Fix msg for verbose suggestions with confusable capitalization

When encountering a verbose/multipart suggestion that has changes
that are only caused by different capitalization of ASCII letters that have
little differenciation, expand the message to highlight that fact (like we
already do for inline suggestions).

The logic to do this was already present, but implemented incorrectly.

compiler/rustc_errors/src/emitter.rs

@@ -1742,9 +1742,17 @@ fn emit_suggestion_default(
         buffer.append(0, level.to_str(), Style::Level(*level));
         buffer.append(0, ": ", Style::HeaderMsg);
 
+        let mut msg = vec![(suggestion.msg.to_owned(), Style::NoStyle)];
+        if suggestions
+            .iter()
+            .take(MAX_SUGGESTIONS)
+            .any(|(_, _, _, only_capitalization)| *only_capitalization)
+        {
+            msg.push((" (notice the capitalization difference)".into(), Style::NoStyle));
+        }
         self.msgs_to_buffer(
             &mut buffer,
-            &[(suggestion.msg.to_owned(), Style::NoStyle)],
+            &msg,
             args,
             max_line_num_len,
             "suggestion",
@@ -1753,12 +1761,8 @@ fn emit_suggestion_default(
 
         let mut row_num = 2;
         draw_col_separator_no_space(&mut buffer, 1, max_line_num_len + 1);
-        let mut notice_capitalization = false;
+        for (complete, parts, highlights, _) in suggestions.iter().take(MAX_SUGGESTIONS) {
-        for (complete, parts, highlights, only_capitalization) in
-            suggestions.iter().take(MAX_SUGGESTIONS)
-        {
             debug!(?complete, ?parts, ?highlights);
-            notice_capitalization |= only_capitalization;
 
             let has_deletion = parts.iter().any(|p| p.is_deletion(sm));
             let is_multiline = complete.lines().count() > 1;
@@ -2057,9 +2061,6 @@ fn emit_suggestion_default(
             let others = suggestions.len() - MAX_SUGGESTIONS;
             let msg = format!("and {} other candidate{}", others, pluralize!(others));
             buffer.puts(row_num, max_line_num_len + 3, &msg, Style::NoStyle);
-        } else if notice_capitalization {
-            let msg = "notice the capitalization difference";
-            buffer.puts(row_num, max_line_num_len + 3, msg, Style::NoStyle);
         }
         emit_to_destination(&buffer.render(), level, &mut self.dst, self.short_message)?;
         Ok(())

compiler/rustc_errors/src/lib.rs

@@ -319,7 +319,9 @@ fn push_trailing(
                 // We need to keep track of the difference between the existing code and the added
                 // or deleted code in order to point at the correct column *after* substitution.
                 let mut acc = 0;
+                let mut only_capitalization = false;
                 for part in &substitution.parts {
+                    only_capitalization |= is_case_difference(sm, &part.snippet, part.span);
                     let cur_lo = sm.lookup_char_pos(part.span.lo());
                     if prev_hi.line == cur_lo.line {
                         let mut count =
@@ -392,7 +394,6 @@ fn push_trailing(
                     }
                 }
                 highlights.push(std::mem::take(&mut line_highlight));
-                let only_capitalization = is_case_difference(sm, &buf, bounding_span);
                 // if the replacement already ends with a newline, don't print the next line
                 if !buf.ends_with('\n') {
                     push_trailing(&mut buf, prev_line.as_ref(), &prev_hi, None);

src/tools/clippy/tests/ui/match_str_case_mismatch.stderr

@@ -17,7 +17,7 @@ error: this `match` arm has a differing case than its expression
 LL |         "~!@#$%^&*()-_=+Foo" => {},
    |         ^^^^^^^^^^^^^^^^^^^^
    |
-help: consider changing the case of this arm to respect `to_ascii_lowercase`
+help: consider changing the case of this arm to respect `to_ascii_lowercase` (notice the capitalization difference)
    |
 LL |         "~!@#$%^&*()-_=+foo" => {},
    |         ~~~~~~~~~~~~~~~~~~~~

tests/ui/error-codes/E0423.stderr

@@ -53,7 +53,7 @@ help: use struct literal syntax instead
    |
 LL |     let f = Foo { a: val };
    |             ~~~~~~~~~~~~~~
-help: a function with a similar name exists
+help: a function with a similar name exists (notice the capitalization difference)
    |
 LL |     let f = foo();
    |             ~~~

tests/ui/parser/kw-in-trait-bounds.stderr

@@ -4,7 +4,7 @@ error: expected identifier, found keyword `fn`
 LL | fn _f<F: fn(), G>(_: impl fn(), _: &dyn fn())
    |          ^^
    |
-help: use `Fn` to refer to the trait
+help: use `Fn` to refer to the trait (notice the capitalization difference)
    |
 LL | fn _f<F: Fn(), G>(_: impl fn(), _: &dyn fn())
    |          ~~
@@ -15,7 +15,7 @@ error: expected identifier, found keyword `fn`
 LL | fn _f<F: fn(), G>(_: impl fn(), _: &dyn fn())
    |                           ^^
    |
-help: use `Fn` to refer to the trait
+help: use `Fn` to refer to the trait (notice the capitalization difference)
    |
 LL | fn _f<F: fn(), G>(_: impl Fn(), _: &dyn fn())
    |                           ~~
@@ -26,7 +26,7 @@ error: expected identifier, found keyword `fn`
 LL | fn _f<F: fn(), G>(_: impl fn(), _: &dyn fn())
    |                                         ^^
    |
-help: use `Fn` to refer to the trait
+help: use `Fn` to refer to the trait (notice the capitalization difference)
    |
 LL | fn _f<F: fn(), G>(_: impl fn(), _: &dyn Fn())
    |                                         ~~
@@ -37,7 +37,7 @@ error: expected identifier, found keyword `fn`
 LL | G: fn(),
    |    ^^
    |
-help: use `Fn` to refer to the trait
+help: use `Fn` to refer to the trait (notice the capitalization difference)
    |
 LL | G: Fn(),
    |    ~~

tests/ui/parser/recover/recover-fn-trait-from-fn-kw.stderr

@@ -4,7 +4,7 @@ error: expected identifier, found keyword `fn`
 LL | fn foo(_: impl fn() -> i32) {}
    |                ^^
    |
-help: use `Fn` to refer to the trait
+help: use `Fn` to refer to the trait (notice the capitalization difference)
    |
 LL | fn foo(_: impl Fn() -> i32) {}
    |                ~~
@@ -15,7 +15,7 @@ error: expected identifier, found keyword `fn`
 LL | fn foo2<T: fn(i32)>(_: T) {}
    |            ^^
    |
-help: use `Fn` to refer to the trait
+help: use `Fn` to refer to the trait (notice the capitalization difference)
    |
 LL | fn foo2<T: Fn(i32)>(_: T) {}
    |            ~~

tests/ui/parser/typod-const-in-const-param-def.stderr

@@ -4,7 +4,7 @@ error: `const` keyword was mistyped as `Const`
 LL | pub fn foo<Const N: u8>() {}
    |            ^^^^^
    |
-help: use the `const` keyword
+help: use the `const` keyword (notice the capitalization difference)
    |
 LL | pub fn foo<const N: u8>() {}
    |            ~~~~~
@@ -15,7 +15,7 @@ error: `const` keyword was mistyped as `Const`
 LL | pub fn baz<Const N: u8, T>() {}
    |            ^^^^^
    |
-help: use the `const` keyword
+help: use the `const` keyword (notice the capitalization difference)
    |
 LL | pub fn baz<const N: u8, T>() {}
    |            ~~~~~
@@ -26,7 +26,7 @@ error: `const` keyword was mistyped as `Const`
 LL | pub fn qux<T, Const N: u8>() {}
    |               ^^^^^
    |
-help: use the `const` keyword
+help: use the `const` keyword (notice the capitalization difference)
    |
 LL | pub fn qux<T, const N: u8>() {}
    |               ~~~~~
@@ -37,7 +37,7 @@ error: `const` keyword was mistyped as `Const`
 LL | pub fn quux<T, Const N: u8, U>() {}
    |                ^^^^^
    |
-help: use the `const` keyword
+help: use the `const` keyword (notice the capitalization difference)
    |
 LL | pub fn quux<T, const N: u8, U>() {}
    |                ~~~~~

tests/ui/suggestions/assoc-ct-for-assoc-method.stderr

@@ -8,7 +8,7 @@ LL |     let x: i32 = MyS::foo;
    |
    = note: expected type `i32`
            found fn item `fn() -> MyS {MyS::foo}`
-help: try referring to the associated const `FOO` instead
+help: try referring to the associated const `FOO` instead (notice the capitalization difference)
    |
 LL |     let x: i32 = MyS::FOO;
    |                       ~~~

tests/ui/suggestions/bool_typo_err_suggest.stderr

@@ -15,7 +15,7 @@ error[E0425]: cannot find value `False` in this scope
 LL |     let y = False;
    |             ^^^^^ not found in this scope
    |
-help: you may want to use a bool value instead
+help: you may want to use a bool value instead (notice the capitalization difference)
    |
 LL |     let y = false;
    |             ~~~~~