Make pest_generator / pest_derive no_std compatible.

[01mf02]

This is the follow-up pull request to #498.
It adds "std" feature flags for both pest_generator and pest_derive.
Currently, the correct function of this can only be tested by running cargo test --no-default-features in the derive directory.
Running cargo test --no-default-features in the root directory unfortunately does not run the tests in no_std mode.

[CAD97]

Again, I prefer crates being unconditionally #![no_std] and then conditionally doing extern crate std; Other than that, everything looks good to me.

I was going to say that you can unconditionally use ::core as a path for core items, but since we're emitting tokens with call-site spans, they'll have call-site edition hygiene, and as such won't have core available for edition=2015 without extern crate core. At least as I understand it, anyway.

[01mf02]

Hi @CAD97, I made the tests unconditionally no_std as you suggested, thanks for spotting this.

I also tried using ::core unconditionally, but unfortunately did not get this to work. The original patch (#243) used a similar technique.

[01mf02]

Oh, I now remember why I originally did not make the tests unconditionally #![no_std]:
The reason for this is that #![no_std] makes ::core accessible.
Therefore, indeed, somebody could change the code in generator.rs to use ::core::option::Option regardless of std and believe that everything is fine, because all tests (especially from derive) would still pass.
However, any regular user of the library that does not use #![no_std] would then see failures such as:

error[E0433]: failed to resolve: maybe a missing crate `core`?
  --> derive/tests/grammar.rs:17:10
   |
17 | #[derive(Parser)]
   |          ^^^^^^ not found in `core::option`

That is why I think it is not a good idea to unconditionally use #![no_std] in the derive tests.

[01mf02]

Is there anything that blocks the merge of this PR?
Should I revert my changes that made the tests unconditionally #![no_std]?

[01mf02]

@CAD97, @dragostis, what is the status on this?

[luojia65]

Should this be merged? no_std pest can be used on operating system kernels that will not provide an std crate.

[01mf02]

I now reverted my commit that made the derive tests unconditionally #![no_std], as explained above.
From my point of view, this should be merged. :)

[CAD97]

Sorry that this has sat for so long. If you fix the conflict, I think I can merge this. We're still waiting on @dragostis for a publish, though.

[01mf02]

Ok, I fixed the conflict. From my POV, this is ready to merge.

[CAD97]

LGTM

bors: r+

[bors]

Build succeeded:

• Minimal Versions Testing
• Unit, Style, and Lint Testing

[01mf02]

Amazing! Thank you for merging this.