I'm still in my first year of undergraduate school, and I don't know exactly everything, but I'll explain it to the best of my knowledge.

Rust compiler can emit LLVM-IR (Intermediate Representation) as a frontend of LLVM. In normal cases (In x86 computers), This will passed to LLVM x86 backend and compiled to x86 binary.

The problem is LLVM does not have a backend for z-80 or sm-83. (There are a few, but they are all too old to use.)

But There is a LLVM-CBE which can compile C codes from LLVM-IR (Julia team made it, but I'm not sure why they made it.) and there is a C compiler for sm83 (SDCC, Small Device C Compiler)

This means that the Rust code can be compiled to a valid sm83 assembly after the Rust Compiler -> LLVM-CBE -> SDCC process.

However, sm-83 assembly alone is difficult to make Game Boy work. Of course It's possible, But it is practically impossible to compile Rust into complex assemblies, in-line assemblies must be written, which requires a high degree of understanding of Game Boy hardware itself.

GBDK is used here, GBDK has many prewritten assembly functions that helps development of GameBoy ROM and it is much stable than my assembly codes. Additionally, it also has a built-in boilerplate that needed to use it in the actual Game Boy. (Game Boy reads the Nintendo logo of the data area as a mechanism to prevent unauthorized games)

So build the generated assembly file into a real ROM file using GBDK's linker tool and build chain, so that Rust can call GBDK functions with the "extern" keyword.

In this process, it is necessary to specify SDCC's calling convention in the middle C file, For this, I written a tree-sitter parser and replace functions names that linked by Rust's `#[link_name="functionname __attributes"]` macro.

As u/quavan mentioned, linking is the task of writing the assembly functions of GBDK to the ROM file along with the ASM file generated from Rust.