format macro argument parsing fix

When the character next to `{}` is "shifted" (when mapping a byte index
in the format string to span) we should avoid shifting the span end
index, so first map the index of `}` to span, then bump the span,
instead of first mapping the next byte index to a span (which causes
bumping the end span too much).

Regression test added.

Fixes #83344

compiler/rustc_parse_format/src/lib.rs

@@ -213,11 +213,13 @@ impl<'a> Iterator for Parser<'a> {
                         Some(String(self.string(pos + 1)))
                     } else {
                         let arg = self.argument();
-                        if let Some(end) = self.must_consume('}') {
-                            let start = self.to_span_index(pos);
-                            let end = self.to_span_index(end + 1);
+                        if let Some(rbrace_byte_idx) = self.must_consume('}') {
+                            let lbrace_inner_offset = self.to_span_index(pos);
+                            let rbrace_inner_offset = self.to_span_index(rbrace_byte_idx);
                             if self.is_literal {
-                                self.arg_places.push(start.to(end));
+                                self.arg_places.push(
+                                    lbrace_inner_offset.to(InnerOffset(rbrace_inner_offset.0 + 1)),
+                                );
                             }
                         }
                         Some(NextArgument(arg))

src/test/ui/macros/issue-83344.rs

@@ -0,0 +1,6 @@
+// chekc-fail
+
+fn main() {
+    println!("{}\
+"); //~^ ERROR: 1 positional argument in format string, but no arguments were given
+}

src/test/ui/macros/issue-83344.stderr

@@ -0,0 +1,8 @@
+error: 1 positional argument in format string, but no arguments were given
+  --> $DIR/issue-83344.rs:4:15
+   |
+LL |     println!("{}\
+   |               ^^
+
+error: aborting due to previous error
+