tl;dr: Trying to port serde to a no_std environment requires proc_macro to also be no_std. The straightforward approach of subbing out the std types in the compiler internals doesn't work.

The specific use case is Intel SGX via rust-sgx-sdk, which provides an alternative std. extern crate sgx_tstd as std; doesn't work in the compiler for some reason. Is this a particularly unreasonable goal?

The most related issue I could find is rust-lang/rfcs#1184 but it looks like there ended up being no easy way to inject no_std into everything!