rustc_parse_format: improve error for missing : before ? in format args

Detect the `{ident?}` pattern where `?` is immediately followed by `}` and emit
a clearer diagnostic explaining that `:` is required for Debug formatting.
This avoids falling back to a generic “invalid format string” error and adds
a targeted UI test for the case.

Signed-off-by: Usman Akinyemi <usmanakinyemi202@gmail.com>

compiler/rustc_builtin_macros/messages.ftl

@@ -182,6 +182,8 @@ builtin_macros_expected_other = expected operand, {$is_inline_asm ->
 
 builtin_macros_export_macro_rules = cannot export macro_rules! macros from a `proc-macro` crate type currently
 
+builtin_macros_format_add_missing_colon = add a colon before the format specifier
+
 builtin_macros_format_duplicate_arg = duplicate argument named `{$ident}`
     .label1 = previously here
     .label2 = duplicate argument

compiler/rustc_builtin_macros/src/errors.rs

@@ -643,6 +643,15 @@ pub(crate) enum InvalidFormatStringSuggestion {
         span: Span,
         replacement: String,
     },
+    #[suggestion(
+        builtin_macros_format_add_missing_colon,
+        code = ":?",
+        applicability = "machine-applicable"
+    )]
+    AddMissingColon {
+        #[primary_span]
+        span: Span,
+    },
 }
 
 #[derive(Diagnostic)]

compiler/rustc_builtin_macros/src/format.rs

@@ -329,6 +329,10 @@ fn make_format_args(
                     replacement,
                 });
             }
+            parse::Suggestion::AddMissingColon(span) => {
+                let span = fmt_span.from_inner(InnerSpan::new(span.start, span.end));
+                e.sugg_ = Some(errors::InvalidFormatStringSuggestion::AddMissingColon { span });
+            }
         }
         let guar = ecx.dcx().emit_err(e);
         return ExpandResult::Ready(Err(guar));

compiler/rustc_parse_format/src/lib.rs

@@ -184,6 +184,9 @@ pub enum Suggestion {
     /// `format!("{foo:?x}")` -> `format!("{foo:x?}")`
     /// `format!("{foo:?X}")` -> `format!("{foo:X?}")`
     ReorderFormatParameter(Range<usize>, String),
+    /// Add missing colon:
+    /// `format!("{foo?}")` -> `format!("{foo:?}")`
+    AddMissingColon(Range<usize>),
 }
 
 /// The parser structure for interpreting the input format string. This is
@@ -453,10 +456,11 @@ impl<'input> Parser<'input> {
             suggestion: Suggestion::None,
         });
 
-        if let Some((_, _, c)) = self.peek() {
-            match c {
-                '?' => self.suggest_format_debug(),
-                '<' | '^' | '>' => self.suggest_format_align(c),
+        if let (Some((_, _, c)), Some((_, _, nc))) = (self.peek(), self.peek_ahead()) {
+            match (c, nc) {
+                ('?', '}') => self.missing_colon_before_debug_formatter(),
+                ('?', _) => self.suggest_format_debug(),
+                ('<' | '^' | '>', _) => self.suggest_format_align(c),
                 _ => self.suggest_positional_arg_instead_of_captured_arg(arg),
             }
         }
@@ -849,6 +853,23 @@ impl<'input> Parser<'input> {
         }
     }
 
+    fn missing_colon_before_debug_formatter(&mut self) {
+        if let Some((range, _)) = self.consume_pos('?') {
+            let span = range.clone();
+            self.errors.insert(
+                0,
+                ParseError {
+                    description: "expected `}`, found `?`".to_owned(),
+                    note: Some(format!("to print `{{`, you can escape it using `{{{{`",)),
+                    label: "expected `:` before `?` to format with `Debug`".to_owned(),
+                    span: range,
+                    secondary_label: None,
+                    suggestion: Suggestion::AddMissingColon(span),
+                },
+            );
+        }
+    }
+
     fn suggest_format_align(&mut self, alignment: char) {
         if let Some((range, _)) = self.consume_pos(alignment) {
             self.errors.insert(

tests/ui/fmt/format-string-error-2.rs

@@ -83,4 +83,7 @@ raw  { \n
 
     println!(r#"\x7B}\u8 {"#, 1);
     //~^ ERROR invalid format string: unmatched `}` found
+
+    println!("{x?}, world!",);
+    //~^ ERROR invalid format string: expected `}`, found `?`
 }

tests/ui/fmt/format-string-error-2.stderr

@@ -177,5 +177,16 @@ LL |     println!(r#"\x7B}\u8 {"#, 1);
    |
    = note: if you intended to print `}`, you can escape it using `}}`
 
-error: aborting due to 18 previous errors
+error: invalid format string: expected `}`, found `?`
+  --> $DIR/format-string-error-2.rs:87:17
+   |
+LL |     println!("{x?}, world!",);
+   |                 ^
+   |                 |
+   |                 expected `:` before `?` to format with `Debug` in format string
+   |                 help: add a colon before the format specifier: `:?`
+   |
+   = note: to print `{`, you can escape it using `{{`
+
+error: aborting due to 19 previous errors