Nightly rejects some macro-generated doc attribute values accepted by stable

[Erutuon]

A version of my crate parse-mediawiki-sql with macro-generated docs builds on stable, but errors out on nightly, where the error suggests that I fix the error by enabling #![feature(extended_key_value_attributes)].

Stable is cargo 1.52.0 (rust-lang/cargo@69767412a 2021-04-21), rustc 1.52.1 (9bc8c42 2021-05-09), nightly is cargo 1.54.0-nightly (rust-lang/cargo@070e459c2 2021-05-11), rustc 1.54.0-nightly (3e99439 2021-05-17).

The error occurs in a macro that generates a struct declaration with an implementation of a trait and calls another macro to take a string generated by another set of macros and put it into a doc attribute above the struct. Here are the lines containing the macro declarations.

The error looks like this:

error[E0658]: arbitrary expressions in key-value attributes are unstable
   --> src/schemas.rs:152:13
    |
152 |               database_table_doc!($table_name $(, $page)?),
    |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
...
274 | / impl_row_from_sql! {
275 | |     image
276 | |     Image<'input> {
277 | |         name: PageTitle,
...   |
293 | |     }
294 | | }
    | |_- in this macro invocation
    |
    = note: see issue #78835 <https://github.com/rust-lang/rust/issues/78835> for more information
    = help: add `#![feature(extended_key_value_attributes)]` to the crate attributes to enable
    = note: this error originates in the macro `impl_row_from_sql` (in Nightly builds, run with -Z macro-backtrace for more info)

There are 22 calls to the macro, but only 13 errors, for the calls those whose contents start with $table_name:ident $output_type:ident<$life:lifetime>. I tried to generate a minimal example by copying the macros and one of the failing calls, but failed: this reduced version compiled on both stable and nightly in the playground.

Somehow this sequence of tokens causes the error in the crate but not in the smaller playground example. I don't know much about the internals of the compiler here so have no idea how to change the playground example to generate the error.

[hameerabbasi]

@rustbot ping cleanup

[rustbot]

Hey Cleanup Crew ICE-breakers! This bug has been identified as a good
"Cleanup ICE-breaking candidate". In case it's useful, here are some
instructions for tackling these sorts of bugs. Maybe take a look?
Thanks! <3

cc @AminArria @camelid @chrissimpkins @contrun @DutchGhost @elshize @h-michael @HallerPatrick @hdhoang @hellow554 @henryboisdequin @imtsuki @JamesPatrickGill @kanru @KarlK90 @LeSeulArtichaut @MAdrianMattocks @matheus-consoli @mental32 @nmccarty @Noah-Kennedy @pard68 @PeytonT @pierreN @Redblueflame @RobbieClarken @RobertoSnap @robjtede @SarthakSingh31 @shekohex @sinato @smmalis37 @steffahn @Stupremee @tamuhey @turboladen @woshilapin @yerke

[hellow554]

While compiling with nightly another (maybe (un-)related error) pops up, which isn't present on stable:

[...]
error[E0308]: mismatched types
    --> /home/marcel/.cargo/registry/src/github.com-1ecc6299db9ec823/lexical-core-0.7.4/src/atof/algorithm/math.rs:2065:27
     |
2065 |     let rs = Limb::BITS - s;
     |                           ^ expected `u32`, found `usize`

error[E0277]: cannot subtract `usize` from `u32`
    --> /home/marcel/.cargo/registry/src/github.com-1ecc6299db9ec823/lexical-core-0.7.4/src/atof/algorithm/math.rs:2065:25
     |
2065 |     let rs = Limb::BITS - s;
     |                         ^ no implementation for `u32 - usize`
     |
     = help: the trait `Sub<usize>` is not implemented for `u32`

---

Never mind, seems to be an issue with the 0.7.4 version of lexical-core. cargo update fixed that by updating to 0.7.6 and now I can see OPs error

[hellow554]

This code errors on nightly, but not on stable:

macro_rules! with_doc_comment {
    ($comment:expr, $item:item) => {
        #[doc = $comment]
        $item
    };
}

macro_rules! database_table_doc {
    () => {
        ""
    };
}

with_doc_comment! {
    database_table_doc!(),
    #[derive(Debug)]
    struct Image {
        #[cfg(test)]
        _f: (),
    }

}

fn main() {}

I had a similar piece of code (I try to post it in the next post) that fails with the exact message on stable as well.

But here's the regression:

searched nightlies: from nightly-2019-10-01 to nightly-2021-05-01
regressed nightly: nightly-2021-04-12
searched commits: from a836d9b to a866124
regressed commit: ba6275b

cc #82608 @Aaron1011

[hellow554]

This code fails on stable with the same message as on nightly:

macro_rules! database_table_doc {
    () => {
        ""
    };
}

#[doc = database_table_doc!()]
#[derive(Debug)]
struct Image {
    #[cfg(test)]
    _f: (),
}

fn main() {}

I just "inlined" the with_doc_comment macro.

---

So in 58d2bad the error message changed from

error: unexpected token: `database_table_doc`
 --> src/main.rs:7:9
  |
7 | #[doc = database_table_doc!()]
  |         ^^^^^^^^^^^^^^^^^^

to

error[E0658]: arbitrary expressions in key-value attributes are unstable
 --> src/main.rs:7:9
  |
7 | #[doc = database_table_doc!()]
  |         ^^^^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #78835 <https://github.com/rust-lang/rust/issues/78835> for more information
  = help: add `#![feature(extended_key_value_attributes)]` to the crate attributes to enable

(that's what we have today)

cc #78837 #78835 @petrochenkov

I think the commit message makes it very clear why the possiblity was added to use macros there

[...] External doc strings (#[doc = include_str!("my_doc.md")], [...]

What to do on this, is above my paygrade ;)

@rustbot modify labels: -E-needs-mcve

[rustbot]

Error: Label ICEBreaker-Cleanup-Crew can only be set by Rust team members

Please let @rust-lang/release know if you're having trouble with this bot.

[hellow554]

Funny enough, in the tracking issue #78835 @alexschrod had the same issue as in this post, so this error is somewhat known. In the paste crate it was reported at dtolnay/paste#29 and fixed in dtolnay/paste#30, so maybe that's something for you @Erutuon if you need a fix right now.

[hameerabbasi]

@rustbot modify labels -E-needs-mcve

[hameerabbasi]

Sorry for the spam!

@rustbot modify labels -ICEBreaker-Cleanup-Crew

[hellow554]

@hameerabbasi you can edit your post and use the rustbot command again without creating a new post ;)

[Erutuon]

@hellow554

This code fails on stable with the same message as on nightly:

macro_rules! database_table_doc {
    () => {
        ""
    };
}

#[doc = database_table_doc!()]
#[derive(Debug)]
struct Image {
    #[cfg(test)]
    _f: (),
}

fn main() {}

I just "inlined" the with_doc_comment macro.

Right, that's the reason for the with_doc_comment macro. On stable, taking a macro-generated string followed by an item and passing them to another macro and having that macro add the string to a doc attribute above the item seems to convince the compiler (somehow) that the doc attribute value is acceptable, even though it's not really a string literal.

Funny enough, in the tracking issue #78835 @alexschrod had the same issue as in this post, so this error is somewhat known. In the paste crate it was reported at dtolnay/paste#29 and fixed in dtolnay/paste#30, so maybe that's something for you @Erutuon if you need a fix right now.

It looks like I'm using the same technique as the regression test, but on nightly now it doesn't compile in 13 out of the 22 macro calls in my code.

[Aaron1011]

This will be fixed by #85445

[hameerabbasi]

Assigning P-critical as discussed in the WG-prioritization discussion on Zulip and removing I-prioritize.

@rustbot modify labels +P-critical -I-prioritize

[pietroalbini]

Fixed on 1.54 by #85445 and fixed on 1.53 by #86225.