Rollup merge of #71018 - lcnr:custom-const-param, r=eddyb

handle ConstValue::ByRef in relate

fixes #68615

r? @eddyb

src/test/ui/const-generics/different_byref.rs

@@ -0,0 +1,11 @@
+#![feature(const_generics)]
+//~^ WARN the feature `const_generics` is incomplete and may cause the compiler to crash
+
+struct Const<const V: [usize; 1]> {}
+
+fn main() {
+    let mut x = Const::<{ [3] }> {};
+    x = Const::<{ [4] }> {};
+    //~^ ERROR mismatched types
+
+}

src/test/ui/const-generics/different_byref.stderr

@@ -0,0 +1,20 @@
+warning: the feature `const_generics` is incomplete and may cause the compiler to crash
+  --> $DIR/different_byref.rs:1:12
+   |
+LL | #![feature(const_generics)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+
+error[E0308]: mismatched types
+  --> $DIR/different_byref.rs:8:9
+   |
+LL |     x = Const::<{ [4] }> {};
+   |         ^^^^^^^^^^^^^^^^^^^ expected `3usize`, found `4usize`
+   |
+   = note: expected struct `Const<[3usize]>`
+              found struct `Const<[4usize]>`
+
+error: aborting due to previous error; 1 warning emitted
+
+For more information about this error, try `rustc --explain E0308`.

src/test/ui/const-generics/issues/issue-68615-adt.rs

@@ -0,0 +1,11 @@
+// check-pass
+#![feature(const_generics)]
+#![allow(incomplete_features)]
+
+struct Const<const V: [usize; 0]> {}
+type MyConst = Const<{ [] }>;
+
+fn main() {
+    let _x = Const::<{ [] }> {};
+    let _y = MyConst {};
+}

src/test/ui/const-generics/issues/issue-68615-array.rs

@@ -0,0 +1,11 @@
+// check-pass
+#![feature(const_generics)]
+#![allow(incomplete_features)]
+
+struct Foo<const V: [usize; 0] > {}
+
+type MyFoo = Foo<{ [] }>;
+
+fn main() {
+    let _ = Foo::<{ [] }> {};
+}