Rollup merge of #70126 - wesleywiser:fix_miri_ice_neg_zst_enum_discr, r=RalfJung,eddyb

Fix ICE caused by truncating a negative ZST enum discriminant

Fixes #70114

r? @oli-obk or @RalfJung

src/test/ui/consts/cast-discriminant-zst-enum.rs

@@ -0,0 +1,47 @@
+// run-pass
+// Test a ZST enum whose dicriminant is ~0i128. This caused an ICE when casting to a i32.
+
+#[derive(Copy, Clone)]
+enum Nums {
+    NegOne = -1,
+}
+
+const NEG_ONE_I8: i8 = Nums::NegOne as i8;
+const NEG_ONE_I16: i16 = Nums::NegOne as i16;
+const NEG_ONE_I32: i32 = Nums::NegOne as i32;
+const NEG_ONE_I64: i64 = Nums::NegOne as i64;
+const NEG_ONE_I128: i128 = Nums::NegOne as i128;
+
+#[inline(never)]
+fn identity<T>(t: T) -> T { t }
+
+fn test_as_arg(n: Nums) {
+    assert_eq!(-1i8, n as i8);
+    assert_eq!(-1i16, n as i16);
+    assert_eq!(-1i32, n as i32);
+    assert_eq!(-1i64, n as i64);
+    assert_eq!(-1i128, n as i128);
+}
+
+fn main() {
+    let kind = Nums::NegOne;
+    assert_eq!(-1i8, kind as i8);
+    assert_eq!(-1i16, kind as i16);
+    assert_eq!(-1i32, kind as i32);
+    assert_eq!(-1i64, kind as i64);
+    assert_eq!(-1i128, kind as i128);
+
+    assert_eq!(-1i8, identity(kind) as i8);
+    assert_eq!(-1i16, identity(kind) as i16);
+    assert_eq!(-1i32, identity(kind) as i32);
+    assert_eq!(-1i64, identity(kind) as i64);
+    assert_eq!(-1i128, identity(kind) as i128);
+
+    test_as_arg(Nums::NegOne);
+
+    assert_eq!(-1i8, NEG_ONE_I8);
+    assert_eq!(-1i16, NEG_ONE_I16);
+    assert_eq!(-1i32, NEG_ONE_I32);
+    assert_eq!(-1i64, NEG_ONE_I64);
+    assert_eq!(-1i128, NEG_ONE_I128);
+}