Unrolled build for rust-lang#117025

Rollup merge of rust-lang#117025 - Urgau:cleanup-improve-check-cfg-impl, r=petrochenkov

Cleanup and improve `--check-cfg` implementation

This PR removes some indentation in the code, as well as preventing some bugs/misusages and fix a nit in the doc.

r? ```@petrochenkov``` (maybe)

Author: rust-timer