From 741d40f327291404172b3f35377d8464688c2fe7 Mon Sep 17 00:00:00 2001
From: Gurinder Singh <frederick.the.fool@gmail.com>
Date: Tue, 30 Apr 2024 15:17:47 +0530
Subject: [PATCH] Remove redundant union check in `KnownPanicsLint const prop

because we are already marking unions `NoPropagation` in
`CanConstProp::check()`. That is enough to prevent any attempts
at const propagating unions and this second check is not needed.

Also improve a comment in `CanConstProp::check()`
---
 .../src/known_panics_lint.rs                  | 47 +++++++------------
 1 file changed, 18 insertions(+), 29 deletions(-)

diff --git a/compiler/rustc_mir_transform/src/known_panics_lint.rs b/compiler/rustc_mir_transform/src/known_panics_lint.rs
index b2030efca8f..90c1c7ba63b 100644
--- a/compiler/rustc_mir_transform/src/known_panics_lint.rs
+++ b/compiler/rustc_mir_transform/src/known_panics_lint.rs
@@ -583,33 +583,21 @@ fn eval_rvalue(&mut self, rvalue: &Rvalue<'tcx>, dest: &Place<'tcx>) -> Option<(
                 val.into()
             }
 
-            Aggregate(ref kind, ref fields) => {
-                // Do not const prop 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)
-                        })
-                        .collect(),
-                    variant: match **kind {
-                        AggregateKind::Adt(_, variant, _, _, _) => variant,
-                        AggregateKind::Array(_)
-                        | AggregateKind::Tuple
-                        | AggregateKind::RawPtr(_, _)
-                        | AggregateKind::Closure(_, _)
-                        | AggregateKind::Coroutine(_, _)
-                        | AggregateKind::CoroutineClosure(_, _) => VariantIdx::ZERO,
-                    },
-                }
-            }
+            Aggregate(ref kind, ref fields) => Value::Aggregate {
+                fields: fields
+                    .iter()
+                    .map(|field| self.eval_operand(field).map_or(Value::Uninit, Value::Immediate))
+                    .collect(),
+                variant: match **kind {
+                    AggregateKind::Adt(_, variant, _, _, _) => variant,
+                    AggregateKind::Array(_)
+                    | AggregateKind::Tuple
+                    | AggregateKind::RawPtr(_, _)
+                    | AggregateKind::Closure(_, _)
+                    | AggregateKind::Coroutine(_, _)
+                    | AggregateKind::CoroutineClosure(_, _) => VariantIdx::ZERO,
+                },
+            },
 
             Repeat(ref op, n) => {
                 trace!(?op, ?n);
@@ -897,8 +885,9 @@ pub fn check<'tcx>(
         for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
             let ty = body.local_decls[local].ty;
             if ty.is_union() {
-                // Do not const prop unions as they can
-                // ICE during layout calc
+                // Unions are incompatible with the current implementation of
+                // const prop because Rust has no concept of an active
+                // variant of a union
                 *val = ConstPropMode::NoPropagation;
             } else {
                 match tcx.layout_of(param_env.and(ty)) {