Mention that type parameters are used recursively

2 files changed, 7 insertions, 5 deletions

diff --git a/tests/ui/variance/variance-unused-type-param.rs b/tests/ui/variance/variance-unused-type-param.rs
index d1114064364..7e35f59fd84 100644
--- a/tests/ui/variance/variance-unused-type-param.rs
+++ b/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
 }
 
diff --git a/tests/ui/variance/variance-unused-type-param.stderr b/tests/ui/variance/variance-unused-type-param.stderr
index 3011b7bd18f..212db564ac4 100644
--- a/tests/ui/variance/variance-unused-type-param.stderr
+++ b/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