Unrolled build for #144655

Rollup merge of #144655 - jdonszelmann:cleanup-codegen-fn-attrs, r=WaffleLapkin

clean up codegen fn attrs

This honestly had become a mess over time and needed some love. No behavior should've changed, and I effectively just moved code around. Though, I can't promise there weren't any bugs in the original code with how randomly it was organised.....

r? `@WaffleLapkin`

Author: rust-timer