Suggest {var:?} when finding {?:var} in inline format strings

compiler/rustc_parse_format/src/lib.rs

@@ -271,7 +271,13 @@ impl<'a> Iterator for Parser<'a> {
                                 );
                             }
                         } else {
-                            self.suggest_positional_arg_instead_of_captured_arg(arg);
+                            if let Some(&(_, maybe)) = self.cur.peek() {
+                                if maybe == '?' {
+                                    self.suggest_format();
+                                } else {
+                                    self.suggest_positional_arg_instead_of_captured_arg(arg);
+                                }
+                            }
                         }
                         Some(NextArgument(Box::new(arg)))
                     }
@@ -823,6 +829,27 @@ impl<'a> Parser<'a> {
         if found { Some(cur) } else { None }
     }
 
+    fn suggest_format(&mut self) {
+        if let (Some(pos), Some(_)) = (self.consume_pos('?'), self.consume_pos(':')) {
+            let word = self.word();
+            let _end = self.current_pos();
+            let pos = self.to_span_index(pos);
+            self.errors.insert(
+                0,
+                ParseError {
+                    description: "expected format parameter to occur after `:`".to_owned(),
+                    note: Some(
+                        format!("`?` comes after `:`, try `{}:{}` instead", word, "?").to_owned(),
+                    ),
+                    label: "expected `?` to occur after `:`".to_owned(),
+                    span: pos.to(pos),
+                    secondary_label: None,
+                    should_be_replaced_with_positional_argument: false,
+                },
+            );
+        }
+    }
+
     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/format-string-wrong-order.rs

@@ -0,0 +1,15 @@
+fn main() {
+    let bar = 3;
+    format!("{?:}", bar);
+    //~^ ERROR invalid format string: expected format parameter to occur after `:`
+    format!("{?:bar}");
+    //~^ ERROR invalid format string: expected format parameter to occur after `:`
+    format!("{?:?}", bar);
+    //~^ ERROR invalid format string: expected format parameter to occur after `:`
+    format!("{??}", bar);
+    //~^ ERROR invalid format string: expected `'}'`, found `'?'`
+    format!("{?;bar}");
+    //~^ ERROR invalid format string: expected `'}'`, found `'?'`
+    format!("{?:#?}", bar);
+    //~^ ERROR invalid format string: expected format parameter to occur after `:`
+}

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

@@ -0,0 +1,54 @@
+error: invalid format string: expected format parameter to occur after `:`
+  --> $DIR/format-string-wrong-order.rs:3:15
+   |
+LL |     format!("{?:}", bar);
+   |               ^ expected `?` to occur after `:` in format string
+   |
+   = note: `?` comes after `:`, try `:?` instead
+
+error: invalid format string: expected format parameter to occur after `:`
+  --> $DIR/format-string-wrong-order.rs:5:15
+   |
+LL |     format!("{?:bar}");
+   |               ^ expected `?` to occur after `:` in format string
+   |
+   = note: `?` comes after `:`, try `bar:?` instead
+
+error: invalid format string: expected format parameter to occur after `:`
+  --> $DIR/format-string-wrong-order.rs:7:15
+   |
+LL |     format!("{?:?}", bar);
+   |               ^ expected `?` to occur after `:` in format string
+   |
+   = note: `?` comes after `:`, try `:?` instead
+
+error: invalid format string: expected `'}'`, found `'?'`
+  --> $DIR/format-string-wrong-order.rs:9:15
+   |
+LL |     format!("{??}", bar);
+   |              -^ expected `}` in format string
+   |              |
+   |              because of this opening brace
+   |
+   = note: if you intended to print `{`, you can escape it using `{{`
+
+error: invalid format string: expected `'}'`, found `'?'`
+  --> $DIR/format-string-wrong-order.rs:11:15
+   |
+LL |     format!("{?;bar}");
+   |              -^ expected `}` in format string
+   |              |
+   |              because of this opening brace
+   |
+   = note: if you intended to print `{`, you can escape it using `{{`
+
+error: invalid format string: expected format parameter to occur after `:`
+  --> $DIR/format-string-wrong-order.rs:13:15
+   |
+LL |     format!("{?:#?}", bar);
+   |               ^ expected `?` to occur after `:` in format string
+   |
+   = note: `?` comes after `:`, try `:?` instead
+
+error: aborting due to 6 previous errors
+