regarding the "security" aspect, I'm interested in what an attack vector would look like against a build system

like, say you are building code, and all the below functions are compilers, and * denotes an evil compiler. Every link in the chain is a compiler building another compiler, until the last node which builds the code.

A() -> B() -> Evil*() -> D() -> E(code) -> binary

how in the world would the evil compiler in this situation inject something malicious into the final binary?