Nightly rejects some macro-generated doc attribute values accepted by stable #85432
Description
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.
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