Mention that type parameters are used recursively on bivariance error

compiler/rustc_hir_analysis/messages.ftl

@@ -382,6 +382,10 @@ hir_analysis_placeholder_not_allowed_item_signatures = the placeholder `_` is no
 hir_analysis_precise_capture_self_alias = `Self` can't be captured in `use<...>` precise captures list, since it is an alias
     .label = `Self` is not a generic argument, but an alias to the type of the {$what}
 
+hir_analysis_recursive_generic_parameter = {$param_def_kind} `{$param_name}` is only used recursively
+    .label = {$param_def_kind} must be used non-recursively in the definition
+    .note = all type parameters must be used in a non-recursive way in order to constrain their variance
+
 hir_analysis_redundant_lifetime_args = unnecessary lifetime parameter `{$victim}`
     .note = you can use the `{$candidate}` lifetime directly, in place of `{$victim}`
 
@@ -549,6 +553,8 @@ hir_analysis_unused_generic_parameter =
     {$param_def_kind} `{$param_name}` is never used
     .label = unused {$param_def_kind}
     .const_param_help = if you intended `{$param_name}` to be a const parameter, use `const {$param_name}: /* Type */` instead
+    .usage_spans = `{$param_name}` is named here, but is likely unused in the containing type
+
 hir_analysis_unused_generic_parameter_adt_help =
     consider removing `{$param_name}`, referring to it in a field, or using a marker such as `{$phantom_data}`
 hir_analysis_unused_generic_parameter_adt_no_phantom_data_help =

compiler/rustc_hir_analysis/src/check/check.rs

@@ -1572,6 +1572,7 @@ fn check_type_alias_type_params_are_used<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalD
                 param_name,
                 param_def_kind: tcx.def_descr(param.def_id),
                 help: errors::UnusedGenericParameterHelp::TyAlias { param_name },
+                usage_spans: vec![],
                 const_param_help,
             });
             diag.code(E0091);

compiler/rustc_hir_analysis/src/check/wfcheck.rs

@@ -4,12 +4,12 @@ use crate::constrained_generic_params::{identify_constrained_generic_params, Par
 use crate::errors;
 use crate::fluent_generated as fluent;
 
-use hir::intravisit::Visitor;
+use hir::intravisit::{self, Visitor};
 use rustc_ast as ast;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet, FxIndexSet};
 use rustc_errors::{codes::*, pluralize, struct_span_code_err, Applicability, ErrorGuaranteed};
 use rustc_hir as hir;
-use rustc_hir::def::DefKind;
+use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::{DefId, LocalDefId, LocalModDefId};
 use rustc_hir::lang_items::LangItem;
 use rustc_hir::ItemKind;
@@ -1799,7 +1799,7 @@ fn receiver_is_implemented<'tcx>(
 
 fn check_variances_for_type_defn<'tcx>(
     tcx: TyCtxt<'tcx>,
-    item: &hir::Item<'tcx>,
+    item: &'tcx hir::Item<'tcx>,
     hir_generics: &hir::Generics<'tcx>,
 ) {
     let identity_args = ty::GenericArgs::identity_for_item(tcx, item.owner_id);
@@ -1886,21 +1886,21 @@ fn check_variances_for_type_defn<'tcx>(
             hir::ParamName::Error => {}
             _ => {
                 let has_explicit_bounds = explicitly_bounded_params.contains(&parameter);
-                report_bivariance(tcx, hir_param, has_explicit_bounds, item.kind);
+                report_bivariance(tcx, hir_param, has_explicit_bounds, item);
             }
         }
     }
 }
 
-fn report_bivariance(
-    tcx: TyCtxt<'_>,
-    param: &rustc_hir::GenericParam<'_>,
+fn report_bivariance<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    param: &'tcx hir::GenericParam<'tcx>,
     has_explicit_bounds: bool,
-    item_kind: ItemKind<'_>,
+    item: &'tcx hir::Item<'tcx>,
 ) -> ErrorGuaranteed {
     let param_name = param.name.ident();
 
-    let help = match item_kind {
+    let help = match item.kind {
         ItemKind::Enum(..) | ItemKind::Struct(..) | ItemKind::Union(..) => {
             if let Some(def_id) = tcx.lang_items().phantom_data() {
                 errors::UnusedGenericParameterHelp::Adt {
@@ -1915,6 +1915,49 @@ fn report_bivariance(
         item_kind => bug!("report_bivariance: unexpected item kind: {item_kind:?}"),
     };
 
+    let mut usage_spans = vec![];
+    intravisit::walk_item(
+        &mut CollectUsageSpans { spans: &mut usage_spans, param_def_id: param.def_id.to_def_id() },
+        item,
+    );
+
+    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.
+        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...
+        //
+        // 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 {
+                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();
+        }
+    }
+
     let const_param_help =
         matches!(param.kind, hir::GenericParamKind::Type { .. } if !has_explicit_bounds)
             .then_some(());
@@ -1923,13 +1966,85 @@ fn report_bivariance(
         span: param.span,
         param_name,
         param_def_kind: tcx.def_descr(param.def_id.to_def_id()),
+        usage_spans,
         help,
         const_param_help,
     });
     diag.code(E0392);
     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
+/// cyclical.
+///
+/// Notably, we don't consider substitutions here, so this may have false positives.
+struct IsProbablyCyclical<'tcx> {
+    tcx: TyCtxt<'tcx>,
+    adt_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)?;
+        }
+
+        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 {
+                return ControlFlow::Break(());
+            }
+
+            if self.seen.insert(adt_def.did()) {
+                self.visit_all_fields(adt_def)?;
+            }
+        }
+
+        t.super_visit_with(self)
+    }
+}
+
+/// Collect usages of the `param_def_id` and `Res::SelfTyAlias` in the HIR.
+///
+/// This is used to report places where the user has used parameters in a
+/// non-variance-constraining way for better bivariance errors.
+struct CollectUsageSpans<'a> {
+    spans: &'a mut Vec<Span>,
+    param_def_id: DefId,
+}
+
+impl<'tcx> Visitor<'tcx> for CollectUsageSpans<'_> {
+    type Result = ();
+
+    fn visit_generics(&mut self, _g: &'tcx rustc_hir::Generics<'tcx>) -> Self::Result {
+        // Skip the generics. We only care about fields, not where clause/param bounds.
+    }
+
+    fn visit_ty(&mut self, t: &'tcx hir::Ty<'tcx>) -> Self::Result {
+        if let hir::TyKind::Path(hir::QPath::Resolved(None, qpath)) = t.kind {
+            if let Res::Def(DefKind::TyParam, def_id) = qpath.res
+                && def_id == self.param_def_id
+            {
+                self.spans.push(t.span);
+                return;
+            } else if let Res::SelfTyAlias { .. } = qpath.res {
+                self.spans.push(t.span);
+                return;
+            }
+        }
+        intravisit::walk_ty(self, t);
+    }
+}
+
 impl<'tcx> WfCheckingCtxt<'_, 'tcx> {
     /// Feature gates RFC 2056 -- trivial bounds, checking for global bounds that
     /// aren't true.

compiler/rustc_hir_analysis/src/errors.rs

@@ -1597,12 +1597,29 @@ pub(crate) struct UnusedGenericParameter {
     pub span: Span,
     pub param_name: Ident,
     pub param_def_kind: &'static str,
+    #[label(hir_analysis_usage_spans)]
+    pub usage_spans: Vec<Span>,
     #[subdiagnostic]
     pub help: UnusedGenericParameterHelp,
     #[help(hir_analysis_const_param_help)]
     pub const_param_help: Option<()>,
 }
 
+#[derive(Diagnostic)]
+#[diag(hir_analysis_recursive_generic_parameter)]
+pub(crate) struct RecursiveGenericParameter {
+    #[primary_span]
+    pub spans: Vec<Span>,
+    #[label]
+    pub param_span: Span,
+    pub param_name: Ident,
+    pub param_def_kind: &'static str,
+    #[subdiagnostic]
+    pub help: UnusedGenericParameterHelp,
+    #[note]
+    pub note: (),
+}
+
 #[derive(Subdiagnostic)]
 pub(crate) enum UnusedGenericParameterHelp {
     #[help(hir_analysis_unused_generic_parameter_adt_help)]

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

@@ -8,7 +8,9 @@ error[E0392]: type parameter `T` is never used
   --> $DIR/inherent-impls-overflow.rs:14:12
    |
 LL | type Poly0<T> = Poly1<(T,)>;
-   |            ^ unused type parameter
+   |            ^           - `T` is named here, but is likely unused in the containing type
+   |            |
+   |            unused type parameter
    |
    = 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
@@ -17,7 +19,9 @@ error[E0392]: type parameter `T` is never used
   --> $DIR/inherent-impls-overflow.rs:17:12
    |
 LL | type Poly1<T> = Poly0<(T,)>;
-   |            ^ unused type parameter
+   |            ^           - `T` is named here, but is likely unused in the containing type
+   |            |
+   |            unused type parameter
    |
    = 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

tests/ui/traits/issue-105231.rs

@@ -1,9 +1,9 @@
 //~ ERROR overflow evaluating the requirement `A<A<A<A<A<A<A<...>>>>>>>: Send`
 struct A<T>(B<T>);
 //~^ ERROR recursive types `A` and `B` have infinite size
-//~| ERROR `T` is never used
+//~| ERROR `T` is only used recursively
 struct B<T>(A<A<T>>);
-//~^ ERROR `T` is never used
+//~^ ERROR `T` is only used recursively
 trait Foo {}
 impl<T> Foo for T where T: Send {}
 impl Foo for B<u8> {}

tests/ui/traits/issue-105231.stderr

@@ -15,23 +15,27 @@ LL |
 LL ~ struct B<T>(Box<A<A<T>>>);
    |
 
-error[E0392]: type parameter `T` is never used
-  --> $DIR/issue-105231.rs:2:10
+error: type parameter `T` is only used recursively
+  --> $DIR/issue-105231.rs:2:15
    |
 LL | struct A<T>(B<T>);
-   |          ^ unused type parameter
+   |          -    ^
+   |          |
+   |          type parameter must be used non-recursively in the definition
    |
    = help: consider removing `T`, referring to it in a field, or using a marker such as `PhantomData`
-   = 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/issue-105231.rs:5:10
+error: type parameter `T` is only used recursively
+  --> $DIR/issue-105231.rs:5:17
    |
 LL | struct B<T>(A<A<T>>);
-   |          ^ unused type parameter
+   |          -      ^
+   |          |
+   |          type parameter must be used non-recursively in the definition
    |
    = help: consider removing `T`, referring to it in a field, or using a marker such as `PhantomData`
-   = 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 `A<A<A<A<A<A<A<...>>>>>>>: Send`
    |
@@ -44,5 +48,5 @@ LL | struct B<T>(A<A<T>>);
 
 error: aborting due to 4 previous errors
 
-Some errors have detailed explanations: E0072, E0275, E0392.
+Some errors have detailed explanations: E0072, E0275.
 For more information about an error, try `rustc --explain E0072`.

tests/ui/variance/variance-unused-type-param.rs

@@ -11,11 +11,14 @@ enum SomeEnum<A> { Nothing }
 
 // Here T might *appear* used, but in fact it isn't.
 enum ListCell<T> {
-//~^ ERROR parameter `T` is never used
     Cons(Box<ListCell<T>>),
+    //~^ ERROR parameter `T` is only used recursively
     Nil
 }
 
+struct SelfTyAlias<T>(Box<Self>);
+//~^ ERROR parameter `T` is only used recursively
+
 struct WithBounds<T: Sized> {}
 //~^ ERROR parameter `T` is never used
 
@@ -25,4 +28,9 @@ struct WithWhereBounds<T> where T: Sized {}
 struct WithOutlivesBounds<T: 'static> {}
 //~^ ERROR parameter `T` is never used
 
+struct DoubleNothing<T> {
+//~^ ERROR parameter `T` is never used
+    s: SomeStruct<T>,
+}
+
 fn main() {}