RFC: pass macro call location to macro-function

[ihnorton]

This PR adds an implicit __LOCATION__ &meta argument (line/file info) to all macros, automatically added in lowering for macro definitions, and in the parser for macro calls. Fixes #9577. The main goal is better Cxx/Gallium error reporting (location information for C++ string macros), but it may also be useful for macro-heavy users such as JuMP.

The main downsides I know of are:

• makes AST round-tripping more complicated (one block of tests in show.jl is currently commented-out for this reason, but those should be "fixable" with some string-replace hackery)
• it is really ugly

Comments?

(FWIW, Clojure also accomplishes this goal using implicit macro arguments: &env and &form; but I am quite sure that this implementation is ... considerably less graceful)

[StefanKarpinski]

The fact that Clojure does this with implicit arguments is actually the most compelling argument (see what I did there?) that I've heard for doing it this way.

[ihnorton]

For kicks, I renamed the argument to &meta after Clojure. It seems less ugly now. The downside to naming it that way is that the argument can only be accessed through constructed symbol names (... possibly a good thing?). I also re-implemented @__LINE__ and @__FILE__ using &meta.

[timholy]

Any chance we can do more with @generated functions? It may be a consequence of nested-genfunctions, but with Interpolations.jl I get nothing useful in terms of profiling backtraces.

[ihnorton]

@timholy I see what you are doing here! 😄

I haven't fully grokked the isstaged handling yet, but a quick look leads me to doubt that missing debug information is directly related to the fact that @generated is a macro. From playing with the @generated example in the manual, it seems like the AST does at least get top-level line information. In a more complicated example, missing info could well be due to the fact that we strip line info during inlining (I need to get back to that).

If you wouldn't mind to file an issue with a small example, I'll be happy to take a look (requiring Interpolations.jl or whatever other package is fine).

[vtjnash]

i think this has been resolved and can be deleted?

[tkelman]

By what?

[StefanKarpinski]

#9577, #13339, #21746