Rollup merge of #120556 - fmease:improve-unused-generic-param-diags, r=oli-obk

Improve the diagnostics for unused generic parameters

* Don't emit two errors (namely E0091 *and* E0392) for unused type parameters on *lazy* type aliases
* Fix the diagnostic help message of E0392 for *lazy* type aliases: Don't talk about the “fields” of lazy type aliases (use the term “body” instead) and don't suggest `PhantomData` for them, it doesn't make much sense
* Consolidate the diagnostics for E0091 (unused type parameters in type aliases) and E0392 (unused generic parameters due to bivariance) and make it translatable
  * Still keep the error codes distinct (for now)
  * Naturally leads to better diagnostics for E0091

r? ```@oli-obk``` (to ballast your review load :P) or compiler

38 files changed, 246 insertions, 114 deletions

diff --git a/tests/ui/const-generics/generic_arg_infer/infer-arg-test.stderr b/tests/ui/const-generics/generic_arg_infer/infer-arg-test.stderr
index a6b736261e0..6d8dd017734 100644
--- a/tests/ui/const-generics/generic_arg_infer/infer-arg-test.stderr
+++ b/tests/ui/const-generics/generic_arg_infer/infer-arg-test.stderr
@@ -10,14 +10,14 @@ error: expected identifier, found reserved identifier `_`
 LL | fn bad_infer_fn<_>() {}
    |                 ^ expected identifier, found reserved identifier
 
-error[E0392]: parameter `_` is never used
+error[E0392]: type parameter `_` is never used
   --> $DIR/infer-arg-test.rs:7:17
    |
 LL | struct BadInfer<_>;
-   |                 ^ unused parameter
+   |                 ^ unused type parameter
    |
    = help: consider removing `_`, referring to it in a field, or using a marker such as `PhantomData`
-   = help: if you intended `_` to be a const parameter, use `const _: usize` instead
+   = help: if you intended `_` to be a const parameter, use `const _: /* Type */` instead
 
 error[E0107]: struct takes 2 generic arguments but 3 generic arguments were supplied
   --> $DIR/infer-arg-test.rs:18:10
diff --git a/tests/ui/const-generics/issue-46511.stderr b/tests/ui/const-generics/issue-46511.stderr
index d57295fa2fa..75d59ee40b3 100644
--- a/tests/ui/const-generics/issue-46511.stderr
+++ b/tests/ui/const-generics/issue-46511.stderr
@@ -7,11 +7,11 @@ LL |     _a: [u8; std::mem::size_of::<&'a mut u8>()]
    = note: lifetime parameters may not be used in const expressions
    = help: add `#![feature(generic_const_exprs)]` to allow generic const expressions
 
-error[E0392]: parameter `'a` is never used
+error[E0392]: lifetime parameter `'a` is never used
   --> $DIR/issue-46511.rs:3:12
    |
 LL | struct Foo<'a>
-   |            ^^ unused parameter
+   |            ^^ unused lifetime parameter
    |
    = help: consider removing `'a`, referring to it in a field, or using a marker such as `PhantomData`
 
diff --git a/tests/ui/const-generics/issues/issue-67375.min.stderr b/tests/ui/const-generics/issues/issue-67375.min.stderr
index 7671e3c4688..e871203ed9b 100644
--- a/tests/ui/const-generics/issues/issue-67375.min.stderr
+++ b/tests/ui/const-generics/issues/issue-67375.min.stderr
@@ -7,14 +7,14 @@ LL |     inner: [(); { [|_: &T| {}; 0].len() }],
    = note: type parameters may not be used in const expressions
    = help: add `#![feature(generic_const_exprs)]` to allow generic const expressions
 
-error[E0392]: parameter `T` is never used
+error[E0392]: type parameter `T` is never used
   --> $DIR/issue-67375.rs:5:12
    |
 LL | struct Bug<T> {
-   |            ^ unused parameter
+   |            ^ unused type parameter
    |
    = 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: usize` instead
+   = help: if you intended `T` to be a const parameter, use `const T: /* Type */` instead
 
 error: aborting due to 2 previous errors
 
diff --git a/tests/ui/const-generics/issues/issue-67945-1.min.stderr b/tests/ui/const-generics/issues/issue-67945-1.min.stderr
index 1d071da903f..1de607644f5 100644
--- a/tests/ui/const-generics/issues/issue-67945-1.min.stderr
+++ b/tests/ui/const-generics/issues/issue-67945-1.min.stderr
@@ -16,14 +16,14 @@ LL |         let b = &*(&x as *const _ as *const S);
    = note: type parameters may not be used in const expressions
    = help: add `#![feature(generic_const_exprs)]` to allow generic const expressions
 
-error[E0392]: parameter `S` is never used
+error[E0392]: type parameter `S` is never used
   --> $DIR/issue-67945-1.rs:7:12
    |
 LL | struct Bug<S> {
-   |            ^ unused parameter
+   |            ^ unused type parameter
    |
    = help: consider removing `S`, referring to it in a field, or using a marker such as `PhantomData`
-   = help: if you intended `S` to be a const parameter, use `const S: usize` instead
+   = help: if you intended `S` to be a const parameter, use `const S: /* Type */` instead
 
 error: aborting due to 3 previous errors
 
diff --git a/tests/ui/const-generics/issues/issue-67945-3.min.stderr b/tests/ui/const-generics/issues/issue-67945-3.min.stderr
index e34869c7938..0ccba18e953 100644
--- a/tests/ui/const-generics/issues/issue-67945-3.min.stderr
+++ b/tests/ui/const-generics/issues/issue-67945-3.min.stderr
@@ -7,14 +7,14 @@ LL |         let x: Option<S> = None;
    = note: type parameters may not be used in const expressions
    = help: add `#![feature(generic_const_exprs)]` to allow generic const expressions
 
-error[E0392]: parameter `S` is never used
+error[E0392]: type parameter `S` is never used
   --> $DIR/issue-67945-3.rs:9:12
    |
 LL | struct Bug<S> {
-   |            ^ unused parameter
+   |            ^ unused type parameter
    |
    = help: consider removing `S`, referring to it in a field, or using a marker such as `PhantomData`
-   = help: if you intended `S` to be a const parameter, use `const S: usize` instead
+   = help: if you intended `S` to be a const parameter, use `const S: /* Type */` instead
 
 error: aborting due to 2 previous errors
 
diff --git a/tests/ui/const-generics/issues/issue-67945-4.min.stderr b/tests/ui/const-generics/issues/issue-67945-4.min.stderr
index 280c6f4f2cd..83ae68e2dbf 100644
--- a/tests/ui/const-generics/issues/issue-67945-4.min.stderr
+++ b/tests/ui/const-generics/issues/issue-67945-4.min.stderr
@@ -7,14 +7,14 @@ LL |         let x: Option<Box<S>> = None;
    = note: type parameters may not be used in const expressions
    = help: add `#![feature(generic_const_exprs)]` to allow generic const expressions
 
-error[E0392]: parameter `S` is never used
+error[E0392]: type parameter `S` is never used
   --> $DIR/issue-67945-4.rs:8:12
    |
 LL | struct Bug<S> {
-   |            ^ unused parameter
+   |            ^ unused type parameter
    |
    = help: consider removing `S`, referring to it in a field, or using a marker such as `PhantomData`
-   = help: if you intended `S` to be a const parameter, use `const S: usize` instead
+   = help: if you intended `S` to be a const parameter, use `const S: /* Type */` instead
 
 error: aborting due to 2 previous errors
 
diff --git a/tests/ui/const-generics/unused-type-param-suggestion.rs b/tests/ui/const-generics/unused-type-param-suggestion.rs
index 2251512c459..fb0ccb4fdcd 100644
--- a/tests/ui/const-generics/unused-type-param-suggestion.rs
+++ b/tests/ui/const-generics/unused-type-param-suggestion.rs
@@ -1,4 +1,27 @@
 #![crate_type="lib"]
 
-struct Example<N>;
-//~^ ERROR parameter
+struct S<N>;
+//~^ ERROR type parameter `N` is never used
+//~| HELP consider removing `N`
+//~| HELP if you intended `N` to be a const parameter
+
+// Ensure that we don't emit the const param suggestion here:
+struct T<N: Copy>;
+//~^ ERROR type parameter `N` is never used
+//~| HELP consider removing `N`
+
+type A<N> = ();
+//~^ ERROR type parameter `N` is never used
+//~| HELP consider removing `N`
+//~| HELP if you intended `N` to be a const parameter
+
+// Ensure that we don't emit the const param suggestion here:
+type B<N: Copy> = ();
+//~^ ERROR type parameter `N` is never used
+//~| HELP consider removing `N`
+type C<N: Sized> = ();
+//~^ ERROR type parameter `N` is never used
+//~| HELP consider removing `N`
+type D<N: ?Sized> = ();
+//~^ ERROR type parameter `N` is never used
+//~| HELP consider removing `N`
diff --git a/tests/ui/const-generics/unused-type-param-suggestion.stderr b/tests/ui/const-generics/unused-type-param-suggestion.stderr
index 6e985f56666..67b704d8bc7 100644
--- a/tests/ui/const-generics/unused-type-param-suggestion.stderr
+++ b/tests/ui/const-generics/unused-type-param-suggestion.stderr
@@ -1,12 +1,54 @@
-error[E0392]: parameter `N` is never used
-  --> $DIR/unused-type-param-suggestion.rs:3:16
+error[E0392]: type parameter `N` is never used
+  --> $DIR/unused-type-param-suggestion.rs:3:10
    |
-LL | struct Example<N>;
-   |                ^ unused parameter
+LL | struct S<N>;
+   |          ^ unused type parameter
    |
    = help: consider removing `N`, referring to it in a field, or using a marker such as `PhantomData`
-   = help: if you intended `N` to be a const parameter, use `const N: usize` instead
+   = help: if you intended `N` to be a const parameter, use `const N: /* Type */` instead
 
-error: aborting due to 1 previous error
+error[E0392]: type parameter `N` is never used
+  --> $DIR/unused-type-param-suggestion.rs:9:10
+   |
+LL | struct T<N: Copy>;
+   |          ^ unused type parameter
+   |
+   = help: consider removing `N`, referring to it in a field, or using a marker such as `PhantomData`
+
+error[E0091]: type parameter `N` is never used
+  --> $DIR/unused-type-param-suggestion.rs:13:8
+   |
+LL | type A<N> = ();
+   |        ^ unused type parameter
+   |
+   = help: consider removing `N` or referring to it in the body of the type alias
+   = help: if you intended `N` to be a const parameter, use `const N: /* Type */` instead
+
+error[E0091]: type parameter `N` is never used
+  --> $DIR/unused-type-param-suggestion.rs:19:8
+   |
+LL | type B<N: Copy> = ();
+   |        ^ unused type parameter
+   |
+   = help: consider removing `N` or referring to it in the body of the type alias
+
+error[E0091]: type parameter `N` is never used
+  --> $DIR/unused-type-param-suggestion.rs:22:8
+   |
+LL | type C<N: Sized> = ();
+   |        ^ unused type parameter
+   |
+   = help: consider removing `N` or referring to it in the body of the type alias
+
+error[E0091]: type parameter `N` is never used
+  --> $DIR/unused-type-param-suggestion.rs:25:8
+   |
+LL | type D<N: ?Sized> = ();
+   |        ^ unused type parameter
+   |
+   = help: consider removing `N` or referring to it in the body of the type alias
+
+error: aborting due to 6 previous errors
 
-For more information about this error, try `rustc --explain E0392`.
+Some errors have detailed explanations: E0091, E0392.
+For more information about an error, try `rustc --explain E0091`.
diff --git a/tests/ui/enum-discriminant/issue-70453-generics-in-discr-ice.stderr b/tests/ui/enum-discriminant/issue-70453-generics-in-discr-ice.stderr
index fac3ce07aeb..1341b03cb56 100644
--- a/tests/ui/enum-discriminant/issue-70453-generics-in-discr-ice.stderr
+++ b/tests/ui/enum-discriminant/issue-70453-generics-in-discr-ice.stderr
@@ -6,14 +6,14 @@ LL |     Some = std::mem::size_of::<T>(),
    |
    = note: type parameters may not be used in enum discriminant values
 
-error[E0392]: parameter `T` is never used
+error[E0392]: type parameter `T` is never used
   --> $DIR/issue-70453-generics-in-discr-ice.rs:7:20
    |
 LL | enum MyWeirdOption<T> {
-   |                    ^ unused parameter
+   |                    ^ unused type parameter
    |
    = 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: usize` instead
+   = help: if you intended `T` to be a const parameter, use `const T: /* Type */` instead
 
 error: aborting due to 2 previous errors
 
diff --git a/tests/ui/enum/issue-67945-1.stderr b/tests/ui/enum/issue-67945-1.stderr
index 878fa322f02..ce0ea777636 100644
--- a/tests/ui/enum/issue-67945-1.stderr
+++ b/tests/ui/enum/issue-67945-1.stderr
@@ -6,14 +6,14 @@ LL |         let x: S = 0;
    |
    = note: type parameters may not be used in enum discriminant values
 
-error[E0392]: parameter `S` is never used
+error[E0392]: type parameter `S` is never used
   --> $DIR/issue-67945-1.rs:1:10
    |
 LL | enum Bug<S> {
-   |          ^ unused parameter
+   |          ^ unused type parameter
    |
    = help: consider removing `S`, referring to it in a field, or using a marker such as `PhantomData`
-   = help: if you intended `S` to be a const parameter, use `const S: usize` instead
+   = help: if you intended `S` to be a const parameter, use `const S: /* Type */` instead
 
 error: aborting due to 2 previous errors
 
diff --git a/tests/ui/enum/issue-67945-2.stderr b/tests/ui/enum/issue-67945-2.stderr
index f8ec12d470a..96bd08f841a 100644
--- a/tests/ui/enum/issue-67945-2.stderr
+++ b/tests/ui/enum/issue-67945-2.stderr
@@ -6,14 +6,14 @@ LL |     Var = type_ascribe!(0, S),
    |
    = note: type parameters may not be used in enum discriminant values
 
-error[E0392]: parameter `S` is never used
+error[E0392]: type parameter `S` is never used
   --> $DIR/issue-67945-2.rs:3:10
    |
 LL | enum Bug<S> {
-   |          ^ unused parameter
+   |          ^ unused type parameter
    |
    = help: consider removing `S`, referring to it in a field, or using a marker such as `PhantomData`
-   = help: if you intended `S` to be a const parameter, use `const S: usize` instead
+   = help: if you intended `S` to be a const parameter, use `const S: /* Type */` instead
 
 error: aborting due to 2 previous errors
 
diff --git a/tests/ui/error-codes/E0091.stderr b/tests/ui/error-codes/E0091.stderr
index a596b75e481..c1427ec6686 100644
--- a/tests/ui/error-codes/E0091.stderr
+++ b/tests/ui/error-codes/E0091.stderr
@@ -1,14 +1,20 @@
-error[E0091]: type parameter `T` is unused
+error[E0091]: type parameter `T` is never used
   --> $DIR/E0091.rs:1:10
    |
 LL | type Foo<T> = u32;
    |          ^ 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
 
-error[E0091]: type parameter `B` is unused
+error[E0091]: type parameter `B` is never used
   --> $DIR/E0091.rs:2:14
    |
 LL | type Foo2<A, B> = Box<A>;
    |              ^ unused type parameter
+   |
+   = help: consider removing `B` or referring to it in the body of the type alias
+   = help: if you intended `B` to be a const parameter, use `const B: /* Type */` instead
 
 error: aborting due to 2 previous errors
 
diff --git a/tests/ui/error-codes/E0392.stderr b/tests/ui/error-codes/E0392.stderr
index ecbfd5584d5..9971267e927 100644
--- a/tests/ui/error-codes/E0392.stderr
+++ b/tests/ui/error-codes/E0392.stderr
@@ -1,11 +1,11 @@
-error[E0392]: parameter `T` is never used
+error[E0392]: type parameter `T` is never used
   --> $DIR/E0392.rs:1:10
    |
 LL | enum Foo<T> { Bar }
-   |          ^ unused parameter
+   |          ^ unused type parameter
    |
    = 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: usize` instead
+   = help: if you intended `T` to be a const parameter, use `const T: /* Type */` instead
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/inner-static-type-parameter.stderr b/tests/ui/inner-static-type-parameter.stderr
index ff6558e494b..6c7f1ffbe16 100644
--- a/tests/ui/inner-static-type-parameter.stderr
+++ b/tests/ui/inner-static-type-parameter.stderr
@@ -6,14 +6,14 @@ LL | fn foo<T>() {
 LL |     static a: Bar<T> = Bar::What;
    |                   ^ use of generic parameter from outer item
 
-error[E0392]: parameter `T` is never used
+error[E0392]: type parameter `T` is never used
   --> $DIR/inner-static-type-parameter.rs:3:10
    |
 LL | enum Bar<T> { What }
-   |          ^ unused parameter
+   |          ^ unused type parameter
    |
    = 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: usize` instead
+   = help: if you intended `T` to be a const parameter, use `const T: /* Type */` instead
 
 error: aborting due to 2 previous errors
 
diff --git a/tests/ui/issues/issue-17904-2.stderr b/tests/ui/issues/issue-17904-2.stderr
index 102c8537f8e..9965106d140 100644
--- a/tests/ui/issues/issue-17904-2.stderr
+++ b/tests/ui/issues/issue-17904-2.stderr
@@ -1,8 +1,8 @@
-error[E0392]: parameter `T` is never used
+error[E0392]: type parameter `T` is never used
   --> $DIR/issue-17904-2.rs:4:12
    |
 LL | struct Foo<T> where T: Copy;
-   |            ^ unused parameter
+   |            ^ unused type parameter
    |
    = help: consider removing `T`, referring to it in a field, or using a marker such as `PhantomData`
 
diff --git a/tests/ui/issues/issue-17994.rs b/tests/ui/issues/issue-17994.rs
index 39b0a7ebe74..ab37a172eaa 100644
--- a/tests/ui/issues/issue-17994.rs
+++ b/tests/ui/issues/issue-17994.rs
@@ -1,3 +1,3 @@
 trait Tr {}
-type Huh<T> where T: Tr = isize; //~  ERROR type parameter `T` is unused
+type Huh<T> where T: Tr = isize; //~ ERROR type parameter `T` is never used
 fn main() {}
diff --git a/tests/ui/issues/issue-17994.stderr b/tests/ui/issues/issue-17994.stderr
index ba3def64dfb..f149e5d08fa 100644
--- a/tests/ui/issues/issue-17994.stderr
+++ b/tests/ui/issues/issue-17994.stderr
@@ -1,8 +1,10 @@
-error[E0091]: type parameter `T` is unused
+error[E0091]: type parameter `T` is never used
   --> $DIR/issue-17994.rs:2:10
    |
 LL | type Huh<T> where T: Tr = isize;
    |          ^ unused type parameter
+   |
+   = help: consider removing `T` or referring to it in the body of the type alias
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/issues/issue-20413.stderr b/tests/ui/issues/issue-20413.stderr
index 8793029dd22..5d442eb9898 100644
--- a/tests/ui/issues/issue-20413.stderr
+++ b/tests/ui/issues/issue-20413.stderr
@@ -1,11 +1,11 @@
-error[E0392]: parameter `T` is never used
+error[E0392]: type parameter `T` is never used
   --> $DIR/issue-20413.rs:6:15
    |
 LL | struct NoData<T>;
-   |               ^ unused parameter
+   |               ^ unused type parameter
    |
    = 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: usize` instead
+   = help: if you intended `T` to be a const parameter, use `const T: /* Type */` instead
 
 error[E0275]: overflow evaluating the requirement `NoData<NoData<NoData<NoData<NoData<NoData<NoData<...>>>>>>>: Foo`
   --> $DIR/issue-20413.rs:9:36
diff --git a/tests/ui/issues/issue-30236.rs b/tests/ui/issues/issue-30236.rs
index 9c2d855076d..08d08a54402 100644
--- a/tests/ui/issues/issue-30236.rs
+++ b/tests/ui/issues/issue-30236.rs
@@ -1,5 +1,5 @@
 type Foo<
-    Unused //~ ERROR type parameter `Unused` is unused
+    Unused //~ ERROR type parameter `Unused` is never used
     > = u8;
 
 fn main() {
diff --git a/tests/ui/issues/issue-30236.stderr b/tests/ui/issues/issue-30236.stderr
index 0f69f49f505..bfe374a653f 100644
--- a/tests/ui/issues/issue-30236.stderr
+++ b/tests/ui/issues/issue-30236.stderr
@@ -1,8 +1,11 @@
-error[E0091]: type parameter `Unused` is unused
+error[E0091]: type parameter `Unused` is never used
   --> $DIR/issue-30236.rs:2:5
    |
 LL |     Unused
    |     ^^^^^^ unused type parameter
+   |
+   = help: consider removing `Unused` or referring to it in the body of the type alias
+   = help: if you intended `Unused` to be a const parameter, use `const Unused: /* Type */` instead
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/issues/issue-31910.stderr b/tests/ui/issues/issue-31910.stderr
index 89a6d5574a1..ca2d2f619e6 100644
--- a/tests/ui/issues/issue-31910.stderr
+++ b/tests/ui/issues/issue-31910.stderr
@@ -4,11 +4,11 @@ error[E0308]: mismatched types
 LL |     X = Trait::Number,
    |         ^^^^^^^^^^^^^ expected `isize`, found `i32`
 
-error[E0392]: parameter `T` is never used
+error[E0392]: type parameter `T` is never used
   --> $DIR/issue-31910.rs:1:11
    |
 LL | enum Enum<T: Trait> {
-   |           ^ unused parameter
+   |           ^ unused type parameter
    |
    = help: consider removing `T`, referring to it in a field, or using a marker such as `PhantomData`
 
diff --git a/tests/ui/issues/issue-34373.stderr b/tests/ui/issues/issue-34373.stderr
index 1a1cfc925b7..13667cd920e 100644
--- a/tests/ui/issues/issue-34373.stderr
+++ b/tests/ui/issues/issue-34373.stderr
@@ -45,14 +45,14 @@ help: alternatively, consider constraining `foo` so it does not apply to trait o
 LL |     fn foo(_: T) where Self: Sized {}
    |                  +++++++++++++++++
 
-error[E0392]: parameter `T` is never used
+error[E0392]: type parameter `T` is never used
   --> $DIR/issue-34373.rs:7:16
    |
 LL | pub struct Foo<T = Box<Trait<DefaultFoo>>>;
-   |                ^^^^^^^^^^^^^^^^^^^^^^^^^^ unused parameter
+   |                ^^^^^^^^^^^^^^^^^^^^^^^^^^ unused type parameter
    |
    = 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: usize` instead
+   = help: if you intended `T` to be a const parameter, use `const T: /* Type */` instead
 
 error: aborting due to 3 previous errors
 
diff --git a/tests/ui/issues/issue-36299.stderr b/tests/ui/issues/issue-36299.stderr
index dc24fb353f4..29e8d7ca59a 100644
--- a/tests/ui/issues/issue-36299.stderr
+++ b/tests/ui/issues/issue-36299.stderr
@@ -1,19 +1,19 @@
-error[E0392]: parameter `'a` is never used
+error[E0392]: lifetime parameter `'a` is never used
   --> $DIR/issue-36299.rs:1:12
    |
 LL | struct Foo<'a, A> {}
-   |            ^^ unused parameter
+   |            ^^ unused lifetime parameter
    |
    = help: consider removing `'a`, referring to it in a field, or using a marker such as `PhantomData`
 
-error[E0392]: parameter `A` is never used
+error[E0392]: type parameter `A` is never used
   --> $DIR/issue-36299.rs:1:16
    |
 LL | struct Foo<'a, A> {}
-   |                ^ unused parameter
+   |                ^ unused type parameter
    |
    = 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: usize` instead
+   = help: if you intended `A` to be a const parameter, use `const A: /* Type */` instead
 
 error: aborting due to 2 previous errors
 
diff --git a/tests/ui/issues/issue-37534.stderr b/tests/ui/issues/issue-37534.stderr
index 03fea2c1648..a687e733d3d 100644
--- a/tests/ui/issues/issue-37534.stderr
+++ b/tests/ui/issues/issue-37534.stderr
@@ -15,11 +15,11 @@ warning: relaxing a default bound only does something for `?Sized`; all other tr
 LL | struct Foo<T: ?Hash> {}
    |               ^^^^^
 
-error[E0392]: parameter `T` is never used
+error[E0392]: type parameter `T` is never used
   --> $DIR/issue-37534.rs:1:12
    |
 LL | struct Foo<T: ?Hash> {}
-   |            ^ unused parameter
+   |            ^ unused type parameter
    |
    = help: consider removing `T`, referring to it in a field, or using a marker such as `PhantomData`
 
diff --git a/tests/ui/lang-items/lang-item-generic-requirements.stderr b/tests/ui/lang-items/lang-item-generic-requirements.stderr
index 8072e6797e4..30abdf84046 100644
--- a/tests/ui/lang-items/lang-item-generic-requirements.stderr
+++ b/tests/ui/lang-items/lang-item-generic-requirements.stderr
@@ -50,23 +50,23 @@ LL |
 LL | fn start(_: *const u8, _: isize, _: *const *const u8) -> isize {
    |         - this function has 0 generic arguments
 
-error[E0392]: parameter `T` is never used
+error[E0392]: type parameter `T` is never used
   --> $DIR/lang-item-generic-requirements.rs:24:22
    |
 LL | struct MyPhantomData<T, U>;
-   |                      ^ unused parameter
+   |                      ^ unused type parameter
    |
    = help: consider removing `T` or referring to it in a field
-   = help: if you intended `T` to be a const parameter, use `const T: usize` instead
+   = help: if you intended `T` to be a const parameter, use `const T: /* Type */` instead
 
-error[E0392]: parameter `U` is never used
+error[E0392]: type parameter `U` is never used
   --> $DIR/lang-item-generic-requirements.rs:24:25
    |
 LL | struct MyPhantomData<T, U>;
-   |                         ^ unused parameter
+   |                         ^ unused type parameter
    |
    = help: consider removing `U` or referring to it in a field
-   = help: if you intended `U` to be a const parameter, use `const U: usize` instead
+   = help: if you intended `U` to be a const parameter, use `const U: /* Type */` instead
 
 error[E0369]: cannot add `{integer}` to `{integer}`
   --> $DIR/lang-item-generic-requirements.rs:44:7
diff --git a/tests/ui/lazy-type-alias/unused-generic-parameters.rs b/tests/ui/lazy-type-alias/unused-generic-parameters.rs
new file mode 100644
index 00000000000..9d02de7a721
--- /dev/null
+++ b/tests/ui/lazy-type-alias/unused-generic-parameters.rs
@@ -0,0 +1,22 @@
+// Check that we reject bivariant generic parameters as unused.
+// Furthermore, check that we only emit a single diagnostic for unused type parameters:
+// Previously, we would emit *two* errors, namely E0392 and E0091.
+
+#![feature(lazy_type_alias)]
+#![allow(incomplete_features)]
+
+type A<'a> = ();
+//~^ ERROR lifetime parameter `'a` is never used
+//~| HELP consider removing `'a`
+
+type B<T> = ();
+//~^ ERROR type parameter `T` is never used
+//~| HELP consider removing `T`
+//~| HELP if you intended `T` to be a const parameter
+
+// Check that we don't emit the const param help message here:
+type C<T: Copy> = ();
+//~^ ERROR type parameter `T` is never used
+//~| HELP consider removing `T`
+
+fn main() {}
diff --git a/tests/ui/lazy-type-alias/unused-generic-parameters.stderr b/tests/ui/lazy-type-alias/unused-generic-parameters.stderr
new file mode 100644
index 00000000000..484e21b0a03
--- /dev/null
+++ b/tests/ui/lazy-type-alias/unused-generic-parameters.stderr
@@ -0,0 +1,28 @@
+error[E0392]: lifetime parameter `'a` is never used
+  --> $DIR/unused-generic-parameters.rs:8:8
+   |
+LL | type A<'a> = ();
+   |        ^^ unused lifetime parameter
+   |
+   = help: consider removing `'a` or referring to it in the body of the type alias
+
+error[E0392]: type parameter `T` is never used
+  --> $DIR/unused-generic-parameters.rs:12:8
+   |
+LL | type B<T> = ();
+   |        ^ 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
+
+error[E0392]: type parameter `T` is never used
+  --> $DIR/unused-generic-parameters.rs:18:8
+   |
+LL | type C<T: Copy> = ();
+   |        ^ unused type parameter
+   |
+   = help: consider removing `T` or referring to it in the body of the type alias
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0392`.
diff --git a/tests/ui/lifetimes/issue-64173-unused-lifetimes.stderr b/tests/ui/lifetimes/issue-64173-unused-lifetimes.stderr
index ec4aea62391..534ba933ba5 100644
--- a/tests/ui/lifetimes/issue-64173-unused-lifetimes.stderr
+++ b/tests/ui/lifetimes/issue-64173-unused-lifetimes.stderr
@@ -13,19 +13,19 @@ error: generic `Self` types are currently not permitted in anonymous constants
 LL |     array: [(); size_of::<&Self>()],
    |                            ^^^^
 
-error[E0392]: parameter `'s` is never used
+error[E0392]: lifetime parameter `'s` is never used
   --> $DIR/issue-64173-unused-lifetimes.rs:3:12
    |
 LL | struct Foo<'s> {
-   |            ^^ unused parameter
+   |            ^^ unused lifetime parameter
    |
    = help: consider removing `'s`, referring to it in a field, or using a marker such as `PhantomData`
 
-error[E0392]: parameter `'a` is never used
+error[E0392]: lifetime parameter `'a` is never used
   --> $DIR/issue-64173-unused-lifetimes.rs:15:12
    |
 LL | struct Bar<'a> {
-   |            ^^ unused parameter
+   |            ^^ unused lifetime parameter
    |
    = help: consider removing `'a`, referring to it in a field, or using a marker such as `PhantomData`
 
diff --git a/tests/ui/parser/item-free-type-bounds-semantic-fail.rs b/tests/ui/parser/item-free-type-bounds-semantic-fail.rs
index 9db4111fbab..062b51e5978 100644
--- a/tests/ui/parser/item-free-type-bounds-semantic-fail.rs
+++ b/tests/ui/parser/item-free-type-bounds-semantic-fail.rs
@@ -13,8 +13,8 @@ fn semantics() {
     //~| ERROR free type alias without body
     type E<_T>: Ord = u8;
     //~^ ERROR bounds on `type`s in this context have no effect
-    //~| ERROR type parameter `_T` is unused
+    //~| ERROR type parameter `_T` is never used
     type F<_T>: Ord where 'static: 'static = u8;
     //~^ ERROR bounds on `type`s in this context have no effect
-    //~| ERROR type parameter `_T` is unused
+    //~| ERROR type parameter `_T` is never used
 }
diff --git a/tests/ui/parser/item-free-type-bounds-semantic-fail.stderr b/tests/ui/parser/item-free-type-bounds-semantic-fail.stderr
index 1b086512891..1077c103928 100644
--- a/tests/ui/parser/item-free-type-bounds-semantic-fail.stderr
+++ b/tests/ui/parser/item-free-type-bounds-semantic-fail.stderr
@@ -50,17 +50,23 @@ error: bounds on `type`s in this context have no effect
 LL |     type F<_T>: Ord where 'static: 'static = u8;
    |                 ^^^
 
-error[E0091]: type parameter `_T` is unused
+error[E0091]: type parameter `_T` is never used
   --> $DIR/item-free-type-bounds-semantic-fail.rs:14:12
    |
 LL |     type E<_T>: Ord = u8;
    |            ^^ 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
 
-error[E0091]: type parameter `_T` is unused
+error[E0091]: type parameter `_T` is never used
   --> $DIR/item-free-type-bounds-semantic-fail.rs:17:12
    |
 LL |     type F<_T>: Ord where 'static: 'static = u8;
    |            ^^ 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
 
 error: aborting due to 10 previous errors
 
diff --git a/tests/ui/regions/region-bounds-on-objects-and-type-parameters.stderr b/tests/ui/regions/region-bounds-on-objects-and-type-parameters.stderr
index 2ec2ca49b11..b15d2affeea 100644
--- a/tests/ui/regions/region-bounds-on-objects-and-type-parameters.stderr
+++ b/tests/ui/regions/region-bounds-on-objects-and-type-parameters.stderr
@@ -21,11 +21,11 @@ note: but lifetime parameter must outlive the lifetime `'a` as defined here
 LL | struct Foo<'a,'b,'c> {
    |            ^^
 
-error[E0392]: parameter `'c` is never used
+error[E0392]: lifetime parameter `'c` is never used
   --> $DIR/region-bounds-on-objects-and-type-parameters.rs:11:18
    |
 LL | struct Foo<'a,'b,'c> {
-   |                  ^^ unused parameter
+   |                  ^^ unused lifetime parameter
    |
    = help: consider removing `'c`, referring to it in a field, or using a marker such as `PhantomData`
 
diff --git a/tests/ui/rfcs/rfc-2632-const-trait-impl/tilde-const-invalid-places.stderr b/tests/ui/rfcs/rfc-2632-const-trait-impl/tilde-const-invalid-places.stderr
index d57f5702a63..92a9c347a07 100644
--- a/tests/ui/rfcs/rfc-2632-const-trait-impl/tilde-const-invalid-places.stderr
+++ b/tests/ui/rfcs/rfc-2632-const-trait-impl/tilde-const-invalid-places.stderr
@@ -272,11 +272,11 @@ LL |     type Type<T: ~const Trait> = ();
    = help: add `#![feature(inherent_associated_types)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
-error[E0392]: parameter `T` is never used
+error[E0392]: type parameter `T` is never used
   --> $DIR/tilde-const-invalid-places.rs:11:19
    |
 LL | struct UnitStruct<T: ~const Trait>;
-   |                   ^ unused parameter
+   |                   ^ unused type parameter
    |
    = help: consider removing `T`, referring to it in a field, or using a marker such as `PhantomData`
 
diff --git a/tests/ui/self/self_type_keyword.stderr b/tests/ui/self/self_type_keyword.stderr
index fed853a7e1f..4909a9cdc7f 100644
--- a/tests/ui/self/self_type_keyword.stderr
+++ b/tests/ui/self/self_type_keyword.stderr
@@ -72,11 +72,11 @@ note: unit struct `foo::Self` exists but is inaccessible
 LL |   struct Self;
    |   ^^^^^^^^^^^^ not accessible
 
-error[E0392]: parameter `'Self` is never used
+error[E0392]: lifetime parameter `'Self` is never used
   --> $DIR/self_type_keyword.rs:6:12
    |
 LL | struct Bar<'Self>;
-   |            ^^^^^ unused parameter
+   |            ^^^^^ unused lifetime parameter
    |
    = help: consider removing `'Self`, referring to it in a field, or using a marker such as `PhantomData`
 
diff --git a/tests/ui/tag-type-args.stderr b/tests/ui/tag-type-args.stderr
index 80ffd3a2f05..a1136f61cc5 100644
--- a/tests/ui/tag-type-args.stderr
+++ b/tests/ui/tag-type-args.stderr
@@ -14,14 +14,14 @@ help: add missing generic argument
 LL | fn foo(c: Quux<T>) { assert!((false)); }
    |               +++
 
-error[E0392]: parameter `T` is never used
+error[E0392]: type parameter `T` is never used
   --> $DIR/tag-type-args.rs:1:11
    |
 LL | enum Quux<T> { Bar }
-   |           ^ unused parameter
+   |           ^ unused type parameter
    |
    = 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: usize` instead
+   = help: if you intended `T` to be a const parameter, use `const T: /* Type */` instead
 
 error: aborting due to 2 previous errors
 
diff --git a/tests/ui/variance/variance-regions-unused-direct.stderr b/tests/ui/variance/variance-regions-unused-direct.stderr
index 1a600f5b058..4dc2af6ce2c 100644
--- a/tests/ui/variance/variance-regions-unused-direct.stderr
+++ b/tests/ui/variance/variance-regions-unused-direct.stderr
@@ -1,16 +1,16 @@
-error[E0392]: parameter `'a` is never used
+error[E0392]: lifetime parameter `'a` is never used
   --> $DIR/variance-regions-unused-direct.rs:5:18
    |
 LL | struct Bivariant<'a>;
-   |                  ^^ unused parameter
+   |                  ^^ unused lifetime parameter
    |
    = help: consider removing `'a`, referring to it in a field, or using a marker such as `PhantomData`
 
-error[E0392]: parameter `'d` is never used
+error[E0392]: lifetime parameter `'d` is never used
   --> $DIR/variance-regions-unused-direct.rs:7:19
    |
 LL | struct Struct<'a, 'd> {
-   |                   ^^ unused parameter
+   |                   ^^ unused lifetime parameter
    |
    = help: consider removing `'d`, referring to it in a field, or using a marker such as `PhantomData`
 
diff --git a/tests/ui/variance/variance-regions-unused-indirect.stderr b/tests/ui/variance/variance-regions-unused-indirect.stderr
index 14fdd849294..ec4d480baab 100644
--- a/tests/ui/variance/variance-regions-unused-indirect.stderr
+++ b/tests/ui/variance/variance-regions-unused-indirect.stderr
@@ -21,19 +21,19 @@ LL | enum Bar<'a> {
 LL ~     Bar1(Box<Foo<'a>>)
    |
 
-error[E0392]: parameter `'a` is never used
+error[E0392]: lifetime parameter `'a` is never used
   --> $DIR/variance-regions-unused-indirect.rs:3:10
    |
 LL | enum Foo<'a> {
-   |          ^^ unused parameter
+   |          ^^ unused lifetime parameter
    |
    = help: consider removing `'a`, referring to it in a field, or using a marker such as `PhantomData`
 
-error[E0392]: parameter `'a` is never used
+error[E0392]: lifetime parameter `'a` is never used
   --> $DIR/variance-regions-unused-indirect.rs:8:10
    |
 LL | enum Bar<'a> {
-   |          ^^ unused parameter
+   |          ^^ unused lifetime parameter
    |
    = help: consider removing `'a`, referring to it in a field, or using a marker such as `PhantomData`
 
diff --git a/tests/ui/variance/variance-unused-region-param.stderr b/tests/ui/variance/variance-unused-region-param.stderr
index 7c7ec40ba35..b9c08bd43c4 100644
--- a/tests/ui/variance/variance-unused-region-param.stderr
+++ b/tests/ui/variance/variance-unused-region-param.stderr
@@ -1,16 +1,16 @@
-error[E0392]: parameter `'a` is never used
+error[E0392]: lifetime parameter `'a` is never used
   --> $DIR/variance-unused-region-param.rs:3:19
    |
 LL | struct SomeStruct<'a> { x: u32 }
-   |                   ^^ unused parameter
+   |                   ^^ unused lifetime parameter
    |
    = help: consider removing `'a`, referring to it in a field, or using a marker such as `PhantomData`
 
-error[E0392]: parameter `'a` is never used
+error[E0392]: lifetime parameter `'a` is never used
   --> $DIR/variance-unused-region-param.rs:4:15
    |
 LL | enum SomeEnum<'a> { Nothing }
-   |               ^^ unused parameter
+   |               ^^ unused lifetime parameter
    |
    = help: consider removing `'a`, referring to it in a field, or using a marker such as `PhantomData`
 
diff --git a/tests/ui/variance/variance-unused-type-param.stderr b/tests/ui/variance/variance-unused-type-param.stderr
index e612da118f0..3011b7bd18f 100644
--- a/tests/ui/variance/variance-unused-type-param.stderr
+++ b/tests/ui/variance/variance-unused-type-param.stderr
@@ -1,51 +1,51 @@
-error[E0392]: parameter `A` is never used
+error[E0392]: type parameter `A` is never used
   --> $DIR/variance-unused-type-param.rs:6:19
    |
 LL | struct SomeStruct<A> { x: u32 }
-   |                   ^ unused parameter
+   |                   ^ unused type parameter
    |
    = 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: usize` instead
+   = help: if you intended `A` to be a const parameter, use `const A: /* Type */` instead
 
-error[E0392]: parameter `A` is never used
+error[E0392]: type parameter `A` is never used
   --> $DIR/variance-unused-type-param.rs:9:15
    |
 LL | enum SomeEnum<A> { Nothing }
-   |               ^ unused parameter
+   |               ^ unused type parameter
    |
    = 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: usize` instead
+   = help: if you intended `A` to be a const parameter, use `const A: /* Type */` instead
 
-error[E0392]: parameter `T` is never used
+error[E0392]: type parameter `T` is never used
   --> $DIR/variance-unused-type-param.rs:13:15
    |
 LL | enum ListCell<T> {
-   |               ^ unused parameter
+   |               ^ unused type parameter
    |
    = 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: usize` instead
+   = help: if you intended `T` to be a const parameter, use `const T: /* Type */` instead
 
-error[E0392]: parameter `T` is never used
+error[E0392]: type parameter `T` is never used
   --> $DIR/variance-unused-type-param.rs:19:19
    |
 LL | struct WithBounds<T: Sized> {}
-   |                   ^ unused parameter
+   |                   ^ unused type parameter
    |
    = help: consider removing `T`, referring to it in a field, or using a marker such as `PhantomData`
 
-error[E0392]: parameter `T` is never used
+error[E0392]: type parameter `T` is never used
   --> $DIR/variance-unused-type-param.rs:22:24
    |
 LL | struct WithWhereBounds<T> where T: Sized {}
-   |                        ^ unused parameter
+   |                        ^ unused type parameter
    |
    = help: consider removing `T`, referring to it in a field, or using a marker such as `PhantomData`
 
-error[E0392]: parameter `T` is never used
+error[E0392]: type parameter `T` is never used
   --> $DIR/variance-unused-type-param.rs:25:27
    |
 LL | struct WithOutlivesBounds<T: 'static> {}
-   |                           ^ unused parameter
+   |                           ^ unused type parameter
    |
    = help: consider removing `T`, referring to it in a field, or using a marker such as `PhantomData`