Implement #[define_opaque] attribute for functions.

Author: oli-obk

core/src/macros/mod.rs

@@ -1743,6 +1743,21 @@ pub(crate) mod builtin {
         /* compiler built-in */
     }
 
+    /// Provide a list of type aliases and other opaque-type-containing type definitions.
+    /// This list will be used in the body of the item it is applied to to define opaque
+    /// types' hidden types.
+    /// Can only be applied to things that have bodies.
+    #[unstable(
+        feature = "type_alias_impl_trait",
+        issue = "63063",
+        reason = "`type_alias_impl_trait` has open design concerns"
+    )]
+    #[rustc_builtin_macro]
+    #[cfg(not(bootstrap))]
+    pub macro define_opaque($($tt:tt)*) {
+        /* compiler built-in */
+    }
+
     /// Unstable placeholder for type ascription.
     #[allow_internal_unstable(builtin_syntax)]
     #[unstable(

core/src/prelude/v1.rs

@@ -111,3 +111,11 @@ pub use crate::macros::builtin::type_ascribe;
     reason = "placeholder syntax for deref patterns"
 )]
 pub use crate::macros::builtin::deref;
+
+#[unstable(
+    feature = "type_alias_impl_trait",
+    issue = "63063",
+    reason = "`type_alias_impl_trait` has open design concerns"
+)]
+#[cfg(not(bootstrap))]
+pub use crate::macros::builtin::define_opaque;

std/src/backtrace.rs

@@ -432,6 +432,7 @@ mod helper {
     use super::*;
     pub(super) type LazyResolve = impl (FnOnce() -> Capture) + Send + Sync + UnwindSafe;
 
+    #[cfg_attr(not(bootstrap), define_opaque(LazyResolve))]
     pub(super) fn lazy_resolve(mut capture: Capture) -> LazyResolve {
         move || {
             // Use the global backtrace lock to synchronize this as it's a

std/src/prelude/v1.rs

@@ -103,6 +103,15 @@ pub use core::prelude::v1::type_ascribe;
 )]
 pub use core::prelude::v1::deref;
 
+// Do not `doc(no_inline)` either.
+#[unstable(
+    feature = "type_alias_impl_trait",
+    issue = "63063",
+    reason = "`type_alias_impl_trait` has open design concerns"
+)]
+#[cfg(not(bootstrap))]
+pub use core::prelude::v1::define_opaque;
+
 // The file so far is equivalent to core/src/prelude/v1.rs. It is duplicated
 // rather than glob imported because we want docs to show these re-exports as
 // pointing to within `std`.