rustdoc doesn't support reading from STDIN via the fake path `-` (dash)

[fmease]

While rustc supports reading from STDIN if the fake path - (dash) is provided (a CLI convention), rustdoc does not.

This is definitely not something I need, it's merely an inconsistency between rustc and rustdoc I've noticed and that I'm recording here.

rustc Example

printf 'pub struct F;' | rustc - --crate-type=lib

Crate name: rust_out; virtual file path: <anon>.

Reproducer

printf 'pub struct F;' | rustdoc -

Currently fails with:

error: couldn't read -: No such file or directory (os error 2)