feat: Add 'frontmatter' unstable feature

Author: epage

Diff for: compiler/rustc_feature/src/unstable.rs

@@ -500,6 +500,8 @@ declare_features! (
     (incomplete, fn_delegation, "1.76.0", Some(118212)),
     /// Allows impls for the Freeze trait.
     (internal, freeze_impls, "1.78.0", Some(121675)),
+    /// Frontmatter blocks for build tool metadata.
+    (unstable, frontmatter, "CURRENT_RUSTC_VERSION", Some(136889)),
     /// Allows defining gen blocks and `gen fn`.
     (unstable, gen_blocks, "1.75.0", Some(117078)),
     /// Infer generic args for both consts and types.

Diff for: compiler/rustc_span/src/symbol.rs

@@ -1014,6 +1014,7 @@ symbols! {
         from_str_method,
         from_usize,
         from_yeet,
+        frontmatter,
         fs_create_dir,
         fsub_algebraic,
         fsub_fast,

Diff for: src/doc/unstable-book/src/language-features/frontmatter.md

@@ -0,0 +1,10 @@
+# `frontmatter`
+
+The tracking issue for this feature is: [#136889]
+
+[#136889]: https://github.com/rust-lang/rust/issues/136889
+
+------------------------
+
+The `frontmatter` feature adds support for a specialized and simplified attribute syntax
+intended for external tools to consume.

Diff for: tests/ui/frontmatter/frontmatter-invalid-in-fn.rs

@@ -1,5 +1,4 @@
 #![feature(frontmatter)]
-//~^ unknown feature `frontmatter`
 
 fn main() {
 ---

Diff for: tests/ui/frontmatter/frontmatter-invalid-in-fn.stderr

@@ -1,15 +1,8 @@
 error: expected expression, found `}`
-  --> $DIR/frontmatter-invalid-in-fn.rs:7:1
+  --> $DIR/frontmatter-invalid-in-fn.rs:6:1
    |
 LL | }
    | ^ expected expression
 
-error[E0635]: unknown feature `frontmatter`
-  --> $DIR/frontmatter-invalid-in-fn.rs:1:12
-   |
-LL | #![feature(frontmatter)]
-   |            ^^^^^^^^^^^
-
-error: aborting due to 2 previous errors
+error: aborting due to 1 previous error
 
-For more information about this error, try `rustc --explain E0635`.