rustc_parse_format: improve the error diagnostic for + sign flag

compiler/rustc_parse_format/src/lib.rs

@@ -491,6 +491,7 @@ impl<'input> Parser<'input> {
                 ('<' | '^' | '>', _) => self.suggest_format_align(c),
                 (',', _) => self.suggest_unsupported_python_numeric_grouping(),
                 ('=', '}') => self.suggest_rust_debug_printing_macro(),
+                ('+', _) => self.suggest_format_missing_colon_for_sign(),
                 _ => self.suggest_positional_arg_instead_of_captured_arg(arg),
             }
         }
@@ -939,6 +940,23 @@ impl<'input> Parser<'input> {
         }
     }
 
+    fn suggest_format_missing_colon_for_sign(&mut self) {
+        if let Some((range, _)) = self.consume_pos('+') {
+            self.errors.insert(
+                0,
+                ParseError {
+                    description: "the `+` sign flag must appear after `:` in a format string"
+                        .to_owned(),
+                    note: Some("`+` comes after `:`, try `{:+}` instead of `{+}`".to_owned()),
+                    label: "expected `:` before `+` sign flag".to_owned(),
+                    span: range,
+                    secondary_label: None,
+                    suggestion: Suggestion::None,
+                },
+            );
+        }
+    }
+
     fn suggest_positional_arg_instead_of_captured_arg(&mut self, arg: &Argument<'_>) {
         // If the argument is not an identifier, it is not a field access.
         if !arg.is_identifier() {

compiler/rustc_parse_format/src/tests.rs

@@ -595,3 +595,13 @@ fn diagnostic_format_mod() {
     assert_eq!(parser.line_spans, &[]);
     assert!(parser.errors.is_empty());
 }
+#[test]
+fn format_plus_sign_missing_colon_error() {
+    // `{+}` should produce an error suggesting `:` before `+`
+    let mut p = Parser::new("{+}", None, None, false, ParseMode::Format);
+    let _ = p.by_ref().collect::<Vec<Piece<'static>>>();
+    assert!(!p.errors.is_empty());
+    assert!(p.errors[0].description.contains("`+` sign flag must appear after `:`"));
+    assert!(p.errors[0].label.contains("expected `:` before `+` sign flag"));
+    assert_eq!(p.errors[0].span, 2..3);
+}

tests/ui/fmt/format-string-wrong-order.rs

@@ -20,4 +20,8 @@ fn main() {
     //~^ ERROR invalid format string: expected alignment specifier after `:` in format string; example: `{:>?}`
     println!("{0:#X>18}", 12345);
     //~^ ERROR invalid format string: expected alignment specifier after `:` in format string; example: `{:>?}`
+    format!("{+:}");
+    //~^ ERROR invalid format string: the `+` sign flag must appear after `:` in a format string
+    format!("{bar+:}");
+    //~^ ERROR invalid format string: the `+` sign flag must appear after `:` in a format string
 }

tests/ui/fmt/format-string-wrong-order.stderr

@@ -74,5 +74,21 @@ error: invalid format string: expected alignment specifier after `:` in format s
 LL |     println!("{0:#X>18}", 12345);
    |                    ^ expected `>` to occur after `:` in format string
 
-error: aborting due to 10 previous errors
+error: invalid format string: the `+` sign flag must appear after `:` in a format string
+  --> $DIR/format-string-wrong-order.rs:23:15
+   |
+LL |     format!("{+:}");
+   |               ^ expected `:` before `+` sign flag in format string
+   |
+   = note: `+` comes after `:`, try `{:+}` instead of `{+}`
+
+error: invalid format string: the `+` sign flag must appear after `:` in a format string
+  --> $DIR/format-string-wrong-order.rs:25:18
+   |
+LL |     format!("{bar+:}");
+   |                  ^ expected `:` before `+` sign flag in format string
+   |
+   = note: `+` comes after `:`, try `{:+}` instead of `{+}`
+
+error: aborting due to 12 previous errors