Amend RFC 550 with (expanded) abstract specification rather than algorithm

[pnkfelix]

(tracking issue: rust-lang/rust#30450)

Massive redo of Detailed Design of RFC #550; expanded parts of spec in passing.

As noted in the Edit History addition:

• replaced detailed design with a specification-oriented presentation rather than an implementation-oriented algorithm.
• fixed some oversights in the specification that led to matchers like $e:expr { stuff } being accepted (which match fragments like break { stuff }, significantly limiting future language extensions),
• expanded the follows sets for ty to include OpenDelim(Brace), Ident(where), Or (since Rust's grammar already requires all of |foo:TY| {}, fn foo() -> TY {} and fn foo() -> TY where {} to work).
• expanded the follow set for pat to include Or (since Rust's grammar already requires match (true,false) { PAT | PAT => {} } and |PAT| {} to work). See also RFC issue Allow pat macro matcher be followed by | #1336.

Not noted in Edit History addition:

• expanded/revised terminology section to fit new detailed design
• added "examples of valid and invalid matchers" subsection, that uses the
  specification from detailed design to explain why each is valid/invalid.
• rewrote the algorithm to actually implement the (new) specification, and
  moved the discussion of the algorithm to a non-binding appendix.

---

diff of amendment (split view)

[pnkfelix]

I forgot to mention it in amendment as currently written, but: This change is strongly motivated by rust-lang/rust#25658

(that is, this change, along with an implementation I am prototyping, is meant to fix that issue).

One thing I have not addressed at all here is whether the change described here would go through warning cycle. I am not sure whether I can easily do that (though in principle I guess I could just run both analyses, and set the new one to warn for the initial release cycle).

[pnkfelix]

cc @cmr @rust-lang/lang

[pnkfelix]

One thing that we may want, in order to land this in practice, is a fragment specifier for the subclass of expressions that corresponds to expressions that can be used as a test in an if; i.e., expressions that cannot be followed by a { token.

The reason that we may want such a fragment specifier is because a crater run with my prototype signaled that some crates in the wild are relying on the ability to put { after an $t:expr NT, which was not supposed to be allowed under the original intent of this RFC.

Adding a new fragment specifier that those crates could use instead of expr would solve this; maybe call it $t:pred, for "predicate" ? Or if you prefer, $t:moelarry (which is of course naturally followed by "curly") 😉.

• (This is not the right place for this bike-shed, of course.)
• Opened Add macro_rules fragment class for "expressions that can be followed by {" #1463 for the bikeshed. :)

[emberian]

This is excellent work @pnkfelix.

[emberian]

I found a typo, but I can't find it again...

[emberian]

Here it is! s/inco/incor/

[pnkfelix]

So hopefully I'll have a chance soon to incorporate this properly into the RFC, but two new things:

• I prototyped a way to warning cycle this. (I think I wrote this idea somewhere else.) Just run both analyses: if the new one accepts, then accept; if they both reject, then reject; and if the old accepts but the new rejects, then issue a warning (and note that it will be promoted to a hard error).
• I did a crater run on my most recent implementation, with the warning above promoted to an error so that we actually see regressions reported). I think this latest prototype follows the specification given here.
  • Update: removed links to erroneous crater run, which is superceded by more recent run linked in comment below.

[pnkfelix]

Did a new crater run (the previous one was bad because I was comparing against an old baseline, and so the set of crates had changed in the meantime). Very promising results.

analysis here

Summary:

• 10 total root regressions reported; 1 is probably a false positive, leaving 9 legitimate root regressions.
• 3 regressions are due to expr is followed by { (which I addressed in an earlier comment).
• 2 regressions show expr followed by ..., which is just a bad macro (see In macros, $($x:expr),* fragments can be used to bypass future-proofing restrictions rust#25658 ); i.e. we expect to break this.
• 2 regressions show expr or ty followed by semi-randomly selected identifiers (functions, varargs, copy, swap, perm, drop). This is again just a bad macro
• 2 regressions show ty followed by [. (In theory we could add [ to FOLLOW(ty), but its not clear if its well motivated.)
• 1 regression due to expr followed by _. (In theory we could add _ to FOLLOW(expr), but its not clear if its well motivated.)
• (The bullets add up to more than 9 because one of the regressions, dlib-0.2.0, falls into two of the buckets above.)

As a fun aside, the prototype does fix one crate (though I suspect its not a terribly vital one): https://github.com/arthurtw/echo-rs/blob/master/src/lib.rs#L37

[huonw]

It's not obvious to me if we want to add | here, #1336 (comment) .

[pnkfelix]

so how would that work in the presence of closures, i.e. let f = |VariantA(x, _)|VariantB(_, x)| { ... }; ? Seems hard to parse to me.

[pnkfelix]

(though I guess we would just not allow the generalized pattern form in that context?)

[pnkfelix]

((which is guess is basically what you said in the comment you linked...))

[nikomatsakis]

Hear ye, hear ye! This RFC amendment is now entering final comment period.

[SimonSapin]

For those like me coming from the subteams report and lacking context: RFC 550 is Macro future-proofing.

[pnkfelix]

@nikomatsakis @huonw so, what do you two think: Should I take | back out of FOLLOW(pat), and let us resolve that issue separately (it is already tracked on #1336) ? How high are the chances of us wanting to extent pat to support a | b as a pat, while being unwilling to force the user to write it as (a | b) instead?

My personal inclination is to add | to FOLLOW(pat)`, and use the "require surrounding parens" as the solution for future extension.

[nikomatsakis]

@pnkfelix I say add it

[pnkfelix]

lang team discussed, and decided RFC amendment is good as-is (i.e., it is okay that it is adding | to the FOLLOW set for pat).

[pnkfelix]

and also durka42 on irc asked about the $e:moelarry idea; the short answer there is that we need not couple that extension with this RFC. We can put in the stricter rules that we had intended to have all along, and orthogonally add extensions like that to the classes of macro_rules fragments.

[SimonSapin]

This is making invalid a macro that I’ve been using for about a year, and I don’t know how to fix it. I’d appreciate some help: https://users.rust-lang.org/t/macro-rules-used-like-a-match-expression/4328

[durka]

I replied in the Discourse thread. It's not pretty :)

On Mon, Jan 18, 2016 at 12:50 PM, Simon Sapin notifications@github.com
wrote:

This is making invalid a macro that I’ve been using for about a year, and
I don’t know how to fix it. I’d appreciate some help:
https://users.rust-lang.org/t/macro-rules-used-like-a-match-expression/4328

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