Suggest flipping reversed ?#/?x/?X in format specifiers

compiler/rustc_builtin_macros/messages.ftl

@@ -162,8 +162,12 @@ builtin_macros_format_redundant_args = redundant {$n ->
     }
     .suggestion = this can be removed
 
+builtin_macros_format_remove = consider removing the format specifier instead
+
 builtin_macros_format_remove_raw_ident = remove the `r#`
 
+builtin_macros_format_reorder = consider reordering the format specifier instead
+
 builtin_macros_format_requires_string = requires at least a format string argument
 
 builtin_macros_format_string_invalid = invalid format string: {$desc}

compiler/rustc_builtin_macros/src/errors.rs

@@ -551,6 +551,25 @@ pub(crate) enum InvalidFormatStringSuggestion {
         #[primary_span]
         span: Span,
     },
+    #[multipart_suggestion(
+        builtin_macros_format_reorder,
+        style = "tool-only",
+        applicability = "machine-applicable"
+    )]
+    ReorderFormat {
+        #[suggestion_part(code = "{string}")]
+        span: Span,
+        string: String,
+    },
+    #[multipart_suggestion(
+        builtin_macros_format_remove,
+        style = "tool-only",
+        applicability = "machine-applicable"
+    )]
+    RemoveCharacter {
+        #[suggestion_part(code = "")]
+        span: Span,
+    },
 }
 
 #[derive(Diagnostic)]

compiler/rustc_builtin_macros/src/format.rs

@@ -302,6 +302,20 @@ fn make_format_args(
                     e.sugg_ = Some(errors::InvalidFormatStringSuggestion::RemoveRawIdent { span })
                 }
             }
+            parse::Suggestion::ReorderFormat(span) => {
+                let span = fmt_span.from_inner(InnerSpan::new(span.start, span.end));
+                if let Ok(original_string) = ecx.source_map().span_to_snippet(span) {
+                    let reversed_string: String = original_string.chars().rev().collect();
+                    e.sugg_ = Some(errors::InvalidFormatStringSuggestion::ReorderFormat {
+                        span: span,
+                        string: reversed_string,
+                    });
+                }
+            }
+            parse::Suggestion::RemoveCharacter(span) => {
+                let span = fmt_span.from_inner(InnerSpan::new(span.start, span.end));
+                e.sugg_ = Some(errors::InvalidFormatStringSuggestion::RemoveCharacter { span })
+            }
         }
         let guar = ecx.dcx().emit_err(e);
         return ExpandResult::Ready(Err(guar));

compiler/rustc_parse_format/src/lib.rs

@@ -221,6 +221,10 @@ pub enum Suggestion {
     /// Remove `r#` from identifier:
     /// `format!("{r#foo}")` -> `format!("{foo}")`
     RemoveRawIdent(InnerSpan),
+    // Reorder `?#` to `#?`.
+    ReorderFormat(InnerSpan),
+    // Remove possibly unnecessary character.
+    RemoveCharacter(InnerSpan),
 }
 
 /// The parser structure for interpreting the input format string. This is
@@ -731,6 +735,13 @@ impl<'a> Parser<'a> {
             }
         } else if self.consume('?') {
             spec.ty = "?";
+            if let Some(&(_, char)) = self.cur.peek() {
+                match char {
+                    '#' => self.suggest_format_reorder(char),
+                    'x' | 'X' => self.suggest_format_remove(char),
+                    _ => {}
+                }
+            }
         } else {
             spec.ty = self.word();
             if !spec.ty.is_empty() {
@@ -895,6 +906,33 @@ impl<'a> Parser<'a> {
         }
     }
 
+    fn suggest_format_reorder(&mut self, reorder: char) {
+        if let Some(pos) = self.consume_pos(reorder) {
+            let pos = self.span(pos - 1, pos + 1);
+            self.errors.insert(0, ParseError {
+                description: format!("unknown format identifier '?{}'", reorder),
+                note: Some("if you intended to print `{`, you can escape it using `{{`".to_owned()),
+                label: format!("help: the format parameter should be written `{}?`", reorder),
+                span: pos,
+                secondary_label: None,
+                suggestion: Suggestion::ReorderFormat(pos),
+            });
+        }
+    }
+    fn suggest_format_remove(&mut self, remove: char) {
+        if let Some(pos) = self.consume_pos(remove) {
+            let pos = self.span(pos, pos + 1);
+            self.errors.insert(0, ParseError {
+                description: format!("unknown format identifier '?{}'", remove),
+                note: Some("if you intended to print `{`, you can escape it using `{{`".to_owned()),
+                label: format!("help: consider removing `{}`", remove),
+                span: pos,
+                secondary_label: None,
+                suggestion: Suggestion::RemoveCharacter(pos),
+            });
+        }
+    }
+
     fn suggest_positional_arg_instead_of_captured_arg(&mut self, arg: Argument<'a>) {
         if let Some(end) = self.consume_pos('.') {
             let byte_pos = self.to_span_index(end);

tests/ui/fmt/help-message-issue-129966-reorder-remove-format-specifier.rs

@@ -0,0 +1,14 @@
+// Test checks for the help messages in the error output/
+//@ check-run-results
+
+struct Foo(u8, u8);
+
+fn main() {
+    let f = Foo(1, 2);
+    format!("{f:?#}");
+//~^ ERROR invalid format string: unknown format identifier '?#'
+    format!("{f:?x}");
+//~^ ERROR invalid format string: unknown format identifier '?x'
+    format!("{f:?X}");
+//~^ ERROR invalid format string: unknown format identifier '?X'
+}

tests/ui/fmt/help-message-issue-129966-reorder-remove-format-specifier.stderr

@@ -0,0 +1,26 @@
+error: invalid format string: unknown format identifier '?#'
+  --> $DIR/help-message-issue-129966-reorder-remove-format-specifier.rs:8:17
+   |
+LL |     format!("{f:?#}");
+   |                 ^^ help: the format parameter should be written `#?` in format string
+   |
+   = note: if you intended to print `{`, you can escape it using `{{`
+
+error: invalid format string: unknown format identifier '?x'
+  --> $DIR/help-message-issue-129966-reorder-remove-format-specifier.rs:10:18
+   |
+LL |     format!("{f:?x}");
+   |                  ^ help: consider removing `x` in format string
+   |
+   = note: if you intended to print `{`, you can escape it using `{{`
+
+error: invalid format string: unknown format identifier '?X'
+  --> $DIR/help-message-issue-129966-reorder-remove-format-specifier.rs:12:18
+   |
+LL |     format!("{f:?X}");
+   |                  ^ help: consider removing `X` in format string
+   |
+   = note: if you intended to print `{`, you can escape it using `{{`
+
+error: aborting due to 3 previous errors
+