nil diagnostics only accepts a single filename, so the obvious glob doesn't work:

$ nil diagnostics **/*.nix
Unrecognized argument: my-file.nix

Run nil --help for more information.

This works, but it's a bit cumbersome:

find . -path '*.nix' -exec nil diagnostics {} \;

It would also be nice if nil diagnostics with no arguments automatically discovered all .nix files in the current directory and its subdirectories.