Rollup merge of rust-lang#127976 - fmease:lta-cyclic-bivariant-param-…

…better-err, r=compiler-errors

Lazy type aliases: Diagostics: Detect bivariant ty params that are only used recursively

Follow-up to errs's rust-lang#127871. Extends the logic to cover LTAs, too, not just ADTs.
This change only takes effect with the next-gen solver enabled as cycle errors like
the one we have here are fatal in the old solver. That's my explanation anyways.

r? compiler-errors

compiler/rustc_hir_analysis/src/check/wfcheck.rs

@@ -1922,39 +1922,31 @@ fn report_bivariance<'tcx>(
     );
 
     if !usage_spans.is_empty() {
-        // First, check if the ADT is (probably) cyclical. We say probably here, since
-        // we're not actually looking into substitutions, just walking through fields.
-        // And we only recurse into the fields of ADTs, and not the hidden types of
-        // opaques or anything else fancy.
+        // First, check if the ADT/LTA is (probably) cyclical. We say probably here, since we're
+        // not actually looking into substitutions, just walking through fields / the "RHS".
+        // We don't recurse into the hidden types of opaques or anything else fancy.
         let item_def_id = item.owner_id.to_def_id();
-        let is_probably_cyclical = if matches!(
-            tcx.def_kind(item_def_id),
-            DefKind::Struct | DefKind::Union | DefKind::Enum
-        ) {
-            IsProbablyCyclical { tcx, adt_def_id: item_def_id, seen: Default::default() }
-                .visit_all_fields(tcx.adt_def(item_def_id))
-                .is_break()
-        } else {
-            false
-        };
-        // If the ADT is cyclical, then if at least one usage of the type parameter or
-        // the `Self` alias is present in the, then it's probably a cyclical struct, and
-        // we should call those parameter usages recursive rather than just saying they're
-        // unused...
+        let is_probably_cyclical =
+            IsProbablyCyclical { tcx, item_def_id, seen: Default::default() }
+                .visit_def(item_def_id)
+                .is_break();
+        // If the ADT/LTA is cyclical, then if at least one usage of the type parameter or
+        // the `Self` alias is present in the, then it's probably a cyclical struct/ type
+        // alias, and we should call those parameter usages recursive rather than just saying
+        // they're unused...
         //
         // We currently report *all* of the parameter usages, since computing the exact
         // subset is very involved, and the fact we're mentioning recursion at all is
         // likely to guide the user in the right direction.
         if is_probably_cyclical {
-            let diag = tcx.dcx().create_err(errors::RecursiveGenericParameter {
+            return tcx.dcx().emit_err(errors::RecursiveGenericParameter {
                 spans: usage_spans,
                 param_span: param.span,
                 param_name,
                 param_def_kind: tcx.def_descr(param.def_id.to_def_id()),
                 help,
                 note: (),
             });
-            return diag.emit();
         }
     }
 
@@ -1974,42 +1966,51 @@ fn report_bivariance<'tcx>(
     diag.emit()
 }
 
-/// Detects cases where an ADT is trivially cyclical -- we want to detect this so
-/// /we only mention that its parameters are used cyclically if the ADT is truly
+/// Detects cases where an ADT/LTA is trivially cyclical -- we want to detect this so
+/// we only mention that its parameters are used cyclically if the ADT/LTA is truly
 /// cyclical.
 ///
 /// Notably, we don't consider substitutions here, so this may have false positives.
 struct IsProbablyCyclical<'tcx> {
     tcx: TyCtxt<'tcx>,
-    adt_def_id: DefId,
+    item_def_id: DefId,
     seen: FxHashSet<DefId>,
 }
 
 impl<'tcx> IsProbablyCyclical<'tcx> {
-    fn visit_all_fields(&mut self, adt_def: ty::AdtDef<'tcx>) -> ControlFlow<(), ()> {
-        for field in adt_def.all_fields() {
-            self.tcx.type_of(field.did).instantiate_identity().visit_with(self)?;
+    fn visit_def(&mut self, def_id: DefId) -> ControlFlow<(), ()> {
+        match self.tcx.def_kind(def_id) {
+            DefKind::Struct | DefKind::Enum | DefKind::Union => {
+                self.tcx.adt_def(def_id).all_fields().try_for_each(|field| {
+                    self.tcx.type_of(field.did).instantiate_identity().visit_with(self)
+                })
+            }
+            DefKind::TyAlias if self.tcx.type_alias_is_lazy(def_id) => {
+                self.tcx.type_of(def_id).instantiate_identity().visit_with(self)
+            }
+            _ => ControlFlow::Continue(()),
         }
-
-        ControlFlow::Continue(())
     }
 }
 
 impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for IsProbablyCyclical<'tcx> {
     type Result = ControlFlow<(), ()>;
 
-    fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<(), ()> {
-        if let Some(adt_def) = t.ty_adt_def() {
-            if adt_def.did() == self.adt_def_id {
+    fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<(), ()> {
+        let def_id = match ty.kind() {
+            ty::Adt(adt_def, _) => Some(adt_def.did()),
+            ty::Alias(ty::Weak, alias_ty) => Some(alias_ty.def_id),
+            _ => None,
+        };
+        if let Some(def_id) = def_id {
+            if def_id == self.item_def_id {
                 return ControlFlow::Break(());
             }
-
-            if self.seen.insert(adt_def.did()) {
-                self.visit_all_fields(adt_def)?;
+            if self.seen.insert(def_id) {
+                self.visit_def(def_id)?;
             }
         }
-
-        t.super_visit_with(self)
+        ty.super_visit_with(self)
     }
 }
 

tests/ui/lazy-type-alias/inherent-impls-overflow.next.stderr

@@ -4,27 +4,27 @@ error[E0275]: overflow evaluating the requirement `Loop == _`
 LL | impl Loop {}
    |      ^^^^
 
-error[E0392]: type parameter `T` is never used
-  --> $DIR/inherent-impls-overflow.rs:14:12
+error: type parameter `T` is only used recursively
+  --> $DIR/inherent-impls-overflow.rs:14:24
    |
 LL | type Poly0<T> = Poly1<(T,)>;
-   |            ^           - `T` is named here, but is likely unused in the containing type
+   |            -           ^
    |            |
-   |            unused type parameter
+   |            type parameter must be used non-recursively in the definition
    |
    = help: consider removing `T` or referring to it in the body of the type alias
-   = help: if you intended `T` to be a const parameter, use `const T: /* Type */` instead
+   = note: all type parameters must be used in a non-recursive way in order to constrain their variance
 
-error[E0392]: type parameter `T` is never used
-  --> $DIR/inherent-impls-overflow.rs:17:12
+error: type parameter `T` is only used recursively
+  --> $DIR/inherent-impls-overflow.rs:17:24
    |
 LL | type Poly1<T> = Poly0<(T,)>;
-   |            ^           - `T` is named here, but is likely unused in the containing type
+   |            -           ^
    |            |
-   |            unused type parameter
+   |            type parameter must be used non-recursively in the definition
    |
    = help: consider removing `T` or referring to it in the body of the type alias
-   = help: if you intended `T` to be a const parameter, use `const T: /* Type */` instead
+   = note: all type parameters must be used in a non-recursive way in order to constrain their variance
 
 error[E0275]: overflow evaluating the requirement `Poly0<()> == _`
   --> $DIR/inherent-impls-overflow.rs:21:6
@@ -36,5 +36,4 @@ LL | impl Poly0<()> {}
 
 error: aborting due to 4 previous errors
 
-Some errors have detailed explanations: E0275, E0392.
-For more information about an error, try `rustc --explain E0275`.
+For more information about this error, try `rustc --explain E0275`.

tests/ui/lazy-type-alias/inherent-impls-overflow.rs

@@ -13,10 +13,10 @@ impl Loop {}
 
 type Poly0<T> = Poly1<(T,)>;
 //[current]~^ ERROR overflow normalizing the type alias `Poly0<(((((((...,),),),),),),)>`
-//[next]~^^ ERROR type parameter `T` is never used
+//[next]~^^ ERROR type parameter `T` is only used recursively
 type Poly1<T> = Poly0<(T,)>;
 //[current]~^ ERROR  overflow normalizing the type alias `Poly1<(((((((...,),),),),),),)>`
-//[next]~^^ ERROR type parameter `T` is never used
+//[next]~^^ ERROR type parameter `T` is only used recursively
 
 impl Poly0<()> {}
 //[current]~^ ERROR overflow normalizing the type alias `Poly1<(((((((...,),),),),),),)>`