Indentation with tab characters in complete for fish shell - Why?

[BaumiCoder]

I encountered that the scripts for completion in the fish shell uses tab characters for indentation. An example output is in the rust-lang/rust project. If I found the correct source file in clap_complete, the reason are tab characters in this multi line string:

clap/clap_complete/src/aot/shells/fish.rs

Line 253 in 8387c81

\tstring join \\n{optspecs}\n\

All code for all other shells are using 4 spaces for indentation. For example the counterpart for zsh is here:

clap/clap_complete/src/aot/shells/zsh.rs

Line 34 in 8387c81

typeset -A opt_args

The official fish_indent tool uses 4 spaces (tested with 4.7.1). Is there some reason for using a different kind of indentation for the fish shell in clap_complete or is it a bug?