[inferiorhuman]

  Rust assumes a runtime, the standard library assumes a stack exists, a heap
  exists, and that main() is called by an OS;

Wrong.

Source: I'm writing Rust without a runtime without a heap and without a main function. You can too.

[xobs]

The Rust runtime will, at a minimum, set up the stack pointer, zero out the .bss, and fill in the .data section. You're right in that a heap is optional, but Rust will get very cranky if you don't set up the .data or .bss sections.

[comex]

As will C.