Support running doc tests serially

[llamicron]

This crate is very useful for my use-case (interacting with hardware), but I have quite a few doc tests that can't be run unless serially. Does this crate support running doc tests serially? If not, could that be supported?

[palfrey]

I've just been experimenting with this, and the following seems to work ok (for values of "doesn't seem to fail"). OTOH, I've been trying a few experiments, and so far I think rustdoc tests are maybe run separately enough that the serial_test usage of a common lock structure just doesn't actually work. Not sure how to fix this barring digging into the rustdoc internals and trying to figure out how to share data between rustdoc tests.

//! ```
//! # #[macro_use] extern crate serial_test;
//! # #[serial_test::serial]
//! # fn main() {
//! assert!(true)
//! # }
//! ```

I've marked the boilerplate bits (everything other than the assert! in this minimal check) as "don't display".

[palfrey]

Yup, I've just confirmed this. Between https://quietmisdreavus.net/code/2018/02/23/how-the-doctests-get-made/ and https://github.com/rust-lang/rust/blob/c4e33b51c1a2d5e599b949fa3006467b88df253a/src/librustdoc/doctest.rs#L233 I've managed to confirm that rustdoc basically makes separate executables for each doctest, and so the standard serial_test methods of locking won't work.

OTOH, I could just use something like https://docs.rs/fslock and make a new attribute that lets you do file-based locking, but that's a notable change that would take me a bit to do and I've got a few other things on my plate at the moment. I might get around to doing this eventually, but if you felt like contributing patches for that it would be appreciated :)

[palfrey]

#43 should solve this.