Forbid non-structural_match types in const generics

src/librustc_typeck/collect.rs

@@ -1532,6 +1532,17 @@ pub fn checked_type_of(tcx: TyCtxt<'_>, def_id: DefId, fail: bool) -> Option<Ty<
                         );
                     };
                 }
+                if ty::search_for_adt_without_structural_match(tcx, ty).is_some() {
+                    struct_span_err!(
+                        tcx.sess,
+                        hir_ty.span,
+                        E0739,
+                        "the types of const generic parameters must derive `PartialEq` and `Eq`",
+                    ).span_label(
+                        hir_ty.span,
+                        format!("`{}` doesn't derive both `PartialEq` and `Eq`", ty),
+                    ).emit();
+                }
                 ty
             }
             x => {

src/librustc_typeck/error_codes.rs

@@ -4978,6 +4978,30 @@ the future, [RFC 2091] prohibits their implementation without a follow-up RFC.
 [RFC 2091]: https://github.com/rust-lang/rfcs/blob/master/text/2091-inline-semantic.md
 "##,
 
+E0739: r##"
+Only `structural_match` types (that is, types that derive `PartialEq` and `Eq`)
+may be used as the types of const generic parameters.
+
+```compile_fail,E0739
+#![feature(const_generics)]
+
+struct A;
+
+struct B<const X: A>; // error!
+```
+
+To fix this example, we derive `PartialEq` and `Eq`.
+
+```
+#![feature(const_generics)]
+
+#[derive(PartialEq, Eq)]
+struct A;
+
+struct B<const X: A>; // ok!
+```
+"##,
+
 ;
 //  E0035, merged into E0087/E0089
 //  E0036, merged into E0087/E0089

src/test/ui/const-generics/forbid-non-structural_match-types.rs

@@ -0,0 +1,13 @@
+#![feature(const_generics)]
+//~^ WARN the feature `const_generics` is incomplete and may cause the compiler to crash
+
+#[derive(PartialEq, Eq)]
+struct A;
+
+struct B<const X: A>; // ok
+
+struct C;
+
+struct D<const X: C>; //~ ERROR the types of const generic parameters must derive
+
+fn main() {}

src/test/ui/const-generics/forbid-non-structural_match-types.stderr

@@ -0,0 +1,17 @@
+warning: the feature `const_generics` is incomplete and may cause the compiler to crash
+  --> $DIR/forbid-non-structural_match-types.rs:1:12
+   |
+LL | #![feature(const_generics)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+
+error[E0739]: the types of const generic parameters must derive `PartialEq` and `Eq`
+  --> $DIR/forbid-non-structural_match-types.rs:11:19
+   |
+LL | struct D<const X: C>;
+   |                   ^ `C` doesn't derive both `PartialEq` and `Eq`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0739`.