I can't remember enough to name names but one obviously didn't to the point that it mentioned what the GNU code does in comments.

I'm really not certain this is enough to be a copyright problem.

For instance, GNU and POSIX both publish their specs for the coreutils. If a coder were to take a look at the actual GNU code (which BTW is published for everyone to see), copyright law has a well trodden distinction between the idea and the expression -- that is, ideas are not copyrightable. If the "idea" simply amounts to what would be a more a detailed specification, I'm not sure there is a problem, like ... GNU uses this kernel facility for X. The problem would be vast amounts of "expression", especially "creative expression", directly copied and reimplemented in Rust. If the code is meat and potatoes, not 10xer galaxy brain fare ("I wrote a custom allocator which is suspiciously like the custom allocator implemented by GNU"), there shouldn't be an issue.

Think about what copyright to a play, or a novel, or a screenplay is. Now imagine a comment in the text/source: "This is how Toni Morrison did her characterizations in Beloved". This obviously isn't a copyright violation, unless you're copying the actual expression or a translation of the actual expression found in Beloved.