Add help message for unused type param

22 files changed, 49 insertions, 3 deletions

diff --git a/compiler/rustc_typeck/src/check/wfcheck.rs b/compiler/rustc_typeck/src/check/wfcheck.rs
index d25dd9a6e83..4914f196afb 100644
--- a/compiler/rustc_typeck/src/check/wfcheck.rs
+++ b/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
diff --git a/src/test/ui/const-generics/const-param-type-depends-on-type-param.full.stderr b/src/test/ui/const-generics/const-param-type-depends-on-type-param.full.stderr
index a83ee627187..32f7dea8263 100644
--- a/src/test/ui/const-generics/const-param-type-depends-on-type-param.full.stderr
+++ b/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
 
diff --git a/src/test/ui/const-generics/const-param-type-depends-on-type-param.min.stderr b/src/test/ui/const-generics/const-param-type-depends-on-type-param.min.stderr
index a83ee627187..32f7dea8263 100644
--- a/src/test/ui/const-generics/const-param-type-depends-on-type-param.min.stderr
+++ b/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
 
diff --git a/src/test/ui/const-generics/issue-67375.full.stderr b/src/test/ui/const-generics/issue-67375.full.stderr
index 2f004f75de5..0fe65272f1b 100644
--- a/src/test/ui/const-generics/issue-67375.full.stderr
+++ b/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
 
diff --git a/src/test/ui/const-generics/issue-67375.min.stderr b/src/test/ui/const-generics/issue-67375.min.stderr
index 337e7bc1409..be81fa92129 100644
--- a/src/test/ui/const-generics/issue-67375.min.stderr
+++ b/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
 
diff --git a/src/test/ui/const-generics/issue-67945-1.full.stderr b/src/test/ui/const-generics/issue-67945-1.full.stderr
index 5cdcefe3501..63c50b5ca54 100644
--- a/src/test/ui/const-generics/issue-67945-1.full.stderr
+++ b/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
 
diff --git a/src/test/ui/const-generics/issue-67945-1.min.stderr b/src/test/ui/const-generics/issue-67945-1.min.stderr
index a3e086ea954..074d36c8ef3 100644
--- a/src/test/ui/const-generics/issue-67945-1.min.stderr
+++ b/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
 
diff --git a/src/test/ui/const-generics/issue-67945-2.full.stderr b/src/test/ui/const-generics/issue-67945-2.full.stderr
index 4d96058b395..b9004060231 100644
--- a/src/test/ui/const-generics/issue-67945-2.full.stderr
+++ b/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
 
diff --git a/src/test/ui/const-generics/issue-67945-2.min.stderr b/src/test/ui/const-generics/issue-67945-2.min.stderr
index 860be4a9b6a..c06df79f842 100644
--- a/src/test/ui/const-generics/issue-67945-2.min.stderr
+++ b/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
 
diff --git a/src/test/ui/const-generics/unused-type-param-suggestion.rs b/src/test/ui/const-generics/unused-type-param-suggestion.rs
new file mode 100644
index 00000000000..2251512c459
--- /dev/null
+++ b/src/test/ui/const-generics/unused-type-param-suggestion.rs
@@ -0,0 +1,4 @@
+#![crate_type="lib"]
+
+struct Example<N>;
+//~^ ERROR parameter
diff --git a/src/test/ui/const-generics/unused-type-param-suggestion.stderr b/src/test/ui/const-generics/unused-type-param-suggestion.stderr
new file mode 100644
index 00000000000..807065ca109
--- /dev/null
+++ b/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`.
diff --git a/src/test/ui/enum-discriminant/issue-70453-generics-in-discr-ice.stderr b/src/test/ui/enum-discriminant/issue-70453-generics-in-discr-ice.stderr
index 1d43903928b..f89be630eeb 100644
--- a/src/test/ui/enum-discriminant/issue-70453-generics-in-discr-ice.stderr
+++ b/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
 
diff --git a/src/test/ui/enum/issue-67945-1.stderr b/src/test/ui/enum/issue-67945-1.stderr
index 32ca94203e6..227899e7535 100644
--- a/src/test/ui/enum/issue-67945-1.stderr
+++ b/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
 
diff --git a/src/test/ui/enum/issue-67945-2.stderr b/src/test/ui/enum/issue-67945-2.stderr
index a738d3b15a5..5a90f00c346 100644
--- a/src/test/ui/enum/issue-67945-2.stderr
+++ b/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
 
diff --git a/src/test/ui/error-codes/E0392.stderr b/src/test/ui/error-codes/E0392.stderr
index 860bf68f018..622402999c3 100644
--- a/src/test/ui/error-codes/E0392.stderr
+++ b/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
 
diff --git a/src/test/ui/inner-static-type-parameter.stderr b/src/test/ui/inner-static-type-parameter.stderr
index 990e4649a58..e4e449e4159 100644
--- a/src/test/ui/inner-static-type-parameter.stderr
+++ b/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
 
diff --git a/src/test/ui/issues/issue-17904-2.stderr b/src/test/ui/issues/issue-17904-2.stderr
index 62b7b79538c..259e029113d 100644
--- a/src/test/ui/issues/issue-17904-2.stderr
+++ b/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
 
diff --git a/src/test/ui/issues/issue-20413.stderr b/src/test/ui/issues/issue-20413.stderr
index 7fb1e3f2bba..6bcff7aff2d 100644
--- a/src/test/ui/issues/issue-20413.stderr
+++ b/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
diff --git a/src/test/ui/issues/issue-36299.stderr b/src/test/ui/issues/issue-36299.stderr
index 8e29a925d80..dc24fb353f4 100644
--- a/src/test/ui/issues/issue-36299.stderr
+++ b/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
 
diff --git a/src/test/ui/issues/issue-36638.stderr b/src/test/ui/issues/issue-36638.stderr
index e5d6f8ec7ad..733fc4af534 100644
--- a/src/test/ui/issues/issue-36638.stderr
+++ b/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
 
diff --git a/src/test/ui/issues/issue-37534.stderr b/src/test/ui/issues/issue-37534.stderr
index 895479986f1..82bb51028c9 100644
--- a/src/test/ui/issues/issue-37534.stderr
+++ b/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
 
diff --git a/src/test/ui/variance/variance-unused-type-param.stderr b/src/test/ui/variance/variance-unused-type-param.stderr
index a4d636bc03c..270233c0c97 100644
--- a/src/test/ui/variance/variance-unused-type-param.stderr
+++ b/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