Get make check working on MSVC

* An apparent bug in VS 2013's implementation of the `exp2` function is worked
  around in one of flt2dec's tests.

Author: alexcrichton

src/libcoretest/num/flt2dec/strategy/dragon.rs

@@ -35,7 +35,22 @@ fn shortest_sanity_test() {
 
 #[test]
 fn exact_sanity_test() {
-    f64_exact_sanity_test(format_exact);
+    // This test ends up running what I can only assume is some corner-ish case
+    // of the `exp2` library function, defined in whatever C runtime we're
+    // using. In VS 2013 this function apparently had a bug as this test fails
+    // when linked, but with VS 2015 the bug appears fixed as the test runs just
+    // fine.
+    //
+    // The bug seems to be a difference in return value of `exp2(-1057)`, where
+    // in VS 2013 it returns a double with the bit pattern 0x2 and in VS 2015 it
+    // returns 0x20000.
+    //
+    // For now just ignore this test entirely on MSVC as it's tested elsewhere
+    // anyway and we're not super interested in testing each platform's exp2
+    // implementation.
+    if !cfg!(target_env = "msvc") {
+        f64_exact_sanity_test(format_exact);
+    }
     f32_exact_sanity_test(format_exact);
 }
 

src/libcoretest/num/flt2dec/strategy/grisu.rs

@@ -77,7 +77,11 @@ fn shortest_f64_hard_random_equivalence_test() {
 
 #[test]
 fn exact_sanity_test() {
-    f64_exact_sanity_test(format_exact);
+    // See comments in dragon.rs's exact_sanity_test for why this test is
+    // ignored on MSVC
+    if !cfg!(target_env = "msvc") {
+        f64_exact_sanity_test(format_exact);
+    }
     f32_exact_sanity_test(format_exact);
 }