I've turned on name mangling for compiler_builtins. Now getting this error:

error[E0658]: use of unstable library feature 'rustc_private': this crate is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead?
   --> c-scape/src/c.rs:896:5
    |
896 |     compiler_builtins::mem::memcmp(a as *const u8, b as *const u8, len)
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: see issue #27812 <https://github.com/rust-lang/rust/issues/27812> for more information
    = help: add `#![feature(rustc_private)]` to the crate attributes to enable

I'm not sure how to proceed since the dependency is in Cargo.toml already and #![feature(rustc_private)] seems like a bad idea. I thought it might be caused by the compiler-builtins feature of compiler_builtins, but disabling that doesn't change anything.

Followed @bjorn3's suggestion and just declared these in an extern "C" block instead of implementing wrappers. memcmp and memmove (edit: and bcmp) are commented out for now to avoid unused warnings, but presumably we'll want them in there once the c module is exposed.

I somehow didn't notice that compiler-builtins directly provides no_mangle extern "C" definitions, but it makes sense!