Add a UI test for rust-lang#50637

This test relies on the fact that restrictions on expressions in `const
fn` do not apply when computing array lengths. It is more difficult to
statically analyze than the simple `loop{}` mentioned in rust-lang#50637.

This test should be updated to ignore the warning after rust-lang#49980 is resolved.

src/test/ui/const-eval/infinite_loop.rs

@@ -0,0 +1,25 @@
+// Copyright 2018 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.
+
+#![feature(const_let)]
+
+fn main() {
+    // Tests the Collatz conjecture with an incorrect base case (0 instead of 1).
+    // The value of `n` will loop indefinitely (4 - 2 - 1 - 4).
+    let _ = [(); {
+        //~^ WARNING Constant evaluating a complex constant, this might take some time
+        //~| ERROR could not evaluate repeat length
+        let mut n = 113383; // #20 in A006884
+        while n != 0 { //~ ERROR constant contains unimplemented expression type
+            n = if n % 2 == 0 { n/2 } else { 3*n + 1 };
+        }
+        n
+    }];
+}

src/test/ui/const-eval/infinite_loop.stderr

@@ -0,0 +1,41 @@
+error[E0019]: constant contains unimplemented expression type
+  --> $DIR/infinite_loop.rs:20:9
+   |
+LL | /         while n != 0 { //~ ERROR constant contains unimplemented expression type
+LL | |             n = if n % 2 == 0 { n/2 } else { 3*n + 1 };
+LL | |         }
+   | |_________^
+
+warning: Constant evaluating a complex constant, this might take some time
+  --> $DIR/infinite_loop.rs:16:18
+   |
+LL |       let _ = [(); {
+   |  __________________^
+LL | |         //~^ WARNING Constant evaluating a complex constant, this might take some time
+LL | |         //~| ERROR could not evaluate repeat length
+LL | |         let mut n = 113383; // #20 in A006884
+...  |
+LL | |         n
+LL | |     }];
+   | |_____^
+
+error[E0080]: could not evaluate repeat length
+  --> $DIR/infinite_loop.rs:16:18
+   |
+LL |       let _ = [(); {
+   |  __________________^
+LL | |         //~^ WARNING Constant evaluating a complex constant, this might take some time
+LL | |         //~| ERROR could not evaluate repeat length
+LL | |         let mut n = 113383; // #20 in A006884
+LL | |         while n != 0 { //~ ERROR constant contains unimplemented expression type
+LL | |             n = if n % 2 == 0 { n/2 } else { 3*n + 1 };
+   | |                    ---------- program will never terminate
+LL | |         }
+LL | |         n
+LL | |     }];
+   | |_____^
+
+error: aborting due to 2 previous errors
+
+Some errors occurred: E0019, E0080.
+For more information about an error, try `rustc --explain E0019`.