Add lazy_type_alias feature gate

Author: GuillaumeGomez

compiler/rustc_feature/src/active.rs

@@ -440,6 +440,8 @@ declare_features! (
     (active, intra_doc_pointers, "1.51.0", Some(80896), None),
     // Allows setting the threshold for the `large_assignments` lint.
     (active, large_assignments, "1.52.0", Some(83518), None),
+    /// Allow to have type alias types for inter-crate use.
+    (active, lazy_type_alias, "CURRENT_RUSTC_VERSION", Some(112792), None),
     /// Allows `if/while p && let q = r && ...` chains.
     (active, let_chains, "1.37.0", Some(53667), None),
     /// Allows using `reason` in lint attributes and the `#[expect(lint)]` lint check.

compiler/rustc_hir_analysis/src/astconv/mod.rs

@@ -896,7 +896,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         let ty = self.tcx().at(span).type_of(did);
 
         if matches!(self.tcx().def_kind(did), DefKind::TyAlias)
-            && ty.skip_binder().has_opaque_types()
+            && (ty.skip_binder().has_opaque_types() || self.tcx().features().lazy_type_alias)
         {
             // Type aliases referring to types that contain opaque types (but aren't just directly
             // referencing a single opaque type) get encoded as a type alias that normalization will

compiler/rustc_span/src/symbol.rs

@@ -870,6 +870,7 @@ symbols! {
         large_assignments,
         lateout,
         lazy_normalization_consts,
+        lazy_type_alias,
         le,
         len,
         let_chains,