Modern compilers generally have a language-specific front and that generates an intermediate representation of the program logic, which is then transformed into

Modern compilers generally have a language-specific front and that generates an intermediate representation of the program logic, which is then transformed into an abstract representation (such as a single static analysis tree) for optimization. That is then transformed into an abstract machine description language, which gets further transformed by the back end into concrete machine instructions or assembly code. Outside of the language-specific front end, compilers generally have no knowledge of the programming language itself. There is no technical advantage to transforming Rust into C when it comes to the middle and back ends, which form the bulk of the compiler. There are no language-specific optimization opportunities. There are, of course, restrictions on what you can do in some languages that eliminate optimization opportunities, but you're not suddenly going to be able to take advantage of those opportunities by transforming your code into a langue that lacks the restrictions, because then you change the semantics of your code.