Auto merge of #28247 - christopherdumas:fix_28243, r=eddyb

as per #28243.

Author: bors

src/libsyntax/parse/parser.rs

@@ -2365,7 +2365,7 @@ impl<'a> Parser<'a> {
                         self.fileline_help(last_span,
                             &format!("try parenthesizing the first index; e.g., `(foo.{}){}`",
                                     float.trunc() as usize,
-                                    &float.fract().to_string()[1..]));
+                                    format!(".{}", fstr.splitn(2, ".").last().unwrap())));
                     }
                     self.abort_if_errors();
 

src/test/parse-fail/tuple-float-index.rs

@@ -0,0 +1,16 @@
+// Copyright 2012 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// compile-flags: -Z parse-only
+
+fn main () {
+    (1, (2, 3)).1.1; //~ ERROR unexpected token
+                     //~^ HELP try parenthesizing the first index; e.g., `(foo.1).1`
+}