Stabilize extended_key_value_attributes

 # Stabilization report

 ## Summary

This stabilizes using macro expansion in key-value attributes, like so:

 ```rust
 #[doc = include_str!("my_doc.md")]
 struct S;

 #[path = concat!(env!("OUT_DIR"), "/generated.rs")]
 mod m;
 ```

See the changes to the reference for details on what macros are allowed;
see Petrochenkov's excellent blog post [on internals](https://internals.rust-lang.org/t/macro-expansion-points-in-attributes/11455)
for alternatives that were considered and rejected ("why accept no more
and no less?")

This has been available on nightly since 1.50 with no major issues.

 ## Notes

 ### Accepted syntax

The parser accepts arbitrary Rust expressions in this position, but any expression other than a macro invocation will ultimately lead to an error because it is not expected by the built-in expression forms (e.g., `#[doc]`).  Note that decorators and the like may be able to observe other expression forms.

 ### Expansion ordering

Expansion of macro expressions in "inert" attributes occurs after decorators have executed, analogously to macro expressions appearing in the function body or other parts of decorator input.

There is currently no way for decorators to accept macros in key-value position if macro expansion must be performed before the decorator executes (if the macro can simply be copied into the output for later expansion, that can work).

 ## Test cases

 - https://github.com/rust-lang/rust/blob/master/src/test/ui/attributes/key-value-expansion-on-mac.rs
 - https://github.com/rust-lang/rust/blob/master/src/test/rustdoc/external-doc.rs

The feature has also been dogfooded extensively in the compiler and
standard library:

- https://github.com/rust-lang/rust/pull/83329
- https://github.com/rust-lang/rust/pull/83230
- https://github.com/rust-lang/rust/pull/82641
- https://github.com/rust-lang/rust/pull/80534

 ## Implementation history

- Initial proposal: https://github.com/rust-lang/rust/issues/55414#issuecomment-554005412
- Experiment to see how much code it would break: https://github.com/rust-lang/rust/pull/67121
- Preliminary work to restrict expansion that would conflict with this
feature: https://github.com/rust-lang/rust/pull/77271
- Initial implementation: https://github.com/rust-lang/rust/pull/78837
- Fix for an ICE: https://github.com/rust-lang/rust/pull/80563

 ## Unresolved Questions

~~https://github.com/rust-lang/rust/pull/83366#issuecomment-805180738 listed some concerns, but they have been resolved as of this final report.~~

 ## Additional Information

 There are two workarounds that have a similar effect for `#[doc]`
attributes on nightly. One is to emulate this behavior by using a limited version of this feature that was stabilized for historical reasons:

```rust
macro_rules! forward_inner_docs {
    ($e:expr => $i:item) => {
        #[doc = $e]
        $i
    };
}

forward_inner_docs!(include_str!("lib.rs") => struct S {});
```

This also works for other attributes (like `#[path = concat!(...)]`).
The other is to use `doc(include)`:

```rust
 #![feature(external_doc)]
 #[doc(include = "lib.rs")]
 struct S {}
```

The first works, but is non-trivial for people to discover, and
difficult to read and maintain. The second is a strange special-case for
a particular use of the macro. This generalizes it to work for any use
case, not just including files.

I plan to remove `doc(include)` when this is stabilized. The
`forward_inner_docs` workaround will still compile without warnings, but
I expect it to be used less once it's no longer necessary.

8 files changed, 14 insertions, 51 deletions

diff --git a/src/doc/rustdoc/README.md b/src/doc/rustdoc/README.md
new file mode 100644
index 00000000000..7d97d5e4ab5
--- /dev/null
+++ b/src/doc/rustdoc/README.md
@@ -0,0 +1,5 @@
+# Rustdoc
+
+This is documentation for rustdoc itself, written in mdbook format.
+To build the book, use `x.py doc src/doc/rustdoc`.
+To run doctests, use `x.py test src/doc/rustdoc`.
diff --git a/src/doc/rustdoc/src/the-doc-attribute.md b/src/doc/rustdoc/src/the-doc-attribute.md
index 52f2a3728fa..d192f7d5ce9 100644
--- a/src/doc/rustdoc/src/the-doc-attribute.md
+++ b/src/doc/rustdoc/src/the-doc-attribute.md
@@ -35,6 +35,13 @@ Which can feel more flexible. Note that this would generate this:
 
 but given that docs are rendered via Markdown, it will remove these newlines.
 
+Another use case is for including external files as documentation:
+
+```rust,no_run
+#[doc = include_str!("../README.md")]
+# fn f() {}
+```
+
 The `doc` attribute has more options though! These don't involve the text of
 the output, but instead, various aspects of the presentation of the output.
 We've split them into two kinds below: attributes that are useful at the
diff --git a/src/test/rustdoc/external-doc.rs b/src/test/rustdoc/external-doc.rs
index befd31a5492..0dadca551a9 100644
--- a/src/test/rustdoc/external-doc.rs
+++ b/src/test/rustdoc/external-doc.rs
@@ -1,5 +1,4 @@
 #![feature(external_doc)]
-#![feature(extended_key_value_attributes)]
 
 // @has external_doc/struct.CanHasDocs.html
 // @has - '//h1' 'External Docs'
diff --git a/src/test/ui/attributes/key-value-expansion-on-mac.rs b/src/test/ui/attributes/key-value-expansion-on-mac.rs
index 1247ff2b230..95bc1c04961 100644
--- a/src/test/ui/attributes/key-value-expansion-on-mac.rs
+++ b/src/test/ui/attributes/key-value-expansion-on-mac.rs
@@ -1,4 +1,3 @@
-#![feature(extended_key_value_attributes)]
 #![feature(rustc_attrs)]
 
 #[rustc_dummy = stringify!(a)] // OK
diff --git a/src/test/ui/attributes/key-value-expansion-on-mac.stderr b/src/test/ui/attributes/key-value-expansion-on-mac.stderr
index b74f3518a7e..fa9ea543765 100644
--- a/src/test/ui/attributes/key-value-expansion-on-mac.stderr
+++ b/src/test/ui/attributes/key-value-expansion-on-mac.stderr
@@ -1,5 +1,5 @@
 error: unexpected token: `stringify!(b)`
-  --> $DIR/key-value-expansion-on-mac.rs:12:17
+  --> $DIR/key-value-expansion-on-mac.rs:11:17
    |
 LL | #[rustc_dummy = stringify!(b)]
    |                 ^^^^^^^^^^^^^
diff --git a/src/test/ui/feature-gates/feature-gate-extended_key_value_attributes.rs b/src/test/ui/feature-gates/feature-gate-extended_key_value_attributes.rs
deleted file mode 100644
index f19fdb45f1f..00000000000
--- a/src/test/ui/feature-gates/feature-gate-extended_key_value_attributes.rs
+++ /dev/null
@@ -1,8 +0,0 @@
-#[cfg(FALSE)]
-#[attr = multi::segment::path] //~ ERROR arbitrary expressions in key-value attributes are unstable
-#[attr = macro_call!()] //~ ERROR arbitrary expressions in key-value attributes are unstable
-#[attr = 1 + 2] //~ ERROR arbitrary expressions in key-value attributes are unstable
-#[attr = what?] //~ ERROR arbitrary expressions in key-value attributes are unstable
-struct S;
-
-fn main() {}
diff --git a/src/test/ui/feature-gates/feature-gate-extended_key_value_attributes.stderr b/src/test/ui/feature-gates/feature-gate-extended_key_value_attributes.stderr
deleted file mode 100644
index 9887814b907..00000000000
--- a/src/test/ui/feature-gates/feature-gate-extended_key_value_attributes.stderr
+++ /dev/null
@@ -1,39 +0,0 @@
-error[E0658]: arbitrary expressions in key-value attributes are unstable
-  --> $DIR/feature-gate-extended_key_value_attributes.rs:2:10
-   |
-LL | #[attr = multi::segment::path]
-   |          ^^^^^^^^^^^^^^^^^^^^
-   |
-   = 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
-
-error[E0658]: arbitrary expressions in key-value attributes are unstable
-  --> $DIR/feature-gate-extended_key_value_attributes.rs:3:10
-   |
-LL | #[attr = macro_call!()]
-   |          ^^^^^^^^^^^^^
-   |
-   = 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
-
-error[E0658]: arbitrary expressions in key-value attributes are unstable
-  --> $DIR/feature-gate-extended_key_value_attributes.rs:4:10
-   |
-LL | #[attr = 1 + 2]
-   |          ^^^^^
-   |
-   = 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
-
-error[E0658]: arbitrary expressions in key-value attributes are unstable
-  --> $DIR/feature-gate-extended_key_value_attributes.rs:5:10
-   |
-LL | #[attr = what?]
-   |          ^^^^^
-   |
-   = 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
-
-error: aborting due to 4 previous errors
-
-For more information about this error, try `rustc --explain E0658`.
diff --git a/src/test/ui/suffixed-literal-meta.rs b/src/test/ui/suffixed-literal-meta.rs
index 319264aec9c..a6531490c01 100644
--- a/src/test/ui/suffixed-literal-meta.rs
+++ b/src/test/ui/suffixed-literal-meta.rs
@@ -1,4 +1,4 @@
-#![feature(rustc_attrs, extended_key_value_attributes)]
+#![feature(rustc_attrs)]
 
 #[rustc_dummy = 1usize] //~ ERROR: suffixed literals are not allowed in attributes
 #[rustc_dummy = 1u8] //~ ERROR: suffixed literals are not allowed in attributes