Permit recursive weak type aliases

compiler/rustc_hir_analysis/src/astconv/mod.rs

@@ -910,19 +910,24 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
     ) -> Ty<'tcx> {
         let tcx = self.tcx();
         let args = self.ast_path_args_for_ty(span, did, item_segment);
-        let ty = tcx.at(span).type_of(did);
 
-        if let DefKind::TyAlias { lazy } = tcx.def_kind(did)
+        if let DefKind::TyAlias { lazy: true } = tcx.def_kind(did) {
-            && (lazy || ty.skip_binder().has_opaque_types())
+            // Type aliases defined in crates that have the
-        {
-            // Type aliases referring to types that contain opaque types (but aren't just directly
-            // referencing a single opaque type) as well as those defined in crates that have the
             // feature `lazy_type_alias` enabled get encoded as a type alias that normalization will
             // then actually instantiate the where bounds of.
             let alias_ty = tcx.mk_alias_ty(did, args);
             Ty::new_alias(tcx, ty::Weak, alias_ty)
         } else {
-            ty.instantiate(tcx, args)
+            let ty = tcx.at(span).type_of(did);
+            if ty.skip_binder().has_opaque_types() {
+                // 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
+                // then actually instantiate the where bounds of.
+                let alias_ty = tcx.mk_alias_ty(did, args);
+                Ty::new_alias(tcx, ty::Weak, alias_ty)
+            } else {
+                ty.instantiate(tcx, args)
+            }
         }
     }
 

compiler/rustc_trait_selection/messages.ftl

@@ -40,4 +40,5 @@ trait_selection_no_value_in_rustc_on_unimplemented = this attribute must have a
     .label = expected value here
     .note = eg `#[rustc_on_unimplemented(message="foo")]`
 
+trait_selection_ty_alias_overflow = in case this is a recursive type alias, consider using a struct, enum, or union instead
 trait_selection_unable_to_construct_constant_value = unable to construct a constant value for the unevaluated constant {$unevaluated}

compiler/rustc_trait_selection/src/traits/project.rs

@@ -659,6 +659,18 @@ impl<'a, 'b, 'tcx> TypeFolder<TyCtxt<'tcx>> for AssocTypeNormalizer<'a, 'b, 'tcx
                 normalized_ty
             }
             ty::Weak => {
+                let recursion_limit = self.interner().recursion_limit();
+                if !recursion_limit.value_within_limit(self.depth) {
+                    self.selcx.infcx.err_ctxt().report_overflow_error(
+                        &ty,
+                        self.cause.span,
+                        false,
+                        |diag| {
+                            diag.note(crate::fluent_generated::trait_selection_ty_alias_overflow);
+                        },
+                    );
+                }
+
                 let infcx = self.selcx.infcx;
                 self.obligations.extend(
                     infcx.tcx.predicates_of(data.def_id).instantiate_own(infcx.tcx, data.args).map(
@@ -678,7 +690,14 @@ impl<'a, 'b, 'tcx> TypeFolder<TyCtxt<'tcx>> for AssocTypeNormalizer<'a, 'b, 'tcx
                         },
                     ),
                 );
-                infcx.tcx.type_of(data.def_id).instantiate(infcx.tcx, data.args).fold_with(self)
+                self.depth += 1;
+                let res = infcx
+                    .tcx
+                    .type_of(data.def_id)
+                    .instantiate(infcx.tcx, data.args)
+                    .fold_with(self);
+                self.depth -= 1;
+                res
             }
 
             ty::Inherent if !data.has_escaping_bound_vars() => {

tests/ui/infinite/infinite-type-alias-mutual-recursion.feature.stderr

@@ -0,0 +1,27 @@
+error[E0275]: overflow evaluating the requirement `X2`
+  --> $DIR/infinite-type-alias-mutual-recursion.rs:6:11
+   |
+LL | type X1 = X2;
+   |           ^^
+   |
+   = note: in case this is a recursive type alias, consider using a struct, enum, or union instead
+
+error[E0275]: overflow evaluating the requirement `X3`
+  --> $DIR/infinite-type-alias-mutual-recursion.rs:9:11
+   |
+LL | type X2 = X3;
+   |           ^^
+   |
+   = note: in case this is a recursive type alias, consider using a struct, enum, or union instead
+
+error[E0275]: overflow evaluating the requirement `X1`
+  --> $DIR/infinite-type-alias-mutual-recursion.rs:11:11
+   |
+LL | type X3 = X1;
+   |           ^^
+   |
+   = note: in case this is a recursive type alias, consider using a struct, enum, or union instead
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0275`.

tests/ui/infinite/infinite-type-alias-mutual-recursion.gated.stderr

@@ -1,16 +1,16 @@
 error[E0391]: cycle detected when expanding type alias `X1`
-  --> $DIR/infinite-type-alias-mutual-recursion.rs:1:11
+  --> $DIR/infinite-type-alias-mutual-recursion.rs:6:11
    |
 LL | type X1 = X2;
    |           ^^
    |
 note: ...which requires expanding type alias `X2`...
-  --> $DIR/infinite-type-alias-mutual-recursion.rs:3:11
+  --> $DIR/infinite-type-alias-mutual-recursion.rs:9:11
    |
 LL | type X2 = X3;
    |           ^^
 note: ...which requires expanding type alias `X3`...
-  --> $DIR/infinite-type-alias-mutual-recursion.rs:4:11
+  --> $DIR/infinite-type-alias-mutual-recursion.rs:11:11
    |
 LL | type X3 = X1;
    |           ^^
@@ -19,12 +19,13 @@ LL | type X3 = X1;
    = help: consider using a struct, enum, or union instead to break the cycle
    = help: see <https://doc.rust-lang.org/reference/types.html#recursive-types> for more information
 note: cycle used when collecting item types in top-level module
-  --> $DIR/infinite-type-alias-mutual-recursion.rs:1:1
+  --> $DIR/infinite-type-alias-mutual-recursion.rs:3:1
    |
-LL | / type X1 = X2;
+LL | / #![cfg_attr(feature, feature(lazy_type_alias))]
+LL | | #![allow(incomplete_features)]
 LL | |
-LL | | type X2 = X3;
+LL | | type X1 = X2;
-LL | | type X3 = X1;
+...  |
 LL | |
 LL | | fn main() {}
    | |____________^

tests/ui/infinite/infinite-type-alias-mutual-recursion.rs

@@ -1,6 +1,14 @@
+// revisions: feature gated
+
+#![cfg_attr(feature, feature(lazy_type_alias))]
+#![allow(incomplete_features)]
+
 type X1 = X2;
-//~^ ERROR cycle detected when expanding type alias `X1`
+//[gated]~^ ERROR cycle detected when expanding type alias `X1`
+//[feature]~^^ ERROR: overflow evaluating the requirement `X2`
 type X2 = X3;
+//[feature]~^ ERROR: overflow evaluating the requirement `X3`
 type X3 = X1;
+//[feature]~^ ERROR: overflow evaluating the requirement `X1`
 
 fn main() {}

tests/ui/infinite/infinite-vec-type-recursion.feature.stderr

@@ -0,0 +1,11 @@
+error[E0275]: overflow evaluating the requirement `X`
+  --> $DIR/infinite-vec-type-recursion.rs:6:10
+   |
+LL | type X = Vec<X>;
+   |          ^^^^^^
+   |
+   = note: in case this is a recursive type alias, consider using a struct, enum, or union instead
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0275`.

tests/ui/infinite/infinite-vec-type-recursion.gated.stderr

@@ -1,5 +1,5 @@
 error[E0391]: cycle detected when expanding type alias `X`
-  --> $DIR/infinite-vec-type-recursion.rs:1:14
+  --> $DIR/infinite-vec-type-recursion.rs:6:14
    |
 LL | type X = Vec<X>;
    |              ^
@@ -9,11 +9,14 @@ LL | type X = Vec<X>;
    = help: consider using a struct, enum, or union instead to break the cycle
    = help: see <https://doc.rust-lang.org/reference/types.html#recursive-types> for more information
 note: cycle used when collecting item types in top-level module
-  --> $DIR/infinite-vec-type-recursion.rs:1:1
+  --> $DIR/infinite-vec-type-recursion.rs:3:1
    |
-LL | / type X = Vec<X>;
+LL | / #![cfg_attr(feature, feature(lazy_type_alias))]
-LL | |
+LL | | #![allow(incomplete_features)]
 LL | |
+LL | | type X = Vec<X>;
+...  |
+LL | | #[rustfmt::skip]
 LL | | fn main() { let b: X = Vec::new(); }
    | |____________________________________^
    = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information

tests/ui/infinite/infinite-vec-type-recursion.rs

@@ -1,4 +1,11 @@
-type X = Vec<X>;
+// revisions: feature gated
-//~^ ERROR cycle detected
 
+#![cfg_attr(feature, feature(lazy_type_alias))]
+#![allow(incomplete_features)]
+
+type X = Vec<X>;
+//[gated]~^ ERROR cycle detected
+//[feature]~^^ ERROR: overflow evaluating the requirement `X`
+
+#[rustfmt::skip]
 fn main() { let b: X = Vec::new(); }