Unstable compilers

Take for example 128-bit integer support. Rust has 128-bit integers. Code to handle these e.g. formatting them for output as text - exists but in a typical Rust application which doesn't actually have any 128-bit integers the code for them never ends up in the resulting binary.

But, Linux currently just says nope, we'll take everything just in case, because a loadable module might use it. A few kilobytes of formatting for types we never use? Give me those too.

So for Stable Rust Linux would get a bunch of 128-bit integer formatting code it has no use for, on the off chance that somebody might some day write a loadable module which expects 128-bit integer formatting to work. In Rust for Linux they've instead just made this code conditional and switched off the condition, it won't compile in your loadable module and the kernel avoids a few kilobytes of "format 128-bit integers" code.

The push back from the Rust community is they're not interested in doing a bunch of extra work to help people who want all the symbols but then wish somebody else would do the work to slim it down for them, pick a lane. So the Rust for Linux change will never land in the stable Rust in this form.