From c677dc26b4d9d5e742ff860ace924a270a2266b6 Mon Sep 17 00:00:00 2001
From: Oli Scherer <github333195615777966@oli-obk.de>
Date: Fri, 6 Jun 2025 16:15:02 +0000
Subject: [PATCH] Allow transmute casts in pre-runtime-MIR

---
 compiler/rustc_middle/src/mir/syntax.rs      |  2 -
 compiler/rustc_mir_transform/src/validate.rs | 48 ++++++++------------
 tests/ui/type/pattern_types/matching.rs      |  1 +
 3 files changed, 20 insertions(+), 31 deletions(-)

diff --git a/compiler/rustc_middle/src/mir/syntax.rs b/compiler/rustc_middle/src/mir/syntax.rs
index bb068f3821db8..f2f975a69683e 100644
--- a/compiler/rustc_middle/src/mir/syntax.rs
+++ b/compiler/rustc_middle/src/mir/syntax.rs
@@ -1531,8 +1531,6 @@ pub enum CastKind {
     ///
     /// MIR is well-formed if the input and output types have different sizes,
     /// but running a transmute between differently-sized types is UB.
-    ///
-    /// Allowed only in [`MirPhase::Runtime`]; Earlier it's a [`TerminatorKind::Call`].
     Transmute,
 }
 
diff --git a/compiler/rustc_mir_transform/src/validate.rs b/compiler/rustc_mir_transform/src/validate.rs
index fd91508cc1148..7dcdd7999f285 100644
--- a/compiler/rustc_mir_transform/src/validate.rs
+++ b/compiler/rustc_mir_transform/src/validate.rs
@@ -1308,37 +1308,27 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                         }
                     }
                     CastKind::Transmute => {
-                        if let MirPhase::Runtime(..) = self.body.phase {
-                            // Unlike `mem::transmute`, a MIR `Transmute` is well-formed
-                            // for any two `Sized` types, just potentially UB to run.
-
-                            if !self
-                                .tcx
-                                .normalize_erasing_regions(self.typing_env, op_ty)
-                                .is_sized(self.tcx, self.typing_env)
-                            {
-                                self.fail(
-                                    location,
-                                    format!("Cannot transmute from non-`Sized` type {op_ty}"),
-                                );
-                            }
-                            if !self
-                                .tcx
-                                .normalize_erasing_regions(self.typing_env, *target_type)
-                                .is_sized(self.tcx, self.typing_env)
-                            {
-                                self.fail(
-                                    location,
-                                    format!("Cannot transmute to non-`Sized` type {target_type:?}"),
-                                );
-                            }
-                        } else {
+                        // Unlike `mem::transmute`, a MIR `Transmute` is well-formed
+                        // for any two `Sized` types, just potentially UB to run.
+
+                        if !self
+                            .tcx
+                            .normalize_erasing_regions(self.typing_env, op_ty)
+                            .is_sized(self.tcx, self.typing_env)
+                        {
                             self.fail(
                                 location,
-                                format!(
-                                    "Transmute is not supported in non-runtime phase {:?}.",
-                                    self.body.phase
-                                ),
+                                format!("Cannot transmute from non-`Sized` type {op_ty}"),
+                            );
+                        }
+                        if !self
+                            .tcx
+                            .normalize_erasing_regions(self.typing_env, *target_type)
+                            .is_sized(self.tcx, self.typing_env)
+                        {
+                            self.fail(
+                                location,
+                                format!("Cannot transmute to non-`Sized` type {target_type:?}"),
                             );
                         }
                     }
diff --git a/tests/ui/type/pattern_types/matching.rs b/tests/ui/type/pattern_types/matching.rs
index b8463a8e82298..21f89b3b673be 100644
--- a/tests/ui/type/pattern_types/matching.rs
+++ b/tests/ui/type/pattern_types/matching.rs
@@ -1,6 +1,7 @@
 #![feature(pattern_types, pattern_type_macro, structural_match)]
 
 //@ check-pass
+//@ compile-flags: -Zvalidate-mir
 
 use std::marker::StructuralPartialEq;
 use std::pat::pattern_type;