EII add feature gate

Author: jdonszelmann

compiler/rustc_feature/src/unstable.rs

@@ -211,6 +211,8 @@ declare_features! (
     (internal, compiler_builtins, "1.13.0", None),
     /// Allows writing custom MIR
     (internal, custom_mir, "1.65.0", None),
+    /// Implementation details of externally implementatble items
+    (unstable, eii_internals, "CURRENT_RUSTC_VERSION", None),
     /// Outputs useful `assert!` messages
     (unstable, generic_assert, "1.63.0", None),
     /// Allows using the #[rustc_intrinsic] attribute.
@@ -482,6 +484,8 @@ declare_features! (
     (unstable, doc_masked, "1.21.0", Some(44027)),
     /// Allows features to allow target_feature to better interact with traits.
     (incomplete, effective_target_features, "CURRENT_RUSTC_VERSION", Some(143352)),
+    /// Externally implementatble items
+    (unstable, eii, "CURRENT_RUSTC_VERSION", Some(125418)),
     /// Allows the .use postfix syntax `x.use` and use closures `use |x| { ... }`
     (incomplete, ergonomic_clones, "1.87.0", Some(132290)),
     /// Allows exhaustive pattern matching on types that contain uninhabited types.

tests/ui/feature-gates/feature-gate-eii-internals.rs

@@ -0,0 +1,11 @@
+#![crate_type = "rlib"]
+#![feature(decl_macro)]
+#![feature(rustc_attrs)]
+
+#[eii_extern_target(bar)] //~ ERROR use of unstable library feature `eii_internals`
+#[rustc_builtin_macro(eii_macro)]
+macro foo() {} //~ ERROR: cannot find a built-in macro with name `foo`
+
+unsafe extern "Rust" {
+    safe fn bar(x: u64) -> u64;
+}

tests/ui/feature-gates/feature-gate-eii-internals.stderr

@@ -0,0 +1,18 @@
+error[E0658]: use of unstable library feature `eii_internals`
+  --> $DIR/feature-gate-eii-internals.rs:5:3
+   |
+LL | #[eii_extern_target(bar)]
+   |   ^^^^^^^^^^^^^^^^^
+   |
+   = help: add `#![feature(eii_internals)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: cannot find a built-in macro with name `foo`
+  --> $DIR/feature-gate-eii-internals.rs:7:1
+   |
+LL | macro foo() {}
+   | ^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0658`.

tests/ui/feature-gates/feature-gate-eii.rs

@@ -0,0 +1,9 @@
+#![crate_type = "rlib"]
+
+#[eii] //~ ERROR use of unstable library feature `eii`
+fn hello(x: u64);
+
+#[hello]
+fn hello_impl(x: u64) {
+    println!("{x:?}")
+}

tests/ui/feature-gates/feature-gate-eii.stderr

@@ -0,0 +1,13 @@
+error[E0658]: use of unstable library feature `eii`
+  --> $DIR/feature-gate-eii.rs:3:3
+   |
+LL | #[eii]
+   |   ^^^
+   |
+   = note: see issue #125418 <https://github.com/rust-lang/rust/issues/125418> for more information
+   = help: add `#![feature(eii)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0658`.