert` with a similar name
    |
 LL |     x.insert();
-   |       ~~~~~~
+   |            +
 
 error[E0599]: no method named `foo` found for struct `rustc_confusables_across_crate::BTreeSet` in the current scope
   --> $DIR/rustc_confusables.rs:15:7

tests/ui/attributes/rustc_confusables_std_cases.stderr

@@ -39,7 +39,7 @@ LL |     let mut x = VecDeque::new();
 help: you might have meant to use `push_back`
    |
 LL |     x.push_back(1);
-   |       ~~~~~~~~~
+   |           +++++
 
 error[E0599]: no method named `length` found for struct `Vec<{integer}>` in the current scope
   --> $DIR/rustc_confusables_std_cases.rs:15:7
@@ -98,7 +98,7 @@ note: method defined here
 help: you might have meant to use `push_str`
    |
 LL |     String::new().push_str("");
-   |                   ~~~~~~~~
+   |                       ++++
 
 error[E0599]: no method named `append` found for struct `String` in the current scope
   --> $DIR/rustc_confusables_std_cases.rs:24:19

tests/ui/borrowck/issue-115259-suggest-iter-mut.stderr

@@ -9,7 +9,7 @@ LL |         self.layers.iter().fold(0, |result, mut layer| result + layer.proce
 help: you may want to use `iter_mut` here
    |
 LL |         self.layers.iter_mut().fold(0, |result, mut layer| result + layer.process())
-   |                     ~~~~~~~~
+   |                         ++++
 
 error: aborting due to 1 previous error