Macro import/export

[eholk]

We need a way to export macros from modules and import them in other modules.

Currently we either prepend a string of macro definitions to every file, such as with debug!, error!, etc., or we just copy and paste the macros we need. This second approach leads to code duplication and lots of subtly different implementations.

As a temporary workaround, we could probably use include!.

[eholk]

This would provide a nice solution for #2247 as well.

[brson]

Wishfully putting this on the 0.4 milestone.

[graydon]

non-critical for 0.6, de-milestoning

[brson]

Nominating this for a milestone.

[pnkfelix]

Does include! actually even function as a work-around for this problem? My brief experiments with using that did not seem to bear fruit.

[isaacaggrey]

@pnkfelix - I can't recall why, but I had to wrap the include! in a function -- see example use case in a bug I am working on #4219 (comment)

[pnkfelix]

@isaacaggrey but in that case the scope of the macro will be limited to that function, yes? (At least, that matches my intuition about the usual intentions for lexically-scoped macros.) So that seems like a problem.

[isaacaggrey]

@pnkfelix IIUC you would think so, but it seems to not limit the macro's scope to the function. If you are trying to import macros from a separate file (in my case, integer-templates.rs), then I can call the imported macros (in my case, int_template!) anywhere in the current file (even outside of the function).

I would link to a gist, but my Rust build is currently not working.

[pnkfelix]

Hmm, I see that this pattern certainly used to work in the code base, but we do not seem to use that pattern any more; @jbclements changed it in SHA: ca147a0

He replaced it with some #[macro_escape] attribute. See also commit SHA: 08b6057 which has a log message that explains the new scoping rules for macros (and also #5120 )

(Let me see if I can use that information to get include to work on macro definitions.)

[pnkfelix]

Or rather, the whole point is to avoid using include and instead use the modules themselves. Which is the original point of this ticket anyway. :)

[isaacaggrey]

Thanks for the update, @pnkfelix . I hadn't followed the last couple months of macro changes and didn't even realize that hack didn't work anymore!

I suppose I have the same question/issue as you then now. :)

[pcwalton]

I do not believe this is backwards incompatible.

[graydon]

accepted for feature-complete milestone

[pnkfelix]

visiting for triage, email 2013-08-05.

I've looked at this in the past. Its something I'd like to see happen, especially coming from my experience working with Racket. (I'm not sure how many of the lessons/caveats from the Scheme community carry over here, though ... it depends in part on whether Rust has an analogue to let{,*,rec}-syntax or not.)

[alexcrichton]

As pointed out in #10723, it may be possible to place #[macro_escape] at the top of a crate to let all the macros get serialized to the crate metadata. This would remove any need for importing/exporting macros, although it's not as general of a solution as having a use statement import a macro.

[pnkfelix]

Accepted for P-low.

[sfackler]

This was done in #11151.

[alexcrichton]

Hurray! nice work @sfackler!