Rustc does a good job of removing unused code, especially with LTO. The trick is to make sure the std library main/panic/backtrace logic doesn't call code you don't want to pay for.
IIRC there's also a mutex somewhere in there used to workaround some threading issues in libc, which brings in a bespoke mutex implementation; I can't remember whether that mutex can be easily disabled, but I think there's a way to use the slower libc mutex implementation instead.
Also, std::fmt is notoriously bad for code size, due to all the dyn vtable shenanigans it does. Avoid using it if you can.
Regardless, the only way to fix many of the problems with std is rebuilding it with the annoying features compiled out. Cargo's build-std feature should make this easy to do in stable Rust soon (and it's available in nightly today).
IIRC there's also a mutex somewhere in there used to workaround some threading issues in libc, which brings in a bespoke mutex implementation; I can't remember whether that mutex can be easily disabled, but I think there's a way to use the slower libc mutex implementation instead.
Also, std::fmt is notoriously bad for code size, due to all the dyn vtable shenanigans it does. Avoid using it if you can.
Regardless, the only way to fix many of the problems with std is rebuilding it with the annoying features compiled out. Cargo's build-std feature should make this easy to do in stable Rust soon (and it's available in nightly today).