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 = {$param_def_kind} must be used non-recursively in the definition
+    .note = all type parameters must be used in a non-recursive way in order to constrain its variance
+
 hir_analysis_redundant_lifetime_args = unnecessary lifetime parameter `{$victim}`
     .note = you can use the `{$candidate}` lifetime directly, in place of `{$victim}`
 
@@ -549,6 +553,7 @@ 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 {$param_name}: /* Type */` instead
+
 hir_analysis_unused_generic_parameter_adt_help =
     consider removing `{$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/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 {
@@ -1915,19 +1915,60 @@ fn report_bivariance(
         item_kind => bug!("report_bivariance: unexpected item kind: {item_kind:?}"),
     };
 
-    let const_param_help =
-        matches!(param.kind, hir::GenericParamKind::Type { .. } if !has_explicit_bounds)
-            .then_some(());
+    let mut usage_spans = vec![];
+    intravisit::walk_item(
+        &mut CollectUsageSpans { spans: &mut usage_spans, param_def_id: param.def_id.to_def_id() },
+        item,
+    );
 
-    let mut diag = tcx.dcx().create_err(errors::UnusedGenericParameter {
-        span: param.span,
-        param_name,
-        param_def_kind: tcx.def_descr(param.def_id.to_def_id()),
-        help,
-        const_param_help,
-    });
-    diag.code(E0392);
-    diag.emit()
+    if usage_spans.is_empty() {
+        let const_param_help =
+            matches!(param.kind, hir::GenericParamKind::Type { .. } if !has_explicit_bounds)
+                .then_some(());
+
+        let mut diag = tcx.dcx().create_err(errors::UnusedGenericParameter {
+            span: param.span,
+            param_name,
+            param_def_kind: tcx.def_descr(param.def_id.to_def_id()),
+            help,
+            const_param_help,
+        });
+        diag.code(E0392);
+        diag.emit()
+    } else {
+        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: (),
+        });
+        diag.emit()
+    }
+}
+
+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
+            && let Res::Def(DefKind::TyParam, def_id) = qpath.res
+            && def_id == self.param_def_id
+        {
+            self.spans.push(t.span);
+        }
+        intravisit::walk_ty(self, t);
+    }
 }
 
 impl<'tcx> WfCheckingCtxt<'_, 'tcx> {

compiler/rustc_hir_analysis/src/errors.rs

@@ -1603,6 +1603,21 @@ pub(crate) struct UnusedGenericParameter {
     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

@@ -4,23 +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,)>;
-   |            ^ 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 its 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,)>;
-   |            ^ 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 its variance
 
 error[E0275]: overflow evaluating the requirement `Poly0<()> == _`
   --> $DIR/inherent-impls-overflow.rs:21:6
@@ -32,5 +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<(((((((...,),),),),),),)>`

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 its 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 its 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

@@ -11,8 +11,8 @@ 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
 }
 

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

@@ -16,14 +16,16 @@ LL | enum SomeEnum<A> { Nothing }
    = help: consider removing `A`, referring to it in a field, or using a marker such as `PhantomData`
    = help: if you intended `A` to be a const parameter, use `const A: /* Type */` instead
 
-error[E0392]: type parameter `T` is never used
-  --> $DIR/variance-unused-type-param.rs:13:15
+error: type parameter `T` is only used recursively
+  --> $DIR/variance-unused-type-param.rs:14:23
    |
 LL | enum ListCell<T> {
-   |               ^ unused type parameter
+   |               - type parameter must be used non-recursively in the definition
+LL |     Cons(Box<ListCell<T>>),
+   |                       ^
    |
    = 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 its variance
 
 error[E0392]: type parameter `T` is never used
   --> $DIR/variance-unused-type-param.rs:19:19