Auto merge of #143460 - JonathanBrouwer:cfg_parser, r=jdonszelmann

Port `#[cfg]` to the new attribute parsing infrastructure

Ports `#[cfg]` to the new attribute parsing infrastructure for https://github.com/rust-lang/rust/issues/131229#issuecomment-2971353197

I've split this PR into commits for reviewability, and left some comments to clarify things

1 files changed, 7 insertions, 9 deletions

diff --git a/compiler/rustc_error_codes/src/error_codes/E0536.md b/compiler/rustc_error_codes/src/error_codes/E0536.md
index c081a3d9cfa..f00d1773944 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0536.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0536.md
@@ -3,22 +3,20 @@ The `not` cfg-predicate was malformed.
 Erroneous code example:
 
 ```compile_fail,E0536
-#[cfg(not())] // error: expected 1 cfg-pattern
-pub fn something() {}
-
-pub fn main() {}
+pub fn main() {
+    if cfg!(not()) { }
+}
 ```
 
 The `not` predicate expects one cfg-pattern. Example:
 
 ```
-#[cfg(not(target_os = "linux"))] // ok!
-pub fn something() {}
-
-pub fn main() {}
+pub fn main() {
+    if cfg!(not(target_os = "linux")) { } // ok!
+}
 ```
 
-For more information about the `cfg` attribute, read the section on
+For more information about the `cfg` macro, read the section on
 [Conditional Compilation][conditional-compilation] in the Reference.
 
 [conditional-compilation]: https://doc.rust-lang.org/reference/conditional-compilation.html