Be accurate on format! parse error expectations

src/libsyntax_ext/format.rs

@@ -138,15 +138,23 @@ fn parse_args<'a>(
     }
 
     let fmtstr = p.parse_expr()?;
+    let mut first = true;
     let mut named = false;
 
     while p.token != token::Eof {
         if !p.eat(&token::Comma) {
-            let mut err = ecx.struct_span_err(p.token.span, "expected token: `,`");
-            err.span_label(p.token.span, "expected `,`");
-            p.maybe_annotate_with_ascription(&mut err, false);
-            return Err(err);
+            if first {
+                // After `format!(""` we always expect *only* a comma...
+                let mut err = ecx.struct_span_err(p.token.span, "expected token: `,`");
+                err.span_label(p.token.span, "expected `,`");
+                p.maybe_annotate_with_ascription(&mut err, false);
+                return Err(err);
+            } else {
+                // ...after that delegate to `expect` to also include the other expected tokens.
+                return Err(p.expect(&token::Comma).err().unwrap());
+            }
         }
+        first = false;
         if p.token == token::Eof {
             break;
         } // accept trailing commas