Rollup merge of #82682 - petrochenkov:cfgeval, r=Aaron1011

Implement built-in attribute macro `#[cfg_eval]` + some refactoring

This PR implements a built-in attribute macro `#[cfg_eval]` as it was suggested in https://github.com/rust-lang/rust/pull/79078 to avoid `#[derive()]` without arguments being abused as a way to configure input for other attributes.

The macro is used for eagerly expanding all `#[cfg]` and `#[cfg_attr]` attributes in its input ("fully configuring" the input).
The effect is identical to effect of `#[derive(Foo, Bar)]` which also fully configures its input before passing it to macros `Foo` and `Bar`, but unlike `#[derive]` `#[cfg_eval]` can be applied to any syntax nodes supporting macro attributes, not only certain items.

`cfg_eval` was the first name suggested in https://github.com/rust-lang/rust/pull/79078, but other alternatives are also possible, e.g. `cfg_expand`.

```rust
#[cfg_eval]
#[my_attr] // Receives `struct S {}` as input, the field is configured away by `#[cfg_eval]`
struct S {
    #[cfg(FALSE)]
    field: u8,
}
```

Tracking issue: https://github.com/rust-lang/rust/issues/82679

1 files changed, 7 insertions, 3 deletions

diff --git a/compiler/rustc_parse/src/parser/attr_wrapper.rs b/compiler/rustc_parse/src/parser/attr_wrapper.rs
index f45d8d6c7a0..7512f46988c 100644
--- a/compiler/rustc_parse/src/parser/attr_wrapper.rs
+++ b/compiler/rustc_parse/src/parser/attr_wrapper.rs
@@ -72,6 +72,10 @@ impl<'a> Parser<'a> {
         let cursor_snapshot = self.token_cursor.clone();
 
         let (mut ret, trailing_token) = f(self, attrs.attrs)?;
+        let tokens = match ret.tokens_mut() {
+            Some(tokens) if tokens.is_none() => tokens,
+            _ => return Ok(ret),
+        };
 
         // Produces a `TokenStream` on-demand. Using `cursor_snapshot`
         // and `num_calls`, we can reconstruct the `TokenStream` seen
@@ -128,14 +132,14 @@ impl<'a> Parser<'a> {
             }
         }
 
-        let lazy_impl = LazyTokenStreamImpl {
+        *tokens = Some(LazyTokenStream::new(LazyTokenStreamImpl {
             start_token,
             num_calls,
             cursor_snapshot,
             desugar_doc_comments: self.desugar_doc_comments,
             append_unglued_token: self.token_cursor.append_unglued_token.clone(),
-        };
-        ret.finalize_tokens(LazyTokenStream::new(lazy_impl));
+        }));
+
         Ok(ret)
     }
 }