macro_rules! should support gensym for creating items

[geofft]

Currently macro_rules! doesn't support the ability to generate items with unique names. This generates an error that I'm defining struct Foo multiple times:

macro_rules! x {
    () => {struct Foo;}
}

x! {}
x! {}

The standard workaround is to accept an ident parameter in the macro, and name everything after that identifier somehow, even if only one item with that name is intended to be visible. lazy_static!, for instance, uses the ability of (non-tuple) structs to share a name with a value. Another option is to name a module after the ident and stuff everything inside that module. But these are incomplete workarounds: the former approach lets you create at most one type and at most one value, and the latter approach causes other hygiene problems because not everything expands the same inside a module. (For instance, ty parameters don't work right because paths to types aren't necessarily valid in the module.)

Having an actual gensym would solve this problem. I think you can do a simple syntax where a macro_rules! parameter of type gensym consumes no tokens and expands to a new, unique identifier per macro invocation, so you'd have e.g.

macro_rules! x {
    ($a:gensym) => {struct $a;}
}

which could still be invoked as x! {}, but can be expanded multiple times, creating a unique structure each time. And you could do $a:gensym $b:gensym if you needed multiple names. If that's sensible, I can write this up more formally or attempt implementing it.

Note that this is different from rust-lang/rust#19700: that one's about hygiene for existing items invoked by the macro, not the creation of new items. (Also I think $crate is a sufficient workaround for that issue in practice, but doesn't help at all here.) However, the backwards-compatibility rationale in that issue applies: we can't simply gensym all literal item names in macros the way we do for let-bindings, since that would break macros that create items with specific names. So there needs to be specific syntax for gensym.

One answer is to put this off until macro! (or whatever you want to call macro_rules! 2.0), but if this can be done simply, clearly, and backwards-compatibly, it seems worth doing now, unless the new system is happening very soon (which it doesn't seem like it is).

[Kimundi]

Alternatively there could be some new macro syntax for declaring a identifier on the RHS of a macro:

macro_rules! x {
    () => {
        $gensym a;
        struct $a;
    }
}

[durka]

To throw out another idea, clojure's syntax for gensyms is a hash after the symbol, like struct $a#.

[durka]

For the record, I think it would be really great to have this functionality in some form.

[kylewlacy]

+1 for the concept, although I think there's a much more sound solution:

First, there'd need to be support for macros in all ident positions (see the discussion around concat_idents! in rust-lang/rust#12249, rust-lang/rust#13294, rust-lang/rust#14266, and #215). Second, a new compiler-provided gensym! macro would be added. With both of these, the above example could like this:

macro_rules! x {
    () => {
        struct gensym!();
    }
}

(Although maybe this is more appropriate as a longer-term solution, following the upcoming and ever-elusive "macro reform"; in which case, I'd be fine with either of the other two syntax proposals)

[durka]

@kylewlacy yes, that would be better. However, as you say if every macro_rules improvement is blocked on another one, none of them will ever happen. Also, as a detail you need to be able to refer back to a gensym (for example struct A; let a = A; where A was gensymmed) so the gensym!() you suggest would need to take a parameter, say an ident or a string literal.

[blaenk]

Yeah, I've run into this many times and expected gensym from lisp languages to have been available, but it wasn't 😞 Accepting an ident is what I've used but I ended up just avoiding creating a macro altogether because I didn't want the user to have to worry about the ident. It's leaking information/work that isn't relevant to the user.

[Stebalien]

@durka

To throw out another idea, clojure's syntax for gensyms is a hash after the symbol, like struct $a#.

$a# would be backwards incompatible but putting a symbol after the dollar sign would be safe ($$a, $&a, $!a...).

[durka]

@Stebalien is it backwards incompatible?

[jonas-schievink]

It might be compatible if you force the metavariable to be unused on the LHS, but that's a bit of a hack

[durka]

Actually I'd think you would want to at least lint if you have a capture
and a gensym with the same identifier, but anyway. As far as I can tell
"$foo#" isn't currently accepted at all, so no compatibility issues, but
maybe there is a situation where it is.

On Sat, Sep 5, 2015 at 1:42 PM, Jonas Schievink notifications@github.com
wrote:

It might be compatible if you force the metavariable to be unused on the
LHS, but that's a bit of a hack

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#1266 (comment).

[jonas-schievink]

This is accepted:

macro_rules! m {
    ( $a:item ) => ( $a# [test] fn t() {} );
}

fn main() {
    m!(fn t() {});
}

[durka]

Yeah, you are right, I just found a similar example.

On Sat, Sep 5, 2015 at 2:02 PM, Jonas Schievink notifications@github.com
wrote:

This is accepted:

macro_rules! m {
( $a:item ) => ( $a# [test] fn t() {} );
}
fn main() {
m!(fn t() {});
}

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#1266 (comment).

[Stebalien]

Also,

macro_rules! m {
    ( $a:ident# ) => ();
    ( $a:ident ) => ( m!($a#) );
}

fn main() {
    m!(test);
}

[geofft]

I feel like I'd prefer either gensym!(a) or $a:gensym for clarity over $a# (you can google what a gensym is, or search for it in the docs), anyway.

[Stebalien]

gensym!(a) is actually a really nice way of doing this because a can be a variable (any ident). This way, if $a == $b, gensym!($a) == gensym!($b).

[durka]

The doc point is a good one, but the question is where to put the
$a:gensym -- it doesn't really fit in the LHS because it's not getting
passed into the macro. Maybe the macro_rules syntax could be extended like
this

macro_rules! m {
    ($a:expr) ($b:gensym) => { foo($a, $b); }
}

On Sat, Sep 5, 2015 at 3:51 PM, Geoffrey Thomas notifications@github.com
wrote:

I feel like I'd prefer either gensym!(a) or $a:gensym for clarity over $a#
(you can google what a gensym is, or search for it in the docs), anyway.

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#1266 (comment).

[durka]

Unfortunately gensym!(a) would be nearly useless for the same reasons
that concat_idents! isn't currently usable.

On Sat, Sep 5, 2015 at 3:57 PM, Steven Allen notifications@github.com
wrote:

gensym!(a) is actually a really nice way of doing this because a can be a
variable (any ident). This way, if $a == $b, gensym!($a) == gensym!($b).

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#1266 (comment).

[nrc]

Would having hygienic items solve this problem? If you write let x = ... in a macro, x is gensym'ed because of the hygiene rules, if we applied hygiene to item names, then the same ought to be true.

[durka]

Seems like it would definitely have to be controllable somehow, because
backwards compatibility.
On Nov 12, 2015 12:59 AM, "Nick Cameron" notifications@github.com wrote:

Would having hygienic items solve this problem? If you write let x = ...
in a macro, x is gensym'ed because of the hygiene rules, if we applied
hygiene to item names, then the same ought to be true.

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#1266 (comment).

[nagisa]

@nrc it is not fully clear where the implicit gensyming should happen. Implicit doesn’t cover all the cases.

I.e. consider macro that generates this

impl A for B {
    fn symbol(){}
}

gensym'ing symbol here would be invalid, and I believe there's a non-trivial amount of macroses that do this. On the other hand gensyming in

impl A {
    fn some_temp_fn1(){}
    fn some_temp_fn2(){}
    fn some_temp_fn3(){}
}

might be desirable.