Auto merge of rust-lang#128440 - oli-obk:defines, r=lcnr

Add `#[define_opaques]` attribute and require it for all type-alias-impl-trait sites that register a hidden type

Instead of relying on the signature of items to decide whether they are constraining an opaque type, the opaque types that the item constrains must be explicitly listed.

A previous version of this PR used an actual attribute, but had to keep the resolved `DefId`s in a side table.

Now we just lower to fields in the AST that have no surface syntax, instead a builtin attribute macro fills in those fields where applicable.

Note that for convenience referencing opaque types in associated types from associated methods on the same impl will not require an attribute. If that causes problems `#[defines()]` can be used to overwrite the default of searching for opaques in the signature.

One wart of this design is that closures and static items do not have generics. So since I stored the opaques in the generics of functions, consts and methods, I would need to add a custom field to closures and statics to track this information. During a T-types discussion we decided to just not do this for now.

fixes rust-lang#131298

Author: bors

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`.