title: How do I build with a custom libstd?
body: <p>I want to make some changes in libstd and then test them with a toy program. It looks like I can build <code>libstd.so</code> by going to <code>rust/src/libstd</code> and doing a (nightly) <code>cargo build</code>. Once I've done that, how do I get a toy program to build with that libstd instead of the regular version installed on my system?</p>