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.