Tracking Issue for #![feature(extended_key_value_attributes)]

[petrochenkov]

Implemented in #78837.

With this feature arbitrary expressions are supported in values of key-value attributes at parse time.

#[my_attr = EXPR]

Previously only unsuffixed literals and interpolated expressions ($expr) were accepted.

If the attribute in question survives expansion, then the value is still restricted to unsuffixed literals by a semantic check. This restriction may also be relaxed in the future.

[jyn514]

On the next bootstrap bump, I plan to switch the standard library from using doc(include) to doc = include_str!. Unfortunately it does need the bump to test since the syntax changed:

error: unexpected token: `include_str`
 --> library/std/src/macros.rs:7:34
  |
7 | #[cfg_attr(not(bootstrap), doc = include_str!("../../core/src/macros/panic.md"))]
  |                                  ^^^^^^^^^^^
  |
  = help: the valid syntax is `#[cfg_attr(condition, attribute, other_attribute, ...)]`
  = note: for more information, visit <https://doc.rust-lang.org/reference/conditional-compilation.html#the-cfg_attr-attribute>

[GuillaumeGomez]

It doesn't work for "sub attributes". For example:

#[doc(alias = $some_expr)]
// or:
#[doc(alias = stringify!(ident))]

I'm sending a little fix so it is supported as well.

[ilyvion]

I'm having a bit of a weird issue. If I compile the project with this file on Windows, everything is fine. But if I compile it on Ubuntu, I get this:

$ cargo build
   Compiling doryen-extra v0.1.0 (/home/alex/doryen-extra)
error[E0658]: arbitrary expressions in key-value attributes are unstable
  --> src/base/def_macro.rs:35:77
   |
35 | ...   define_two_property_arithmetic_struct!(@IMPL $name, $uname, $fname, stringify!($name), $field1, $field2, stringify!($field1), strin...
   |                                                                           ^^^^^^^^^^^^^^^^^
   |
  ::: src/base.rs:39:1
   |
39 | define_two_property_arithmetic_struct!(Position, UPosition, FPosition, x, y, ORIGIN, "({}, {})");
   | ------------------------------------------------------------------------------------------------- in this macro invocation
   |
   = note: see issue #78835 <https://github.com/rust-lang/rust/issues/78835> for more information
   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0658]: arbitrary expressions in key-value attributes are unstable
  --> src/base/def_macro.rs:35:77
   |
35 | ...   define_two_property_arithmetic_struct!(@IMPL $name, $uname, $fname, stringify!($name), $field1, $field2, stringify!($field1), strin...
   |                                                                           ^^^^^^^^^^^^^^^^^
   |
  ::: src/base.rs:40:1
   |
40 | define_two_property_arithmetic_struct!(Size, USize, FSize, width, height, ZERO, "{}x{}");
   | ----------------------------------------------------------------------------------------- in this macro invocation
   |
   = note: see issue #78835 <https://github.com/rust-lang/rust/issues/78835> for more information
   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

I don't understand

1. why it's only unstable on Ubuntu, but not Windows (both have rustc 1.50.0 (cb75ad5db 2021-02-10)) and
2. what I'm supposed to do instead. I made my macro that way so that I could do #[doc = $name_str] on the value from the stringify!($name) invocation, and it was hard enough to get this working as is (#[doc = stringify!($name)] did not work), but now my workaround seems to have been made invalid, too, and I honestly don't know where to go from here...

[petrochenkov]

@alexschrod
Could you make a minimized reproduction and build it with -Z macro-backtrace as the message suggests? (It will work on stable with env var RUSTC_BOOTSTRAP=1 set.)