Rollup merge of #44671 - GuillaumeGomez:run-button, r=steveklabnik

Fix run button

r? @QuietMisdreavus

Before:

![before](https://user-images.githubusercontent.com/3050060/30538927-612c6a44-9c70-11e7-9ca2-f3860d880b95.png)

After:

![after](https://user-images.githubusercontent.com/3050060/30538929-6416ea0e-9c70-11e7-990a-7e4fbf5d7319.png)

Quite urgent.

src/librustdoc/html/static/rustdoc.css

@@ -825,6 +825,7 @@ span.since {
 	position: absolute;
 	left: -1px;
 	margin-top: 7px;
+	z-index: 1;
 }
 
 .tooltip {
@@ -861,3 +862,7 @@ span.since {
 	border-style: solid;
 	border-color: transparent black transparent transparent;
 }
+
+pre.rust {
+	position: relative;
+}