rust-lang · bors · Sep 29, 2023 · Sep 28, 2023
diff --git a/compiler/rustc_hir_analysis/src/check/wfcheck.rs b/compiler/rustc_hir_analysis/src/check/wfcheck.rs
@@ -24,6 +24,9 @@ use rustc_span::symbol::{sym, Ident, Symbol};
 use rustc_span::{Span, DUMMY_SP};
 use rustc_target::spec::abi::Abi;
 use rustc_trait_selection::traits::error_reporting::TypeErrCtxtExt;
+use rustc_trait_selection::traits::misc::{
+    type_allowed_to_implement_const_param_ty, ConstParamTyImplementationError,
+};
 use rustc_trait_selection::traits::outlives_bounds::InferCtxtExt as _;
 use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt as _;
 use rustc_trait_selection::traits::{
@@ -865,43 +868,65 @@ fn check_param_wf(tcx: TyCtxt<'_>, param: &hir::GenericParam<'_>) {
                     );
                 });
             } else {
-                let err_ty_str;
-                let mut is_ptr = true;
-
-                let err = match ty.kind() {
+                let diag = match ty.kind() {
                     ty::Bool | ty::Char | ty::Int(_) | ty::Uint(_) | ty::Error(_) => None,
-                    ty::FnPtr(_) => Some("function pointers"),
-                    ty::RawPtr(_) => Some("raw pointers"),
-                    _ => {
-                        is_ptr = false;
-                        err_ty_str = format!("`{ty}`");
-                        Some(err_ty_str.as_str())
-                    }
+                    ty::FnPtr(_) => Some(tcx.sess.struct_span_err(
+                        hir_ty.span,
+                        "using function pointers as const generic parameters is forbidden",
+                    )),
+                    ty::RawPtr(_) => Some(tcx.sess.struct_span_err(
+                        hir_ty.span,
+                        "using raw pointers as const generic parameters is forbidden",
+                    )),
+                    _ => Some(tcx.sess.struct_span_err(
+                        hir_ty.span,
+                        format!("`{}` is forbidden as the type of a const generic parameter", ty),
+                    )),
                 };
 
-                if let Some(unsupported_type) = err {
-                    if is_ptr {
-                        tcx.sess.span_err(
-                            hir_ty.span,
-                            format!(
-                                "using {unsupported_type} as const generic parameters is forbidden",
-                            ),
-                        );
-                    } else {
-                        let mut err = tcx.sess.struct_span_err(
-                            hir_ty.span,
-                            format!(
-                                "{unsupported_type} is forbidden as the type of a const generic parameter",
-                            ),
-                        );
-                        err.note("the only supported types are integers, `bool` and `char`");
-                        if tcx.sess.is_nightly_build() {
-                            err.help(
-                            "more complex types are supported with `#![feature(adt_const_params)]`",
-                        );
+                if let Some(mut diag) = diag {
+                    diag.note("the only supported types are integers, `bool` and `char`");
+
+                    let cause = ObligationCause::misc(hir_ty.span, param.def_id);
+                    let may_suggest_feature = match type_allowed_to_implement_const_param_ty(
+                        tcx,
+                        tcx.param_env(param.def_id),
+                        ty,
+                        cause,
+                    ) {
+                        // Can never implement `ConstParamTy`, don't suggest anything.
+                        Err(ConstParamTyImplementationError::NotAnAdtOrBuiltinAllowed) => false,
+                        // May be able to implement `ConstParamTy`. Only emit the feature help
+                        // if the type is local, since the user may be able to fix the local type.
+                        Err(ConstParamTyImplementationError::InfrigingFields(..)) => {
+                            fn ty_is_local(ty: Ty<'_>) -> bool {
+                                match ty.kind() {
+                                    ty::Adt(adt_def, ..) => adt_def.did().is_local(),
+                                    // Arrays and slices use the inner type's `ConstParamTy`.
+                                    ty::Array(ty, ..) => ty_is_local(*ty),
+                                    ty::Slice(ty) => ty_is_local(*ty),
+                                    // `&` references use the inner type's `ConstParamTy`.
+                                    // `&mut` are not supported.
+                                    ty::Ref(_, ty, ast::Mutability::Not) => ty_is_local(*ty),
+                                    // Say that a tuple is local if any of its components are local.
+                                    // This is not strictly correct, but it's likely that the user can fix the local component.
+                                    ty::Tuple(tys) => tys.iter().any(|ty| ty_is_local(ty)),
+                                    _ => false,
+                                }
+                            }
+
+                            ty_is_local(ty)
                         }
-                        err.emit();
+                        // Implments `ConstParamTy`, suggest adding the feature to enable.
+                        Ok(..) => true,
+                    };
+                    if may_suggest_feature && tcx.sess.is_nightly_build() {
+                        diag.help(
+                            "add `#![feature(adt_const_params)]` to the crate attributes to enable more complex and user defined types",
+                        );
                     }
+
+                    diag.emit();
                 }
             }
         }

diff --git a/tests/ui/const-generics/adt_const_params/suggest_feature_only_when_possible.rs b/tests/ui/const-generics/adt_const_params/suggest_feature_only_when_possible.rs
@@ -0,0 +1,44 @@
+// Test that when adt_const_params is not enabled, we suggest adding the feature only when
+// it would be possible for the type to be used as a const generic or when it's likely
+// possible for the user to fix their type to be used.
+
+// Can never be used as const generics.
+fn uwu_0<const N: &'static mut ()>() {}
+//~^ ERROR: forbidden as the type of a const generic
+
+// Needs the feature but can be used, so suggest adding the feature.
+fn owo_0<const N: &'static u32>() {}
+//~^ ERROR: forbidden as the type of a const generic
+//~^^ HELP: add `#![feature(adt_const_params)]`
+
+// Can only be used in const generics with changes.
+struct Meow {
+    meow: u8,
+}
+
+fn meow_0<const N: Meow>() {}
+//~^ ERROR: forbidden as the type of a const generic
+//~^^ HELP: add `#![feature(adt_const_params)]`
+fn meow_1<const N: &'static Meow>() {}
+//~^ ERROR: forbidden as the type of a const generic
+//~^^ HELP: add `#![feature(adt_const_params)]`
+fn meow_2<const N: [Meow; 100]>() {}
+//~^ ERROR: forbidden as the type of a const generic
+//~^^ HELP: add `#![feature(adt_const_params)]`
+fn meow_3<const N: (Meow, u8)>() {}
+//~^ ERROR: forbidden as the type of a const generic
+//~^^ HELP: add `#![feature(adt_const_params)]`
+
+// This is suboptimal that it thinks it can be used
+// but better to suggest the feature to the user.
+fn meow_4<const N: (Meow, String)>() {}
+//~^ ERROR: forbidden as the type of a const generic
+//~^^ HELP: add `#![feature(adt_const_params)]`
+
+// Non-local ADT that does not impl `ConstParamTy`
+fn nya_0<const N: String>() {}
+//~^ ERROR: forbidden as the type of a const generic
+fn nya_1<const N: Vec<u32>>() {}
+//~^ ERROR: forbidden as the type of a const generic
+
+fn main() {}
diff --git a/tests/ui/const-generics/adt_const_params/suggest_feature_only_when_possible.stderr b/tests/ui/const-generics/adt_const_params/suggest_feature_only_when_possible.stderr
@@ -0,0 +1,80 @@
+error: `&'static mut ()` is forbidden as the type of a const generic parameter
+  --> $DIR/suggest_feature_only_when_possible.rs:6:19
+   |
+LL | fn uwu_0<const N: &'static mut ()>() {}
+   |                   ^^^^^^^^^^^^^^^
+   |
+   = note: the only supported types are integers, `bool` and `char`
+
+error: `&'static u32` is forbidden as the type of a const generic parameter
+  --> $DIR/suggest_feature_only_when_possible.rs:10:19
+   |
+LL | fn owo_0<const N: &'static u32>() {}
+   |                   ^^^^^^^^^^^^
+   |
+   = note: the only supported types are integers, `bool` and `char`
+   = help: add `#![feature(adt_const_params)]` to the crate attributes to enable more complex and user defined types
+
+error: `Meow` is forbidden as the type of a const generic parameter
+  --> $DIR/suggest_feature_only_when_possible.rs:19:20
+   |
+LL | fn meow_0<const N: Meow>() {}
+   |                    ^^^^
+   |
+   = note: the only supported types are integers, `bool` and `char`
+   = help: add `#![feature(adt_const_params)]` to the crate attributes to enable more complex and user defined types
+
+error: `&'static Meow` is forbidden as the type of a const generic parameter
+  --> $DIR/suggest_feature_only_when_possible.rs:22:20
+   |
+LL | fn meow_1<const N: &'static Meow>() {}
+   |                    ^^^^^^^^^^^^^
+   |
+   = note: the only supported types are integers, `bool` and `char`
+   = help: add `#![feature(adt_const_params)]` to the crate attributes to enable more complex and user defined types
+
+error: `[Meow; 100]` is forbidden as the type of a const generic parameter
+  --> $DIR/suggest_feature_only_when_possible.rs:25:20
+   |
+LL | fn meow_2<const N: [Meow; 100]>() {}
+   |                    ^^^^^^^^^^^
+   |
+   = note: the only supported types are integers, `bool` and `char`
+   = help: add `#![feature(adt_const_params)]` to the crate attributes to enable more complex and user defined types
+
+error: `(Meow, u8)` is forbidden as the type of a const generic parameter
+  --> $DIR/suggest_feature_only_when_possible.rs:28:20
+   |
+LL | fn meow_3<const N: (Meow, u8)>() {}
+   |                    ^^^^^^^^^^
+   |
+   = note: the only supported types are integers, `bool` and `char`
+   = help: add `#![feature(adt_const_params)]` to the crate attributes to enable more complex and user defined types
+
+error: `(Meow, String)` is forbidden as the type of a const generic parameter
+  --> $DIR/suggest_feature_only_when_possible.rs:34:20
+   |
+LL | fn meow_4<const N: (Meow, String)>() {}
+   |                    ^^^^^^^^^^^^^^
+   |
+   = note: the only supported types are integers, `bool` and `char`
+   = help: add `#![feature(adt_const_params)]` to the crate attributes to enable more complex and user defined types
+
+error: `String` is forbidden as the type of a const generic parameter
+  --> $DIR/suggest_feature_only_when_possible.rs:39:19
+   |
+LL | fn nya_0<const N: String>() {}
+   |                   ^^^^^^
+   |
+   = note: the only supported types are integers, `bool` and `char`
+
+error: `Vec<u32>` is forbidden as the type of a const generic parameter
+  --> $DIR/suggest_feature_only_when_possible.rs:41:19
+   |
+LL | fn nya_1<const N: Vec<u32>>() {}
+   |                   ^^^^^^^^
+   |
+   = note: the only supported types are integers, `bool` and `char`
+
+error: aborting due to 9 previous errors
+
diff --git a/tests/ui/const-generics/const-param-elided-lifetime.min.stderr b/tests/ui/const-generics/const-param-elided-lifetime.min.stderr
@@ -35,7 +35,7 @@ LL | struct A<const N: &u8>;
    |                   ^^^
    |
    = note: the only supported types are integers, `bool` and `char`
-   = help: more complex types are supported with `#![feature(adt_const_params)]`
+   = help: add `#![feature(adt_const_params)]` to the crate attributes to enable more complex and user defined types
 
 error: `&u8` is forbidden as the type of a const generic parameter
   --> $DIR/const-param-elided-lifetime.rs:14:15
@@ -44,7 +44,7 @@ LL | impl<const N: &u8> A<N> {
    |               ^^^
    |
    = note: the only supported types are integers, `bool` and `char`
-   = help: more complex types are supported with `#![feature(adt_const_params)]`
+   = help: add `#![feature(adt_const_params)]` to the crate attributes to enable more complex and user defined types
 
 error: `&u8` is forbidden as the type of a const generic parameter
   --> $DIR/const-param-elided-lifetime.rs:22:15
@@ -53,7 +53,7 @@ LL | impl<const N: &u8> B for A<N> {}
    |               ^^^
    |
    = note: the only supported types are integers, `bool` and `char`
-   = help: more complex types are supported with `#![feature(adt_const_params)]`
+   = help: add `#![feature(adt_const_params)]` to the crate attributes to enable more complex and user defined types
 
 error: `&u8` is forbidden as the type of a const generic parameter
   --> $DIR/const-param-elided-lifetime.rs:26:17
@@ -62,7 +62,7 @@ LL | fn bar<const N: &u8>() {}
    |                 ^^^
    |
    = note: the only supported types are integers, `bool` and `char`
-   = help: more complex types are supported with `#![feature(adt_const_params)]`
+   = help: add `#![feature(adt_const_params)]` to the crate attributes to enable more complex and user defined types
 
 error: `&u8` is forbidden as the type of a const generic parameter
   --> $DIR/const-param-elided-lifetime.rs:17:21
@@ -71,7 +71,7 @@ LL |     fn foo<const M: &u8>(&self) {}
    |                     ^^^
    |
    = note: the only supported types are integers, `bool` and `char`
-   = help: more complex types are supported with `#![feature(adt_const_params)]`
+   = help: add `#![feature(adt_const_params)]` to the crate attributes to enable more complex and user defined types
 
 error: aborting due to 10 previous errors
 

diff --git a/tests/ui/const-generics/const-param-type-depends-on-const-param.min.stderr b/tests/ui/const-generics/const-param-type-depends-on-const-param.min.stderr
@@ -21,7 +21,7 @@ LL | pub struct Dependent<const N: usize, const X: [u8; N]>([(); N]);
    |                                               ^^^^^^^
    |
    = note: the only supported types are integers, `bool` and `char`
-   = help: more complex types are supported with `#![feature(adt_const_params)]`
+   = help: add `#![feature(adt_const_params)]` to the crate attributes to enable more complex and user defined types
 
 error: `[u8; N]` is forbidden as the type of a const generic parameter
   --> $DIR/const-param-type-depends-on-const-param.rs:15:35
@@ -30,7 +30,7 @@ LL | pub struct SelfDependent<const N: [u8; N]>;
    |                                   ^^^^^^^
    |
    = note: the only supported types are integers, `bool` and `char`
-   = help: more complex types are supported with `#![feature(adt_const_params)]`
+   = help: add `#![feature(adt_const_params)]` to the crate attributes to enable more complex and user defined types
 
 error: aborting due to 4 previous errors
 

diff --git a/tests/ui/const-generics/float-generic.simple.stderr b/tests/ui/const-generics/float-generic.simple.stderr
@@ -5,7 +5,6 @@ LL | fn foo<const F: f32>() {}
    |                 ^^^
    |
    = note: the only supported types are integers, `bool` and `char`
-   = help: more complex types are supported with `#![feature(adt_const_params)]`
 
 error: aborting due to previous error
 
diff --git a/tests/ui/const-generics/fn-const-param-call.min.stderr b/tests/ui/const-generics/fn-const-param-call.min.stderr
@@ -3,12 +3,16 @@ error: using function pointers as const generic parameters is forbidden
    |
 LL | struct Wrapper<const F: fn() -> u32>;
    |                         ^^^^^^^^^^^
+   |
+   = note: the only supported types are integers, `bool` and `char`
 
 error: using function pointers as const generic parameters is forbidden
   --> $DIR/fn-const-param-call.rs:13:15
    |
 LL | impl<const F: fn() -> u32> Wrapper<F> {
    |               ^^^^^^^^^^^
+   |
+   = note: the only supported types are integers, `bool` and `char`
 
 error: aborting due to 2 previous errors
 
diff --git a/tests/ui/const-generics/fn-const-param-infer.min.stderr b/tests/ui/const-generics/fn-const-param-infer.min.stderr
@@ -3,6 +3,8 @@ error: using function pointers as const generic parameters is forbidden
    |
 LL | struct Checked<const F: fn(usize) -> bool>;
    |                         ^^^^^^^^^^^^^^^^^
+   |
+   = note: the only supported types are integers, `bool` and `char`
 
 error: aborting due to previous error
 
diff --git a/tests/ui/const-generics/generic_const_exprs/array-size-in-generic-struct-param.min.stderr b/tests/ui/const-generics/generic_const_exprs/array-size-in-generic-struct-param.min.stderr
@@ -23,7 +23,7 @@ LL | struct B<const CFG: Config> {
    |                     ^^^^^^
    |
    = note: the only supported types are integers, `bool` and `char`
-   = help: more complex types are supported with `#![feature(adt_const_params)]`
+   = help: add `#![feature(adt_const_params)]` to the crate attributes to enable more complex and user defined types
 
 error: aborting due to 3 previous errors
 
diff --git a/tests/ui/const-generics/intrinsics-type_name-as-const-argument.min.stderr b/tests/ui/const-generics/intrinsics-type_name-as-const-argument.min.stderr
@@ -14,7 +14,7 @@ LL | trait Trait<const S: &'static str> {}
    |                      ^^^^^^^^^^^^
    |
    = note: the only supported types are integers, `bool` and `char`
-   = help: more complex types are supported with `#![feature(adt_const_params)]`
+   = help: add `#![feature(adt_const_params)]` to the crate attributes to enable more complex and user defined types
 
 error: aborting due to 2 previous errors
 
diff --git a/tests/ui/const-generics/issues/issue-56445-1.min.stderr b/tests/ui/const-generics/issues/issue-56445-1.min.stderr
@@ -13,7 +13,7 @@ LL | struct Bug<'a, const S: &'a str>(PhantomData<&'a ()>);
    |                         ^^^^^^^
    |
    = note: the only supported types are integers, `bool` and `char`
-   = help: more complex types are supported with `#![feature(adt_const_params)]`
+   = help: add `#![feature(adt_const_params)]` to the crate attributes to enable more complex and user defined types
 
 error: aborting due to 2 previous errors
 

diff --git a/tests/ui/const-generics/issues/issue-62878.min.stderr b/tests/ui/const-generics/issues/issue-62878.min.stderr
@@ -13,7 +13,7 @@ LL | fn foo<const N: usize, const A: [u8; N]>() {}
    |                                 ^^^^^^^
    |
    = note: the only supported types are integers, `bool` and `char`
-   = help: more complex types are supported with `#![feature(adt_const_params)]`
+   = help: add `#![feature(adt_const_params)]` to the crate attributes to enable more complex and user defined types
 
 error: aborting due to 2 previous errors
 

diff --git a/tests/ui/const-generics/issues/issue-63322-forbid-dyn.min.stderr b/tests/ui/const-generics/issues/issue-63322-forbid-dyn.min.stderr
@@ -5,7 +5,7 @@ LL | fn test<const T: &'static dyn A>() {
    |                  ^^^^^^^^^^^^^^
    |
    = note: the only supported types are integers, `bool` and `char`
-   = help: more complex types are supported with `#![feature(adt_const_params)]`
+   = help: add `#![feature(adt_const_params)]` to the crate attributes to enable more complex and user defined types
 
 error: aborting due to previous error
 
diff --git a/tests/ui/const-generics/issues/issue-68615-adt.min.stderr b/tests/ui/const-generics/issues/issue-68615-adt.min.stderr
@@ -5,7 +5,7 @@ LL | struct Const<const V: [usize; 0]> {}
    |                       ^^^^^^^^^^
    |
    = note: the only supported types are integers, `bool` and `char`
-   = help: more complex types are supported with `#![feature(adt_const_params)]`
+   = help: add `#![feature(adt_const_params)]` to the crate attributes to enable more complex and user defined types
 
 error: aborting due to previous error
 
diff --git a/tests/ui/const-generics/issues/issue-68615-array.min.stderr b/tests/ui/const-generics/issues/issue-68615-array.min.stderr
@@ -5,7 +5,7 @@ LL | struct Foo<const V: [usize; 0] > {}
    |                     ^^^^^^^^^^
    |
    = note: the only supported types are integers, `bool` and `char`
-   = help: more complex types are supported with `#![feature(adt_const_params)]`
+   = help: add `#![feature(adt_const_params)]` to the crate attributes to enable more complex and user defined types
 
 error: aborting due to previous error