Suggest generic_const_args + type const in const operation diagnostics

Point users at `#![feature(generic_const_args)]` and `type const` items
when they hit the "generic parameters may not be used in const operations"
error, not just `generic_const_exprs`.

Author: Dnreikronos

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

@@ -432,13 +432,14 @@ impl<'tcx> ForbidParamUsesFolder<'tcx> {
                 diag.span_note(impl_.self_ty.span, "not a concrete type");
             }
         }
-        if matches!(self.context, ForbidParamContext::ConstArgument)
-            && self.tcx.features().min_generic_const_args()
-        {
-            if !self.tcx.features().generic_const_args() {
-                diag.help("add `#![feature(generic_const_args)]` to allow generic expressions as the RHS of const items");
-            } else {
+        if matches!(self.context, ForbidParamContext::ConstArgument) {
+            if self.tcx.features().generic_const_args() {
                 diag.help("consider factoring the expression into a `type const` item and use it as the const argument instead");
+            } else if self.tcx.features().min_generic_const_args() {
+                diag.help("add `#![feature(generic_const_args)]` and extract the expression into a `type const` item");
+            } else if self.tcx.sess.is_nightly_build() {
+                diag.help("add `#![feature(generic_const_exprs)]` to allow generic const expressions");
+                diag.help("alternatively, you can use `#![feature(generic_const_args)]` and extract the expression into a `type const` item");
             }
         }
         diag.emit()

compiler/rustc_resolve/src/diagnostics.rs

@@ -1071,9 +1071,11 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                     span,
                     name,
                     param_kind: is_type,
-                    help: self.tcx.sess.is_nightly_build(),
+                    help: self.tcx.sess.is_nightly_build()
+                        && !self.tcx.features().min_generic_const_args(),
                     is_gca,
                     help_gca: is_gca,
+                    help_suggest_gca: self.tcx.sess.is_nightly_build() && !is_gca,
                 })
             }
             ResolutionError::ParamInEnumDiscriminant { name, param_kind: is_type } => self

compiler/rustc_resolve/src/errors.rs

@@ -407,6 +407,10 @@ pub(crate) struct ParamInNonTrivialAnonConst {
         "consider factoring the expression into a `type const` item and use it as the const argument instead"
     )]
     pub(crate) help_gca: bool,
+    #[help(
+        "alternatively, you can use `#![feature(generic_const_args)]` and extract the expression into a `type const` item"
+    )]
+    pub(crate) help_suggest_gca: bool,
 }
 
 #[derive(Debug)]

compiler/rustc_resolve/src/late/diagnostics.rs

@@ -3943,9 +3943,12 @@ impl<'ast, 'ra, 'tcx> LateResolutionVisitor<'_, 'ast, 'ra, 'tcx> {
                         span: lifetime_ref.ident.span,
                         name: lifetime_ref.ident.name,
                         param_kind: errors::ParamKindInNonTrivialAnonConst::Lifetime,
-                        help: self.r.tcx.sess.is_nightly_build(),
+                        help: self.r.tcx.sess.is_nightly_build()
+                            && !self.r.tcx.features().min_generic_const_args(),
                         is_gca: self.r.tcx.features().generic_const_args(),
                         help_gca: self.r.tcx.features().generic_const_args(),
+                        help_suggest_gca: self.r.tcx.sess.is_nightly_build()
+                            && !self.r.tcx.features().generic_const_args(),
                     })
                     .emit()
             }

tests/ui/associated-consts/associated-const-type-parameter-arrays.stderr

@@ -6,6 +6,7 @@ LL |     let _array: [u32; <A as Foo>::Y];
    |
    = note: type parameters may not be used in const expressions
    = help: add `#![feature(generic_const_exprs)]` to allow generic const expressions
+   = help: alternatively, you can use `#![feature(generic_const_args)]` and extract the expression into a `type const` item
 
 error: aborting due to 1 previous error
 

tests/ui/associated-consts/issue-47814.stderr

@@ -9,6 +9,8 @@ note: not a concrete type
    |
 LL | impl<'a> ArpIPv4<'a> {
    |          ^^^^^^^^^^^
+   = help: add `#![feature(generic_const_exprs)]` to allow generic const expressions
+   = help: alternatively, you can use `#![feature(generic_const_args)]` and extract the expression into a `type const` item
 
 error: aborting due to 1 previous error
 

tests/ui/associated-item/associated-item-duplicate-bounds.stderr

@@ -6,6 +6,7 @@ LL |     links: [u32; A::LINKS], // Shouldn't suggest bounds already there.
    |
    = note: type parameters may not be used in const expressions
    = help: add `#![feature(generic_const_exprs)]` to allow generic const expressions
+   = help: alternatively, you can use `#![feature(generic_const_args)]` and extract the expression into a `type const` item
 
 error: aborting due to 1 previous error
 

tests/ui/const-generics/adt_const_params/index-oob-ice-83993.stderr

@@ -6,6 +6,7 @@ LL |         let x: &'b ();
    |
    = note: lifetime parameters may not be used in const expressions
    = help: add `#![feature(generic_const_exprs)]` to allow generic const expressions
+   = help: alternatively, you can use `#![feature(generic_const_args)]` and extract the expression into a `type const` item
 
 error: generic parameters may not be used in const operations
   --> $DIR/index-oob-ice-83993.rs:18:17
@@ -15,6 +16,7 @@ LL |         let _: &'b ();
    |
    = note: lifetime parameters may not be used in const expressions
    = help: add `#![feature(generic_const_exprs)]` to allow generic const expressions
+   = help: alternatively, you can use `#![feature(generic_const_args)]` and extract the expression into a `type const` item
 
 error: aborting due to 2 previous errors
 

tests/ui/const-generics/const-arg-in-const-arg.min.stderr

@@ -6,6 +6,7 @@ LL |     let _: [u8; foo::<T>()];
    |
    = note: type parameters may not be used in const expressions
    = help: add `#![feature(generic_const_exprs)]` to allow generic const expressions
+   = help: alternatively, you can use `#![feature(generic_const_args)]` and extract the expression into a `type const` item
 
 error: generic parameters may not be used in const operations
   --> $DIR/const-arg-in-const-arg.rs:16:23
@@ -15,6 +16,7 @@ LL |     let _: [u8; bar::<N>()];
    |
    = help: const parameters may only be used as standalone arguments here, i.e. `N`
    = help: add `#![feature(generic_const_exprs)]` to allow generic const expressions
+   = help: alternatively, you can use `#![feature(generic_const_args)]` and extract the expression into a `type const` item
 
 error: generic parameters may not be used in const operations
   --> $DIR/const-arg-in-const-arg.rs:18:23
@@ -24,6 +26,7 @@ LL |     let _: [u8; faz::<'a>(&())];
    |
    = note: lifetime parameters may not be used in const expressions
    = help: add `#![feature(generic_const_exprs)]` to allow generic const expressions
+   = help: alternatively, you can use `#![feature(generic_const_args)]` and extract the expression into a `type const` item
 
 error: generic parameters may not be used in const operations
   --> $DIR/const-arg-in-const-arg.rs:20:23
@@ -33,6 +36,7 @@ LL |     let _: [u8; baz::<'a>(&())];
    |
    = note: lifetime parameters may not be used in const expressions
    = help: add `#![feature(generic_const_exprs)]` to allow generic const expressions
+   = help: alternatively, you can use `#![feature(generic_const_args)]` and extract the expression into a `type const` item
 
 error: generic parameters may not be used in const operations
   --> $DIR/const-arg-in-const-arg.rs:21:23
@@ -42,6 +46,7 @@ LL |     let _: [u8; faz::<'b>(&())];
    |
    = note: lifetime parameters may not be used in const expressions
    = help: add `#![feature(generic_const_exprs)]` to allow generic const expressions
+   = help: alternatively, you can use `#![feature(generic_const_args)]` and extract the expression into a `type const` item
 
 error: generic parameters may not be used in const operations
   --> $DIR/const-arg-in-const-arg.rs:23:23
@@ -51,6 +56,7 @@ LL |     let _: [u8; baz::<'b>(&())];
    |
    = note: lifetime parameters may not be used in const expressions
    = help: add `#![feature(generic_const_exprs)]` to allow generic const expressions
+   = help: alternatively, you can use `#![feature(generic_const_args)]` and extract the expression into a `type const` item
 
 error: generic parameters may not be used in const operations
   --> $DIR/const-arg-in-const-arg.rs:27:23
@@ -60,6 +66,7 @@ LL |     let _ = [0; bar::<N>()];
    |
    = help: const parameters may only be used as standalone arguments here, i.e. `N`
    = help: add `#![feature(generic_const_exprs)]` to allow generic const expressions
+   = help: alternatively, you can use `#![feature(generic_const_args)]` and extract the expression into a `type const` item
 
 error: generic parameters may not be used in const operations
   --> $DIR/const-arg-in-const-arg.rs:29:23
@@ -69,6 +76,7 @@ LL |     let _ = [0; faz::<'a>(&())];
    |
    = note: lifetime parameters may not be used in const expressions
    = help: add `#![feature(generic_const_exprs)]` to allow generic const expressions
+   = help: alternatively, you can use `#![feature(generic_const_args)]` and extract the expression into a `type const` item
 
 error: generic parameters may not be used in const operations
   --> $DIR/const-arg-in-const-arg.rs:31:23
@@ -78,6 +86,7 @@ LL |     let _ = [0; baz::<'a>(&())];
    |
    = note: lifetime parameters may not be used in const expressions
    = help: add `#![feature(generic_const_exprs)]` to allow generic const expressions
+   = help: alternatively, you can use `#![feature(generic_const_args)]` and extract the expression into a `type const` item
 
 error: generic parameters may not be used in const operations
   --> $DIR/const-arg-in-const-arg.rs:32:23
@@ -87,6 +96,7 @@ LL |     let _ = [0; faz::<'b>(&())];
    |
    = note: lifetime parameters may not be used in const expressions
    = help: add `#![feature(generic_const_exprs)]` to allow generic const expressions
+   = help: alternatively, you can use `#![feature(generic_const_args)]` and extract the expression into a `type const` item
 
 error: generic parameters may not be used in const operations
   --> $DIR/const-arg-in-const-arg.rs:34:23
@@ -96,6 +106,7 @@ LL |     let _ = [0; baz::<'b>(&())];
    |
    = note: lifetime parameters may not be used in const expressions
    = help: add `#![feature(generic_const_exprs)]` to allow generic const expressions
+   = help: alternatively, you can use `#![feature(generic_const_args)]` and extract the expression into a `type const` item
 
 error: generic parameters may not be used in const operations
   --> $DIR/const-arg-in-const-arg.rs:35:24
@@ -105,6 +116,7 @@ LL |     let _: Foo<{ foo::<T>() }>;
    |
    = note: type parameters may not be used in const expressions
    = help: add `#![feature(generic_const_exprs)]` to allow generic const expressions
+   = help: alternatively, you can use `#![feature(generic_const_args)]` and extract the expression into a `type const` item
 
 error: generic parameters may not be used in const operations
   --> $DIR/const-arg-in-const-arg.rs:36:24
@@ -114,6 +126,7 @@ LL |     let _: Foo<{ bar::<N>() }>;
    |
    = help: const parameters may only be used as standalone arguments here, i.e. `N`
    = help: add `#![feature(generic_const_exprs)]` to allow generic const expressions
+   = help: alternatively, you can use `#![feature(generic_const_args)]` and extract the expression into a `type const` item
 
 error: generic parameters may not be used in const operations
   --> $DIR/const-arg-in-const-arg.rs:38:24
@@ -123,6 +136,7 @@ LL |     let _: Foo<{ faz::<'a>(&()) }>;
    |
    = note: lifetime parameters may not be used in const expressions
    = help: add `#![feature(generic_const_exprs)]` to allow generic const expressions
+   = help: alternatively, you can use `#![feature(generic_const_args)]` and extract the expression into a `type const` item
 
 error: generic parameters may not be used in const operations
   --> $DIR/const-arg-in-const-arg.rs:40:24
@@ -132,6 +146,7 @@ LL |     let _: Foo<{ baz::<'a>(&()) }>;
    |
    = note: lifetime parameters may not be used in const expressions
    = help: add `#![feature(generic_const_exprs)]` to allow generic const expressions
+   = help: alternatively, you can use `#![feature(generic_const_args)]` and extract the expression into a `type const` item
 
 error: generic parameters may not be used in const operations
   --> $DIR/const-arg-in-const-arg.rs:41:24
@@ -141,6 +156,7 @@ LL |     let _: Foo<{ faz::<'b>(&()) }>;
    |
    = note: lifetime parameters may not be used in const expressions
    = help: add `#![feature(generic_const_exprs)]` to allow generic const expressions
+   = help: alternatively, you can use `#![feature(generic_const_args)]` and extract the expression into a `type const` item
 
 error: generic parameters may not be used in const operations
   --> $DIR/const-arg-in-const-arg.rs:43:24
@@ -150,6 +166,7 @@ LL |     let _: Foo<{ baz::<'b>(&()) }>;
    |
    = note: lifetime parameters may not be used in const expressions
    = help: add `#![feature(generic_const_exprs)]` to allow generic const expressions
+   = help: alternatively, you can use `#![feature(generic_const_args)]` and extract the expression into a `type const` item
 
 error: generic parameters may not be used in const operations
   --> $DIR/const-arg-in-const-arg.rs:44:27
@@ -159,6 +176,7 @@ LL |     let _ = Foo::<{ foo::<T>() }>;
    |
    = note: type parameters may not be used in const expressions
    = help: add `#![feature(generic_const_exprs)]` to allow generic const expressions
+   = help: alternatively, you can use `#![feature(generic_const_args)]` and extract the expression into a `type const` item
 
 error: generic parameters may not be used in const operations
   --> $DIR/const-arg-in-const-arg.rs:45:27
@@ -168,6 +186,7 @@ LL |     let _ = Foo::<{ bar::<N>() }>;
    |
    = help: const parameters may only be used as standalone arguments here, i.e. `N`
    = help: add `#![feature(generic_const_exprs)]` to allow generic const expressions
+   = help: alternatively, you can use `#![feature(generic_const_args)]` and extract the expression into a `type const` item
 
 error: generic parameters may not be used in const operations
   --> $DIR/const-arg-in-const-arg.rs:47:27
@@ -177,6 +196,7 @@ LL |     let _ = Foo::<{ faz::<'a>(&()) }>;
    |
    = note: lifetime parameters may not be used in const expressions
    = help: add `#![feature(generic_const_exprs)]` to allow generic const expressions
+   = help: alternatively, you can use `#![feature(generic_const_args)]` and extract the expression into a `type const` item
 
 error: generic parameters may not be used in const operations
   --> $DIR/const-arg-in-const-arg.rs:49:27
@@ -186,6 +206,7 @@ LL |     let _ = Foo::<{ baz::<'a>(&()) }>;
    |
    = note: lifetime parameters may not be used in const expressions
    = help: add `#![feature(generic_const_exprs)]` to allow generic const expressions
+   = help: alternatively, you can use `#![feature(generic_const_args)]` and extract the expression into a `type const` item
 
 error: generic parameters may not be used in const operations
   --> $DIR/const-arg-in-const-arg.rs:50:27
@@ -195,6 +216,7 @@ LL |     let _ = Foo::<{ faz::<'b>(&()) }>;
    |
    = note: lifetime parameters may not be used in const expressions
    = help: add `#![feature(generic_const_exprs)]` to allow generic const expressions
+   = help: alternatively, you can use `#![feature(generic_const_args)]` and extract the expression into a `type const` item
 
 error: generic parameters may not be used in const operations
   --> $DIR/const-arg-in-const-arg.rs:52:27
@@ -204,6 +226,7 @@ LL |     let _ = Foo::<{ baz::<'b>(&()) }>;
    |
    = note: lifetime parameters may not be used in const expressions
    = help: add `#![feature(generic_const_exprs)]` to allow generic const expressions
+   = help: alternatively, you can use `#![feature(generic_const_args)]` and extract the expression into a `type const` item
 
 error[E0747]: unresolved item provided when a constant was expected
   --> $DIR/const-arg-in-const-arg.rs:16:23

tests/ui/const-generics/const-argument-if-length.min.stderr

@@ -6,6 +6,7 @@ LL |     pad: [u8; is_zst::<T>()],
    |
    = note: type parameters may not be used in const expressions
    = help: add `#![feature(generic_const_exprs)]` to allow generic const expressions
+   = help: alternatively, you can use `#![feature(generic_const_args)]` and extract the expression into a `type const` item
 
 error[E0277]: the size for values of type `T` cannot be known at compilation time
   --> $DIR/const-argument-if-length.rs:16:12