Provide a more helpful error for tests that fail due to noexec

The rustdoc tests create and execute a file in a temporary directory. By
default on UNIX-like platforms this is in `/tmp`, which some users mount
with the `noexec` option. In those cases, the tests fail in a mysterious
way. This change adds a note that suggests what the problem might be, if
the error looks like it could have been caused by the `noexec` setup.

Closes #12558

src/librustdoc/test.rs

@@ -9,6 +9,7 @@
 // except according to those terms.
 
 use std::cell::RefCell;
+use std::io;
 use std::io::Process;
 use std::local_data;
 use std::os;
@@ -128,7 +129,10 @@ fn runtest(test: &str, cratename: &str, libs: HashSet<Path>, should_fail: bool)
     let exe = outdir.path().join("rust_out");
     let out = Process::output(exe.as_str().unwrap(), []);
     match out {
-        Err(e) => fail!("couldn't run the test: {}", e),
+        Err(e) => fail!("couldn't run the test: {}{}", e,
+                        if e.kind == io::PermissionDenied {
+                            " - maybe your tempdir is mounted with noexec?"
+                        } else { "" }),
         Ok(out) => {
             if should_fail && out.status.success() {
                 fail!("test executable succeeded when it should have failed");