Do not const pop unions

as they can made to produce values whose types don't
match their underlying layout types which can lead to
ICEs on eval

compiler/rustc_mir_transform/src/known_panics_lint.rs

@@ -585,10 +585,21 @@ fn eval_rvalue(&mut self, rvalue: &Rvalue<'tcx>, dest: &Place<'tcx>) -> Option<(
                 val.into()
             }
 
-            Aggregate(ref kind, ref fields) => Value::Aggregate {
+            Aggregate(ref kind, ref fields) => {
+                // Do not const pop union fields as they can be
+                // made to produce values that don't match their
+                // underlying layout's type (see ICE #121534).
+                // If the last element of the `Adt` tuple
+                // is `Some` it indicates the ADT is a union
+                if let AggregateKind::Adt(_, _, _, _, Some(_)) = **kind {
+                    return None;
+                };
+                Value::Aggregate {
                     fields: fields
                         .iter()
-                    .map(|field| self.eval_operand(field).map_or(Value::Uninit, Value::Immediate))
+                        .map(|field| {
+                            self.eval_operand(field).map_or(Value::Uninit, Value::Immediate)
+                        })
                         .collect(),
                     variant: match **kind {
                         AggregateKind::Adt(_, variant, _, _, _) => variant,
@@ -598,7 +609,8 @@ fn eval_rvalue(&mut self, rvalue: &Rvalue<'tcx>, dest: &Place<'tcx>) -> Option<(
                         | AggregateKind::Coroutine(_, _)
                         | AggregateKind::CoroutineClosure(_, _) => VariantIdx::new(0),
                     },
-            },
+                }
+            }
 
             Repeat(ref op, n) => {
                 trace!(?op, ?n);

tests/ui/lint/ice-unions-known-panics-lint-issue-121534.rs

@@ -0,0 +1,21 @@
+// Regression test for #121534
+// Tests that no ICE occurs in KnownPanicsLint when it
+// evaluates an operation whose operands have different
+// layout types even though they have the same type.
+// This situation can be contrived through the use of
+// unions as in this test
+
+//@ build-pass
+union Union {
+    u32_field: u32,
+    i32_field: i32,
+}
+
+pub fn main() {
+    let u32_variant = Union { u32_field: 2 };
+    let i32_variant = Union { i32_field: 3 };
+    let a = unsafe { u32_variant.u32_field };
+    let b = unsafe { i32_variant.u32_field };
+
+    let _diff = a - b;
+}