Right now, I don't see any advantage of embedding the DSL into Rust code, when instead I could have the DSL completely separate from Rust, and just use Rust's F

Right now, I don't see any advantage of embedding the DSL into Rust code, when instead I could have the DSL completely separate from Rust, and just use Rust's FFI to access what the DSL compiler generates. One benefit of the FFI route is that the result wouldn't be Rust-specific. Remember that one of the goals is to become confident that the implementation is correct without also having to analyze whether rustc is correct or whether the Rust language has meaningful semantics. If we involve rustc in it, then we've greatly expanded the scope of what we have to analyze. Even if asm! were stable in Rust, I would still prefer to have my assembly language in separate .s/.asm files. rustc gets updated every 6 weeks, which is great, but I'd really kind of like my assembler to be more static than that. Assembly language benefits massively from having macros, but Rust actually is not as good of a macro language for assembly as existing assembler macro languages. I'd rather see rustc improve at compiling Rust code than have it try to also compile ASM code.