Add dedicated testcase for #15881

Also enable an ignored test case, closes #15881

src/test/run-pass/issue-15881-model-lexer-dotdotdot.rs

@@ -0,0 +1,43 @@
+// Copyright 2015 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.
+//
+// regression test for the model lexer handling the DOTDOTDOT syntax (#15877)
+
+pub fn main() {
+    match 5u {
+      1u...5u => {}
+      _ => panic!("should match range"),
+    }
+    match 5u {
+      6u...7u => panic!("shouldn't match range"),
+      _ => {}
+    }
+    match 5u {
+      1u => panic!("should match non-first range"),
+      2u...6u => {}
+      _ => panic!("math is broken")
+    }
+    match 'c' {
+      'a'...'z' => {}
+      _ => panic!("should suppport char ranges")
+    }
+    match -3i {
+      -7...5 => {}
+      _ => panic!("should match signed range")
+    }
+    match 3.0f64 {
+      1.0...5.0 => {}
+      _ => panic!("should match float range")
+    }
+    match -1.5f64 {
+      -3.6...3.6 => {}
+      _ => panic!("should match negative float range")
+    }
+}

src/test/run-pass/issue-2185.rs

@@ -9,7 +9,6 @@
 // except according to those terms.
 
 // ignore-test
-// ignore-lexer-test FIXME #15881
 
 // notes on this test case:
 // On Thu, Apr 18, 2013-2014 at 6:30 PM, John Clements <clements@brinckerhoff.org> wrote: