Add deprecated_safe feature gate and attribute, cc #94978

compiler/rustc_feature/src/active.rs

@@ -364,6 +364,8 @@ declare_features! (
     (active, default_alloc_error_handler, "1.48.0", Some(66741), None),
     /// Allows default type parameters to influence type inference.
     (active, default_type_parameter_fallback, "1.3.0", Some(27336), None),
+    /// Allows using `#[deprecated_safe]` to deprecate the safeness of a function or trait
+    (active, deprecated_safe, "1.61.0", Some(94978), None),
     /// Allows having using `suggestion` in the `#[deprecated]` attribute.
     (active, deprecated_suggestion, "1.61.0", Some(94785), None),
     /// Allows `#[derive(Default)]` and `#[default]` on enums.

compiler/rustc_feature/src/builtin_attrs.rs

@@ -452,6 +452,11 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         "`default_method_body_is_const` is a temporary placeholder for declaring default bodies \
         as `const`, which may be removed or renamed in the future."
     ),
+    // lang-team MCP 147
+    gated!(
+        deprecated_safe, Normal, template!(List: r#"since = "version", note = "...""#), ErrorFollowing,
+        experimental!(deprecated_safe),
+    ),
 
     // ==========================================================================
     // Internal attributes: Stability, deprecation, and unsafe:

compiler/rustc_span/src/symbol.rs

@@ -563,6 +563,7 @@ symbols! {
         delay_span_bug_from_inside_query,
         deny,
         deprecated,
+        deprecated_safe,
         deprecated_suggestion,
         deref,
         deref_method,

src/test/ui/feature-gates/feature-gate-deprecated_safe.rs

@@ -0,0 +1,7 @@
+#[deprecated_safe(since = "TBD", note = "...")] //~ ERROR: the `#[deprecated_safe]` attribute is an experimental feature
+unsafe fn deprecated_safe_fn() {}
+
+#[deprecated_safe(since = "TBD", note = "...")] //~ ERROR: the `#[deprecated_safe]` attribute is an experimental feature
+unsafe trait DeprecatedSafeTrait {}
+
+fn main() {}

src/test/ui/feature-gates/feature-gate-deprecated_safe.stderr

@@ -0,0 +1,21 @@
+error[E0658]: the `#[deprecated_safe]` attribute is an experimental feature
+  --> $DIR/feature-gate-deprecated_safe.rs:1:1
+   |
+LL | #[deprecated_safe(since = "TBD", note = "...")]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #94978 <https://github.com/rust-lang/rust/issues/94978> for more information
+   = help: add `#![feature(deprecated_safe)]` to the crate attributes to enable
+
+error[E0658]: the `#[deprecated_safe]` attribute is an experimental feature
+  --> $DIR/feature-gate-deprecated_safe.rs:4:1
+   |
+LL | #[deprecated_safe(since = "TBD", note = "...")]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #94978 <https://github.com/rust-lang/rust/issues/94978> for more information
+   = help: add `#![feature(deprecated_safe)]` to the crate attributes to enable
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0658`.