@shadykaty thanks for clarification.
I agree with most of your explanations.
But there is one thing missing, which I implied when said about TLS:
C allows to go even deeper - to replace some code with parts, written in assembly language.
Since not every instruction is available on every CPU, such code usually written in several modifications, wrapped with selection logic.
This is, as far as I know, what compilers usually not doing.
At the same time such code may be very important, for example when it uses hardware crypto (while allowing less optimized code run on CPUs, which do not have such accelerations).
Because of it I wonder if it is possible to effectively mix assembly code and Rust code.
By the way, GCC is so good at it, that allows mixing code even within single function.
While MSVC (al least for x64 code) require assembly and C functions to be in separate files.