Rollup merge of #142124 - oli-obk:transmute-cast, r=scottmcm

Allow transmute casts in pre-runtime-MIR

r? ``@scottmcm``

cc ``@BoxyUwU``

turns out in #138393 I erroneously used transmute casts in https://github.com/rust-lang/rust/blob/fd3da4bebdff63b7529483ff7025986ef16bf463/compiler/rustc_mir_build/src/builder/matches/test.rs#L209

I don't think they have any issues using them before runtime, we just checked for them because we didn't have code exercising those code paths

Author: fmease

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,
 }
 

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:?}"),
                             );
                         }
                     }

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;