Test runner fails if /tmp is mounted with "noexec"

[felixc]

When running make check, it failed during the doc-std target with 244 failures and 0 successes. All of them produced an error of the form "'couldn't run the test: permission denied', [snip]/rust/src/librustdoc/test.rs:131".

Investigating this further, it turns out to be because rustdoc's test.rs attempts to create a file in a temporary directory and then execute it. By default on my platform, this happens in /tmp. My /tmp, however, is mounted with the "noexec" option that prevents executing files from it.

Arguably this is just user error on my part for configuring things this way, but I don't think it's an uncommon setup. It was also quite a mysterious way for things to fail. Perhaps the test runner could check for this and fail more usefully?

In the meantime, a workaround is to run the tests with the environment variable TMPDIR set to something else.