Shorten error message and add link to test

Implements @bjorn3's suggestions.

Author: ecstatic-morse

src/librustc/mir/interpret/error.rs

@@ -400,8 +400,7 @@ impl<'tcx, O> EvalErrorKind<'tcx, O> {
             GeneratorResumedAfterReturn => "generator resumed after completion",
             GeneratorResumedAfterPanic => "generator resumed after panicking",
             InfiniteLoop =>
-                "duplicate interpreter state observed while executing this expression, \
-                const evaluation will never terminate",
+                "duplicate interpreter state observed here, const evaluation will never terminate",
         }
     }
 }

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

@@ -16,7 +16,7 @@ fn main() {
     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
+        let mut n = 113383; // #20 in https://oeis.org/A006884
         while n != 0 { //~ ERROR constant contains unimplemented expression type
             n = if n % 2 == 0 { n/2 } else { 3*n + 1 };
         }

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

@@ -13,7 +13,7 @@ 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 | |         let mut n = 113383; // #20 in https://oeis.org/A006884
 ...  |
 LL | |         n
 LL | |     }];
@@ -26,10 +26,10 @@ 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 | |         let mut n = 113383; // #20 in https://oeis.org/A006884
 LL | |         while n != 0 { //~ ERROR constant contains unimplemented expression type
 LL | |             n = if n % 2 == 0 { n/2 } else { 3*n + 1 };
-   | |                    ---------- duplicate interpreter state observed while executing this expression, const evaluation will never terminate
+   | |                    ---------- duplicate interpreter state observed here, const evaluation will never terminate
 LL | |         }
 LL | |         n
 LL | |     }];