Add an experimental feature gate for global registration

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -560,6 +560,7 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session, features: &Features) {
     gate_all!(postfix_match, "postfix match is experimental");
     gate_all!(mut_ref, "mutable by-reference bindings are experimental");
     gate_all!(precise_capturing, "precise captures on `impl Trait` are experimental");
+    gate_all!(global_registration, "global registration is experimental");
 
     if !visitor.features.never_patterns {
         if let Some(spans) = spans.get(&sym::never_patterns) {

compiler/rustc_feature/src/unstable.rs

@@ -487,6 +487,8 @@ declare_features! (
     (incomplete, generic_const_exprs, "1.56.0", Some(76560)),
     /// Allows generic parameters and where-clauses on free & associated const items.
     (incomplete, generic_const_items, "1.73.0", Some(113521)),
+    /// Allows registring static items globally, possibly across crates, to iterate over at runtime.
+    (unstable, global_registration, "CURRENT_RUSTC_VERSION", Some(125119)),
     /// Allows using `..=X` as a patterns in slices.
     (unstable, half_open_range_patterns_in_slices, "1.66.0", Some(67264)),
     /// Allows `if let` guard in match arms.

compiler/rustc_span/src/symbol.rs

@@ -928,6 +928,7 @@ symbols! {
         global_alloc_ty,
         global_allocator,
         global_asm,
+        global_registration,
         globs,
         gt,
         half_open_range_patterns,

tests/ui/feature-gates/feature-gate-global-registration.rs

@@ -0,0 +1,3 @@
+todo!(); //~ ERROR
+
+fn main() {}

tests/ui/feature-gates/feature-gate-global-registration.stderr

@@ -0,0 +1,13 @@
+error: expected one of `!` or `::`, found `(`
+  --> $DIR/feature-gate-global-registration.rs:1:1
+   |
+LL | todo!();
+   | ^^^^^^^
+   | |
+   | expected one of `!` or `::`
+   | in this macro invocation
+   |
+   = note: this error originates in the macro `todo` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 1 previous error
+