Depending on compiler-builtins creates more unresolved symbols.

[rust-lang]

Depending on compiler-builtins creates more unresolved symbols.

[japaric]

I don't use staticlibs much but I don't think it's going to give you what I think you want to produce: a standalone (archived) (flashable) object file because building staticlibs doesn't involve the linker at all and that's what takes cares of "filling" the undefined references you are seeing.

My understanding is that building staticlibs is just going to (re)archive together all the involved object files but not try to resolve references between them.

What does arm-none-eabi-nm target/arm-none-eabihf/debug/libtyran.a | grep "T " output? I expect you'll see some of the "undefined" symbol in there. e.g. T __aeabi_uidiv (note: that's a "defined" symbol).

[turnage]

I did not know that building static libs did not invoke the linker!

The offending code that was creating references to compiler-rt symbols was:

for i in 0..10 {}

When building an rlib, this code stopped creating references to compiler-rt symbols (I am not depending on this repo anymore).

There were still the following undefined symbols:

undefined reference to `core::panicking::panic::h86aded962ecadfcc'
undefined reference to `__aeabi_unwind_cpp_pr0'
undefined reference to `_Unwind_Resume'

But these references were removed with lto.

Thank you for the response @japaric!