Add help message to suggest const for unused type param

compiler/rustc_typeck/src/check/wfcheck.rs

@@ -1298,12 +1298,14 @@ fn check_variances_for_type_defn<'tcx>(
 
         match param.name {
             hir::ParamName::Error => {}
-            _ => report_bivariance(tcx, param.span, param.name.ident().name),
+            _ => report_bivariance(tcx, param),
         }
     }
 }
 
-fn report_bivariance(tcx: TyCtxt<'_>, span: Span, param_name: Symbol) {
+fn report_bivariance(tcx: TyCtxt<'_>, param: &rustc_hir::GenericParam<'_>) {
+    let span = param.span;
+    let param_name = param.name.ident().name;
     let mut err = error_392(tcx, span, param_name);
 
     let suggested_marker_id = tcx.lang_items().phantom_data();
@@ -1318,7 +1320,14 @@ fn report_bivariance(tcx: TyCtxt<'_>, span: Span, param_name: Symbol) {
         format!("consider removing `{}` or referring to it in a field", param_name)
     };
     err.help(&msg);
-    err.emit();
+
+    if matches!(param.kind, rustc_hir::GenericParamKind::Type { .. }) {
+        err.help(&format!(
+            "if you intended `{0}` to be a const parameter, use `const {0}: usize` instead",
+            param_name
+        ));
+    }
+    err.emit()
 }
 
 /// Feature gates RFC 2056 -- trivial bounds, checking for global bounds that

src/test/ui/const-generics/const-param-type-depends-on-type-param.full.stderr

@@ -11,6 +11,7 @@ LL | pub struct Dependent<T, const X: T>([(); X]);
    |                      ^ unused 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
 
 error: aborting due to 2 previous errors
 

src/test/ui/const-generics/const-param-type-depends-on-type-param.min.stderr

@@ -11,6 +11,7 @@ LL | pub struct Dependent<T, const X: T>([(); X]);
    |                      ^ unused 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
 
 error: aborting due to 2 previous errors
 

src/test/ui/const-generics/issue-67375.full.stderr

@@ -15,6 +15,7 @@ LL | struct Bug<T> {
    |            ^ unused 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
 
 error: aborting due to previous error; 1 warning emitted
 

src/test/ui/const-generics/issue-67375.min.stderr

@@ -14,6 +14,7 @@ LL | struct Bug<T> {
    |            ^ unused 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
 
 error: aborting due to 2 previous errors
 

src/test/ui/const-generics/issue-67945-1.full.stderr

@@ -19,6 +19,7 @@ LL | struct Bug<S> {
    |            ^ unused 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
 
 error: aborting due to 2 previous errors
 

src/test/ui/const-generics/issue-67945-1.min.stderr

@@ -23,6 +23,7 @@ LL | struct Bug<S> {
    |            ^ unused 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
 
 error: aborting due to 3 previous errors
 

src/test/ui/const-generics/issue-67945-2.full.stderr

@@ -19,6 +19,7 @@ LL | struct Bug<S> {
    |            ^ unused 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
 
 error: aborting due to 2 previous errors
 

src/test/ui/const-generics/issue-67945-2.min.stderr

@@ -23,6 +23,7 @@ LL | struct Bug<S> {
    |            ^ unused 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
 
 error: aborting due to 3 previous errors
 

src/test/ui/const-generics/unused-type-param-suggestion.rs

@@ -0,0 +1,4 @@
+#![crate_type="lib"]
+
+struct Example<N>;
+//~^ ERROR parameter

src/test/ui/const-generics/unused-type-param-suggestion.stderr

@@ -0,0 +1,12 @@
+error[E0392]: parameter `N` is never used
+  --> $DIR/unused-type-param-suggestion.rs:3:16
+   |
+LL | struct Example<N>;
+   |                ^ unused 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
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0392`.

src/test/ui/enum-discriminant/issue-70453-generics-in-discr-ice.stderr

@@ -14,6 +14,7 @@ LL | enum MyWeirdOption<T> {
    |                    ^ unused 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
 
 error: aborting due to 2 previous errors
 

src/test/ui/enum/issue-67945-1.stderr

@@ -14,6 +14,7 @@ LL | enum Bug<S> {
    |          ^ unused 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
 
 error: aborting due to 2 previous errors
 

src/test/ui/enum/issue-67945-2.stderr

@@ -14,6 +14,7 @@ LL | enum Bug<S> {
    |          ^ unused 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
 
 error: aborting due to 2 previous errors
 

src/test/ui/error-codes/E0392.stderr

@@ -5,6 +5,7 @@ LL | enum Foo<T> { Bar }
    |          ^ unused 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
 
 error: aborting due to previous error
 

src/test/ui/inner-static-type-parameter.stderr

@@ -13,6 +13,7 @@ LL | enum Bar<T> { What }
    |          ^ unused 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
 
 error: aborting due to 2 previous errors
 

src/test/ui/issues/issue-17904-2.stderr

@@ -5,6 +5,7 @@ LL | struct Foo<T> where T: Copy;
    |            ^ unused 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
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-20413.stderr

@@ -5,6 +5,7 @@ LL | struct NoData<T>;
    |               ^ unused 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
 
 error[E0275]: overflow evaluating the requirement `NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<NoData<T>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>: Foo`
   --> $DIR/issue-20413.rs:8:36

src/test/ui/issues/issue-36299.stderr

@@ -13,6 +13,7 @@ LL | struct Foo<'a, A> {}
    |                ^ unused 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
 
 error: aborting due to 2 previous errors
 

src/test/ui/issues/issue-36638.stderr

@@ -17,6 +17,7 @@ LL | struct Foo<Self>(Self);
    |            ^^^^ unused parameter
    |
    = help: consider removing `Self`, referring to it in a field, or using a marker such as `PhantomData`
+   = help: if you intended `Self` to be a const parameter, use `const Self: usize` instead
 
 error: aborting due to 3 previous errors
 

src/test/ui/issues/issue-37534.stderr

@@ -22,6 +22,7 @@ LL | struct Foo<T: ?Hash> { }
    |            ^ unused 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
 
 error: aborting due to 2 previous errors; 1 warning emitted
 

src/test/ui/variance/variance-unused-type-param.stderr

@@ -5,6 +5,7 @@ LL | struct SomeStruct<A> { x: u32 }
    |                   ^ unused 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
 
 error[E0392]: parameter `A` is never used
   --> $DIR/variance-unused-type-param.rs:9:15
@@ -13,6 +14,7 @@ LL | enum SomeEnum<A> { Nothing }
    |               ^ unused 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
 
 error[E0392]: parameter `T` is never used
   --> $DIR/variance-unused-type-param.rs:13:15
@@ -21,6 +23,7 @@ LL | enum ListCell<T> {
    |               ^ unused 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
 
 error: aborting due to 3 previous errors