auto merge of #8440 : sfackler/rust/env-fix, r=pcwalton

The type of the result of option_env! was not fully specified in the
None case, leading to type check failures in the case where the variable
was not defined (e.g. option_env!("FOO").is_none()).

Also cleaned up some compilation warnings.

src/test/run-pass/extoption_env-not-defined.rs

@@ -9,6 +9,5 @@
 // except according to those terms.
 
 fn main() {
-    let opt: Option<&'static str> = option_env!("__HOPEFULLY_DOESNT_EXIST__");
-    assert!(opt.is_none());
+    assert!(option_env!("__HOPEFULLY_DOESNT_EXIST__").is_none());
 }