Add explicit_extern_abis unstable feature

Author: obeis

compiler/rustc_feature/src/unstable.rs

@@ -479,6 +479,8 @@ declare_features! (
     (incomplete, dyn_star, "1.65.0", Some(102425)),
     /// Allows exhaustive pattern matching on types that contain uninhabited types.
     (unstable, exhaustive_patterns, "1.13.0", Some(51085)),
+    /// Allows explicit extern ABIs.
+    (unstable, explicit_extern_abis, "1.84.2", Some(134986)),
     /// Allows explicit tail calls via `become` expression.
     (incomplete, explicit_tail_calls, "1.72.0", Some(112788)),
     /// Allows defining `extern type`s.

compiler/rustc_span/src/symbol.rs

@@ -853,6 +853,7 @@ symbols! {
         expf16,
         expf32,
         expf64,
+        explicit_extern_abis,
         explicit_generic_args_with_impl_trait,
         explicit_tail_calls,
         export_name,

tests/ui/feature-gates/feature-gate-explicit-extern-abis.rs

@@ -0,0 +1,6 @@
+#![feature(explicit_extern_abis)]
+
+extern fn foo() {}
+//~^ ERROR extern declarations without an explicit ABI are disallowed
+
+fn main() {}

tests/ui/feature-gates/feature-gate-explicit-extern-abis.stderr

@@ -0,0 +1,8 @@
+error: extern declarations without an explicit ABI are disallowed
+  --> $DIR/feature-gate-explicit-extern-abis.rs:3:1
+   |
+LL | extern fn foo() {}
+   | ^^^^^^
+
+error: aborting due to 1 previous error
+