Add CastKind::Transmute to MIR

Nothing actually produces it in this commit.

Author: scottmcm

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -2242,6 +2242,13 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                             }
                         }
                     }
+                    CastKind::Transmute => {
+                        span_mirbug!(
+                            self,
+                            rvalue,
+                            "Unexpected CastKind::Transmute, should only appear after lowering_intrinsics",
+                        );
+                    }
                 }
             }
 

compiler/rustc_codegen_cranelift/src/base.rs

@@ -720,6 +720,10 @@ fn codegen_stmt<'tcx>(
                     let operand = codegen_operand(fx, operand);
                     operand.coerce_dyn_star(fx, lval);
                 }
+                Rvalue::Cast(CastKind::Transmute, ref operand, _to_ty) => {
+                    let operand = codegen_operand(fx, operand);
+                    lval.write_cvalue_transmute(fx, operand);
+                }
                 Rvalue::Discriminant(place) => {
                     let place = codegen_place(fx, place);
                     let value = place.to_cvalue(fx);

compiler/rustc_codegen_ssa/src/mir/block.rs

@@ -1789,7 +1789,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         }
     }
 
-    fn codegen_transmute_into(
+    pub(crate) fn codegen_transmute_into(
         &mut self,
         bx: &mut Bx,
         src: &mir::Operand<'tcx>,

compiler/rustc_codegen_ssa/src/mir/rvalue.rs

@@ -135,6 +135,10 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                 dest.codegen_set_discr(bx, variant_index);
             }
 
+            mir::Rvalue::Cast(mir::CastKind::Transmute, ref operand, _ty) => {
+                self.codegen_transmute_into(bx, operand, dest);
+            }
+
             _ => {
                 assert!(self.rvalue_creates_operand(rvalue, DUMMY_SP));
                 let temp = self.codegen_rvalue_operand(bx, rvalue);
@@ -344,6 +348,9 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                         };
                         OperandValue::Immediate(newval)
                     }
+                    mir::CastKind::Transmute => {
+                        bug!("Transmute operand {:?} in `codegen_rvalue_operand`", operand);
+                    }
                 };
                 OperandRef { val, layout: cast }
             }
@@ -684,6 +691,9 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
 impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
     pub fn rvalue_creates_operand(&self, rvalue: &mir::Rvalue<'tcx>, span: Span) -> bool {
         match *rvalue {
+            mir::Rvalue::Cast(mir::CastKind::Transmute, ..) =>
+                // sometimes this could, but for aggregates it can't
+                false,
             mir::Rvalue::Ref(..) |
             mir::Rvalue::CopyForDeref(..) |
             mir::Rvalue::AddressOf(..) |

compiler/rustc_const_eval/src/interpret/cast.rs

@@ -133,6 +133,10 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                     bug!()
                 }
             }
+
+            Transmute => {
+                self.copy_op(src, dest, /*allow_transmute*/ true)?;
+            }
         }
         Ok(())
     }

compiler/rustc_const_eval/src/transform/validate.rs

@@ -619,6 +619,25 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                             );
                         }
                     }
+                    CastKind::Transmute => {
+                        let src_layout = self.tcx.layout_of(self.param_env.and(op_ty));
+                        let dst_layout = self.tcx.layout_of(self.param_env.and(*target_type));
+                        if let Err(e) = src_layout {
+                            self.fail(
+                                location,
+                                format!("Unable to compute layout for source type {op_ty:?}: {e}"),
+                            );
+                        }
+                        if let Err(e) = dst_layout {
+                            self.fail(location, format!("Unable to compute layout for destination type {target_type:?}: {e}"));
+                        }
+
+                        if let (Ok(src_layout), Ok(dst_layout)) = (src_layout, dst_layout) {
+                            if src_layout.layout.size() != dst_layout.layout.size() {
+                                self.fail(location, format!("Source and destination layouts have different sizes: {src_layout:?} vs {dst_layout:?}"));
+                            }
+                        }
+                    }
                 }
             }
             Rvalue::Repeat(_, _)

compiler/rustc_middle/src/mir/mod.rs

@@ -1963,7 +1963,8 @@ impl<'tcx> Rvalue<'tcx> {
                 | CastKind::PtrToPtr
                 | CastKind::Pointer(_)
                 | CastKind::PointerFromExposedAddress
-                | CastKind::DynStar,
+                | CastKind::DynStar
+                | CastKind::Transmute,
                 _,
                 _,
             )

compiler/rustc_middle/src/mir/syntax.rs

@@ -1189,6 +1189,8 @@ pub enum CastKind {
     IntToFloat,
     PtrToPtr,
     FnPtrToPtr,
+    /// Reinterpret the bits of the input as a different type.
+    Transmute,
 }
 
 #[derive(Clone, Debug, PartialEq, Eq, TyEncodable, TyDecodable, Hash, HashStable)]

compiler/rustc_mir_transform/src/check_unsafety.rs

@@ -132,6 +132,12 @@ impl<'tcx> Visitor<'tcx> for UnsafetyChecker<'_, 'tcx> {
                     self.register_violations(violations, used_unsafe_blocks.iter().copied());
                 }
             },
+            Rvalue::Cast(CastKind::Transmute, ..) => {
+                self.require_unsafe(
+                    UnsafetyViolationKind::General,
+                    UnsafetyViolationDetails::CallToUnsafeFunction,
+                );
+            }
             _ => {}
         }
         self.super_rvalue(rvalue, location);

compiler/rustc_mir_transform/src/lower_intrinsics.rs

@@ -2,6 +2,7 @@
 
 use crate::MirPass;
 use rustc_middle::mir::*;
+use rustc_middle::ty::inhabitedness::inhabited_predicate::InhabitedPredicate;
 use rustc_middle::ty::subst::SubstsRef;
 use rustc_middle::ty::{self, Ty, TyCtxt};
 use rustc_span::symbol::{sym, Symbol};
@@ -162,6 +163,30 @@ impl<'tcx> MirPass<'tcx> for LowerIntrinsics {
                             terminator.kind = TerminatorKind::Goto { target };
                         }
                     }
+                    sym::transmute => {
+                        let dst_ty = destination.ty(local_decls, tcx).ty;
+                        if let Some(target) = *target {
+                            let mut args = args.drain(..);
+                            block.statements.push(Statement {
+                                source_info: terminator.source_info,
+                                kind: StatementKind::Assign(Box::new((
+                                    *destination,
+                                    Rvalue::Cast(CastKind::Transmute, args.next().unwrap(), dst_ty),
+                                ))),
+                            });
+                            assert_eq!(args.next(), None, "Extra argument for transmute intrinsic");
+                            drop(args);
+                            terminator.kind = TerminatorKind::Goto { target };
+                        } else {
+                            debug_assert!(!matches!(
+                                dst_ty.inhabited_predicate(tcx),
+                                InhabitedPredicate::True
+                            ));
+                            // `transmute::<_, !>(x)` is UB for anything inhabited,
+                            // and must be unreachable if `x` is uninhabited.
+                            terminator.kind = TerminatorKind::Unreachable;
+                        }
+                    }
                     _ if intrinsic_name.as_str().starts_with("simd_shuffle") => {
                         validate_simd_shuffle(tcx, args, terminator.source_info.span);
                     }