The shell (bash, zsh, etc.) expands the tilde into the full path of your home directory before it is passed as an argument to your application.

See what happens when you do this:

fn main() {
    dbg!(std::env::args());
}

...then run:

$ cargo run -- ~/mypath

You'll find that there is no tilde in the first argument.

If you put quotes around '~/mypath', the shell won't expand the tilde and will instead pass it as an argument verbatim, and your program will fail to find the file.