String interpolating a relative or absolute path value will cause Nix to copy the contents at that path to the store before constructing the string. (NixOS/nix#9428)

• This has I/O implications for evaluation times, especially for large files/directories
• This will error out if the path does not exist or if the user lacks permissions to view it
• In a flake-controlled project (which is already copied as part of evaluation), this creates duplicate store objects

Solutions:

• Avoid string interpolating path values. Use builtins.toString if you need the values to be a string.
• Avoid using path values in general, and prefer using a generic string ("/var/empty" instead of /var/empty)

nix-repl> "${/var/empty}"
"/nix/store/<hash>-empty"

nix-repl> toString /var/empty
"/var/empty"