From b12151c40d8ad3f1d4c45bd8bd2da9cc94076d5a Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Sat, 10 Feb 2024 15:26:55 +0100
Subject: [PATCH] interpret/write_discriminant: when encoding niched variant,
 ensure the stored value matches

---
 ...um-set-discriminant-niche-variant-wrong.rs | 31 +++++++++++++++++++
 ...et-discriminant-niche-variant-wrong.stderr | 20 ++++++++++++
 2 files changed, 51 insertions(+)
 create mode 100644 tests/fail/enum-set-discriminant-niche-variant-wrong.rs
 create mode 100644 tests/fail/enum-set-discriminant-niche-variant-wrong.stderr

diff --git a/tests/fail/enum-set-discriminant-niche-variant-wrong.rs b/tests/fail/enum-set-discriminant-niche-variant-wrong.rs
new file mode 100644
index 0000000000..7097aa0c43
--- /dev/null
+++ b/tests/fail/enum-set-discriminant-niche-variant-wrong.rs
@@ -0,0 +1,31 @@
+#![feature(core_intrinsics)]
+#![feature(custom_mir)]
+
+use std::intrinsics::mir::*;
+use std::num::NonZeroI32;
+
+// We define our own option type so that we can control the varian indices.
+#[allow(unused)]
+enum Option<T> {
+    None,
+    Some(T),
+}
+use Option::*;
+
+#[custom_mir(dialect = "runtime", phase = "optimized")]
+fn set_discriminant(ptr: &mut Option<NonZeroI32>) {
+    mir! {
+        {
+            // We set the discriminant to `Some`, which is a NOP since this is the niched variant.
+            // However, the enum is actually encoding `None` currently! That's not good...
+            SetDiscriminant(*ptr, 1);
+            //~^ ERROR: trying to set discriminant of a Option<std::num::NonZero<i32>> to the niched variant, but the value does not match
+            Return()
+        }
+    }
+}
+
+pub fn main() {
+    let mut v = None;
+    set_discriminant(&mut v);
+}
diff --git a/tests/fail/enum-set-discriminant-niche-variant-wrong.stderr b/tests/fail/enum-set-discriminant-niche-variant-wrong.stderr
new file mode 100644
index 0000000000..a48a0a993d
--- /dev/null
+++ b/tests/fail/enum-set-discriminant-niche-variant-wrong.stderr
@@ -0,0 +1,20 @@
+error: Undefined Behavior: trying to set discriminant of a Option<std::num::NonZero<i32>> to the niched variant, but the value does not match
+  --> $DIR/enum-set-discriminant-niche-variant-wrong.rs:LL:CC
+   |
+LL |             SetDiscriminant(*ptr, 1);
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^ trying to set discriminant of a Option<std::num::NonZero<i32>> to the niched variant, but the value does not match
+   |
+   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
+   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
+   = note: BACKTRACE:
+   = note: inside `set_discriminant` at $DIR/enum-set-discriminant-niche-variant-wrong.rs:LL:CC
+note: inside `main`
+  --> $DIR/enum-set-discriminant-niche-variant-wrong.rs:LL:CC
+   |
+LL |     set_discriminant(&mut v);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to 1 previous error
+