diff --git a/tests/compile-fail/intrinsics/float_to_int_64_too_big6.rs b/tests/compile-fail/intrinsics/float_to_int_64_too_big6.rs
new file mode 100644
index 00000000000..f008131a6e5
--- /dev/null
+++ b/tests/compile-fail/intrinsics/float_to_int_64_too_big6.rs
@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f64, u128>(f64::MAX); } //~ ERROR: cannot be represented in target type `u128`
+}
diff --git a/tests/compile-fail/intrinsics/float_to_int_64_too_big7.rs b/tests/compile-fail/intrinsics/float_to_int_64_too_big7.rs
new file mode 100644
index 00000000000..69922e60a6b
--- /dev/null
+++ b/tests/compile-fail/intrinsics/float_to_int_64_too_big7.rs
@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f64, i128>(f64::MIN); } //~ ERROR: cannot be represented in target type `i128`
+}
diff --git a/tests/run-pass/float.rs b/tests/run-pass/float.rs
index c56d12a0c00..fc513ead8dd 100644
--- a/tests/run-pass/float.rs
+++ b/tests/run-pass/float.rs
@@ -114,6 +114,8 @@ fn basic() {
     assert_eq(y, 42.0_f32);
 }
 
+/// Many of these test values are taken from
+/// https://github.com/WebAssembly/testsuite/blob/master/conversions.wast.
 fn casts() {
     // f32 -> i8
     test_both_cast::<f32, i8>(127.99, 127);