Remove "type parameter depends on const parameter" error from resolution

Author: varkor

src/librustc_resolve/diagnostics.rs

@@ -367,16 +367,6 @@ impl<'a> Resolver<'a> {
                     span, "`Self` in type parameter default".to_string());
                 err
             }
-            ResolutionError::ConstParamDependentOnTypeParam => {
-                let mut err = struct_span_err!(
-                    self.session,
-                    span,
-                    E0671,
-                    "const parameters cannot depend on type parameters"
-                );
-                err.span_label(span, format!("const parameter depends on type parameter"));
-                err
-            }
         }
     }
 

src/librustc_resolve/error_codes.rs

@@ -1880,13 +1880,14 @@ fn main() {
 "##,
 
 E0671: r##"
+#### Note: this error code is no longer emitted by the compiler.
+
 Const parameters cannot depend on type parameters.
 The following is therefore invalid:
-```compile_fail,E0671
+```compile_fail,E0740
 #![feature(const_generics)]
 
-fn const_id<T, const N: T>() -> T { // error: const parameter
-                                    // depends on type parameter
+fn const_id<T, const N: T>() -> T { // error
     N
 }
 ```

src/librustc_resolve/late.rs

@@ -111,9 +111,6 @@ crate 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,
 }
 
 impl RibKind<'_> {
@@ -128,8 +125,7 @@ impl RibKind<'_> {
             | MacroDefinition(_) => false,
             AssocItemRibKind
             | ItemRibKind(_)
-            | ForwardTyParamBanRibKind
-            | TyParamAsConstParamTy => true,
+            | ForwardTyParamBanRibKind => true,
         }
     }
 }
@@ -483,18 +479,6 @@ impl<'a, 'tcx> Visitor<'tcx> for LateResolutionVisitor<'a, '_> {
             default_ban_rib.bindings.insert(Ident::with_dummy_span(kw::SelfUpper), Res::Err);
         }
 
-        // 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_dummy_span(param.ident.name), Res::Err)));
-
         for param in &generics.params {
             match param.kind {
                 GenericParamKind::Lifetime { .. } => self.visit_generic_param(param),
@@ -513,15 +497,10 @@ impl<'a, 'tcx> Visitor<'tcx> for LateResolutionVisitor<'a, '_> {
                     default_ban_rib.bindings.remove(&Ident::with_dummy_span(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();
                 }
             }
         }

src/librustc_resolve/lib.rs

@@ -215,8 +215,6 @@ enum ResolutionError<'a> {
     ForwardDeclaredTyParam, // FIXME(const_generics:defaults)
     /// Error E0735: type parameters with a default cannot use `Self`
     SelfInTyParamDefault,
-    /// Error E0671: const parameter cannot depend on type parameter.
-    ConstParamDependentOnTypeParam,
 }
 
 // A minimal representation of a path segment. We use this in resolve because
@@ -2169,15 +2167,6 @@ impl<'a> Resolver<'a> {
             return Res::Err;
         }
 
-        // An invalid use of a type parameter as the type of a const parameter.
-        if let TyParamAsConstParamTy = all_ribs[rib_index].kind {
-            if record_used {
-                self.report_error(span, ResolutionError::ConstParamDependentOnTypeParam);
-            }
-            assert_eq!(res, Res::Err);
-            return Res::Err;
-        }
-
         match res {
             Res::Local(_) => {
                 use ResolutionError::*;
@@ -2186,7 +2175,7 @@ impl<'a> Resolver<'a> {
                 for rib in ribs {
                     match rib.kind {
                         NormalRibKind | ModuleRibKind(..) | MacroDefinition(..) |
-                        ForwardTyParamBanRibKind | TyParamAsConstParamTy => {
+                        ForwardTyParamBanRibKind => {
                             // Nothing to do. Continue.
                         }
                         ItemRibKind(_) | FnItemRibKind | AssocItemRibKind => {
@@ -2220,7 +2209,7 @@ impl<'a> Resolver<'a> {
                     let has_generic_params = match rib.kind {
                         NormalRibKind | AssocItemRibKind |
                         ModuleRibKind(..) | MacroDefinition(..) | ForwardTyParamBanRibKind |
-                        ConstantItemRibKind | TyParamAsConstParamTy => {
+                        ConstantItemRibKind => {
                             // Nothing to do. Continue.
                             continue;
                         }

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

@@ -1,6 +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
+//~^ ERROR the types of const generic parameters must derive `PartialEq` and `Eq`
 
 fn main() {}

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

@@ -1,9 +1,3 @@
-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
    |
@@ -13,7 +7,13 @@ 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[E0740]: the types of const generic parameters must derive `PartialEq` and `Eq`
+  --> $DIR/const-param-type-depends-on-type-param-ungated.rs:3:22
+   |
+LL | struct B<T, const N: T>(PhantomData<[T; N]>);
+   |                      ^ `T` doesn't derive both `PartialEq` and `Eq`
+
 error: aborting due to 2 previous errors
 
-Some errors have detailed explanations: E0658, E0671.
+Some errors have detailed explanations: E0658, E0740.
 For more information about an error, try `rustc --explain E0658`.

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

@@ -7,7 +7,6 @@
 // details.
 
 pub struct Dependent<T, const X: T>([(); X]);
-//~^ ERROR const parameters cannot depend on type parameters
-//~^^ ERROR parameter `T` is never used
+//~^ ERROR the types of const generic parameters must derive `PartialEq` and `Eq`
 
 fn main() {}

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

@@ -1,9 +1,3 @@
-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
-
 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
    |
@@ -12,15 +6,12 @@ LL | #![feature(const_generics)]
    |
    = note: `#[warn(incomplete_features)]` on by default
 
-error[E0392]: parameter `T` is never used
-  --> $DIR/const-param-type-depends-on-type-param.rs:9:22
+error[E0740]: the types of const generic parameters must derive `PartialEq` and `Eq`
+  --> $DIR/const-param-type-depends-on-type-param.rs:9:34
    |
 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 `std::marker::PhantomData`
+   |                                  ^ `T` doesn't derive both `PartialEq` and `Eq`
 
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
-Some errors have detailed explanations: E0392, E0671.
-For more information about an error, try `rustc --explain E0392`.
+For more information about this error, try `rustc --explain E0740`.