Do not allow const generics to depend on type parameters

src/librustc_resolve/error_codes.rs

@@ -1642,6 +1642,19 @@ fn main() {
 ```
 "##,
 
+E0671: r##"
+Const parameters cannot depend on type parameters.
+The following is therefore invalid:
+```compile_fail,E0671
+#![feature(const_generics)]
+
+fn const_id<T, const N: T>() -> T { // error: const parameter
+                                    // depends on type parameter
+    N
+}
+```
+"##,
+
 }
 
 register_diagnostics! {

src/librustc_resolve/lib.rs

@@ -185,6 +185,8 @@ enum ResolutionError<'a> {
     BindingShadowsSomethingUnacceptable(&'a str, Name, &'a NameBinding<'a>),
     /// Error E0128: type parameters with a default cannot use forward-declared identifiers.
     ForwardDeclaredTyParam, // FIXME(const_generics:defaults)
+    /// Error E0671: const parameter cannot depend on type parameter.
+    ConstParamDependentOnTypeParam,
 }
 
 /// Combines an error with provided span and emits it.
@@ -440,6 +442,16 @@ fn resolve_struct_error<'sess, 'a>(resolver: &'sess Resolver<'_>,
                 span, "defaulted type parameters cannot be forward declared".to_string());
             err
         }
+        ResolutionError::ConstParamDependentOnTypeParam => {
+            let mut err = struct_span_err!(
+                resolver.session,
+                span,
+                E0671,
+                "const parameters cannot depend on type parameters"
+            );
+            err.span_label(span, format!("const parameter depends on type parameter"));
+            err
+        }
     }
 }
 
@@ -915,6 +927,18 @@ impl<'a, 'tcx> Visitor<'tcx> for Resolver<'a> {
                 }
             }));
 
+        // We also ban access to type parameters for use as the types of const parameters.
+        let mut const_ty_param_ban_rib = Rib::new(TyParamAsConstParamTy);
+        const_ty_param_ban_rib.bindings.extend(generics.params.iter()
+            .filter(|param| {
+                if let GenericParamKind::Type { .. } = param.kind {
+                    true
+                } else {
+                    false
+                }
+            })
+            .map(|param| (Ident::with_empty_ctxt(param.ident.name), Def::Err)));
+
         for param in &generics.params {
             match param.kind {
                 GenericParamKind::Lifetime { .. } => self.visit_generic_param(param),
@@ -933,11 +957,15 @@ impl<'a, 'tcx> Visitor<'tcx> for Resolver<'a> {
                     default_ban_rib.bindings.remove(&Ident::with_empty_ctxt(param.ident.name));
                 }
                 GenericParamKind::Const { ref ty } => {
+                    self.ribs[TypeNS].push(const_ty_param_ban_rib);
+
                     for bound in &param.bounds {
                         self.visit_param_bound(bound);
                     }
 
                     self.visit_ty(ty);
+
+                    const_ty_param_ban_rib = self.ribs[TypeNS].pop().unwrap();
                 }
             }
         }
@@ -994,6 +1022,9 @@ enum RibKind<'a> {
     /// from the default of a type parameter because they're not declared
     /// before said type parameter. Also see the `visit_generics` override.
     ForwardTyParamBanRibKind,
+
+    /// We forbid the use of type parameters as the types of const parameters.
+    TyParamAsConstParamTy,
 }
 
 /// A single local scope.
@@ -3944,6 +3975,15 @@ impl<'a> Resolver<'a> {
             return Def::Err;
         }
 
+        // An invalid use of a type parameter as the type of a const parameter.
+        if let TyParamAsConstParamTy = self.ribs[ns][rib_index].kind {
+            if record_used {
+                resolve_error(self, span, ResolutionError::ConstParamDependentOnTypeParam);
+            }
+            assert_eq!(def, Def::Err);
+            return Def::Err;
+        }
+
         match def {
             Def::Upvar(..) => {
                 span_bug!(span, "unexpected {:?} in bindings", def)
@@ -3955,7 +3995,7 @@ impl<'a> Resolver<'a> {
                 for rib in ribs {
                     match rib.kind {
                         NormalRibKind | ModuleRibKind(..) | MacroDefinition(..) |
-                        ForwardTyParamBanRibKind => {
+                        ForwardTyParamBanRibKind | TyParamAsConstParamTy => {
                             // Nothing to do. Continue.
                         }
                         ClosureRibKind(function_id) => {
@@ -4013,7 +4053,7 @@ impl<'a> Resolver<'a> {
                     match rib.kind {
                         NormalRibKind | TraitOrImplItemRibKind | ClosureRibKind(..) |
                         ModuleRibKind(..) | MacroDefinition(..) | ForwardTyParamBanRibKind |
-                        ConstantItemRibKind => {
+                        ConstantItemRibKind | TyParamAsConstParamTy => {
                             // Nothing to do. Continue.
                         }
                         ItemRibKind | FnItemRibKind => {

src/test/ui/const-generics/const-param-type-depends-on-type-param-ungated.rs

@@ -0,0 +1,6 @@
+use std::marker::PhantomData;
+
+struct B<T, const N: T>(PhantomData<[T; N]>); //~ ERROR const generics are unstable
+//~^ ERROR const parameters cannot depend on type parameters
+
+fn main() {}

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

@@ -0,0 +1,19 @@
+error[E0671]: const parameters cannot depend on type parameters
+  --> $DIR/const-param-type-depends-on-type-param-ungated.rs:3:22
+   |
+LL | struct B<T, const N: T>(PhantomData<[T; N]>);
+   |                      ^ const parameter depends on type parameter
+
+error[E0658]: const generics are unstable
+  --> $DIR/const-param-type-depends-on-type-param-ungated.rs:3:19
+   |
+LL | struct B<T, const N: T>(PhantomData<[T; N]>);
+   |                   ^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/44580
+   = help: add #![feature(const_generics)] to the crate attributes to enable
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0658, E0671.
+For more information about an error, try `rustc --explain E0658`.

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

@@ -0,0 +1,13 @@
+#![feature(const_generics)]
+//~^ WARN the feature `const_generics` is incomplete and may cause the compiler to crash
+
+// Currently, const parameters cannot depend on type parameters, because there is no way to
+// enforce the `structural_match` property on an arbitrary type parameter. This restriction
+// may be relaxed in the future. See https://github.com/rust-lang/rfcs/pull/2000 for more
+// details.
+
+pub struct Dependent<T, const X: T>([(); X]);
+//~^ ERROR const parameters cannot depend on type parameters
+//~^^ ERROR parameter `T` is never used
+
+fn main() {}

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

@@ -0,0 +1,24 @@
+warning: the feature `const_generics` is incomplete and may cause the compiler to crash
+  --> $DIR/const-param-type-depends-on-type-param.rs:1:12
+   |
+LL | #![feature(const_generics)]
+   |            ^^^^^^^^^^^^^^
+
+error[E0671]: const parameters cannot depend on type parameters
+  --> $DIR/const-param-type-depends-on-type-param.rs:9:34
+   |
+LL | pub struct Dependent<T, const X: T>([(); X]);
+   |                                  ^ const parameter depends on type parameter
+
+error[E0392]: parameter `T` is never used
+  --> $DIR/const-param-type-depends-on-type-param.rs:9:22
+   |
+LL | pub struct Dependent<T, const X: T>([(); X]);
+   |                      ^ unused parameter
+   |
+   = help: consider removing `T` or using a marker such as `std::marker::PhantomData`
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0392, E0671.
+For more information about an error, try `rustc --explain E0392`.