add test for repr(128) enum derives

src/test/ui/enum-discriminant/issue-70509-partial_eq.rs

@@ -0,0 +1,17 @@
+// run-pass
+#![feature(repr128, arbitrary_enum_discriminant)]
+
+#[derive(PartialEq, Debug)]
+#[repr(i128)]
+enum Test {
+    A(Box<u64>) = 0,
+    B(usize) = u64::max_value() as i128 + 1,
+}
+
+fn main() {
+    assert_ne!(Test::A(Box::new(2)), Test::B(0));
+    // This previously caused a segfault.
+    //
+    // See https://github.com/rust-lang/rust/issues/70509#issuecomment-620654186
+    // for a detailed explanation.
+}