Add comment explaining restriction

Author: varkor

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

@@ -1,7 +1,13 @@
 #![feature(const_generics)]
 //~^ WARN the feature `const_generics` is incomplete and may cause the compiler to crash
 
-pub struct Dependent<T, const X: T>([(); X]); //~ ERROR const parameters
-//~^ ERROR parameter `T` is never used
+// 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

@@ -5,13 +5,13 @@ LL | #![feature(const_generics)]
    |            ^^^^^^^^^^^^^^
 
 error[E0671]: const parameters cannot depend on type parameters
-  --> $DIR/const-param-type-depends-on-type-param.rs:4:34
+  --> $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:4:22
+  --> $DIR/const-param-type-depends-on-type-param.rs:9:22
    |
 LL | pub struct Dependent<T, const X: T>([(); X]);
    |                      ^ unused parameter