Stop turning transmutes into discriminants in mir-opt

Partially reverts 109612, as after 109993 these aren't actually equivalent any more, and I'm no longer confident this was ever an improvement in the first place.

3 files changed, 49 insertions, 152 deletions

diff --git a/compiler/rustc_mir_transform/src/instsimplify.rs b/compiler/rustc_mir_transform/src/instsimplify.rs
index 6bff535586a..e4dc617620e 100644
--- a/compiler/rustc_mir_transform/src/instsimplify.rs
+++ b/compiler/rustc_mir_transform/src/instsimplify.rs
@@ -5,7 +5,6 @@ use crate::MirPass;
 use rustc_hir::Mutability;
 use rustc_middle::mir::*;
 use rustc_middle::ty::layout::ValidityRequirement;
-use rustc_middle::ty::util::IntTypeExt;
 use rustc_middle::ty::{self, ParamEnv, SubstsRef, Ty, TyCtxt};
 use rustc_span::symbol::Symbol;
 use rustc_target::abi::FieldIdx;
@@ -163,18 +162,6 @@ impl<'tcx> InstSimplifyContext<'tcx, '_> {
                     return;
                 }
 
-                // Transmuting a fieldless enum to its repr is a discriminant read
-                if let ty::Adt(adt_def, ..) = operand_ty.kind()
-                    && adt_def.is_enum()
-                    && adt_def.is_payloadfree()
-                    && let Some(place) = operand.place()
-                    && let Some(repr_int) = adt_def.repr().int
-                    && repr_int.to_ty(self.tcx) == *cast_ty
-                {
-                    *rvalue = Rvalue::Discriminant(place);
-                    return;
-                }
-
                 // Transmuting a transparent struct/union to a field's type is a projection
                 if let ty::Adt(adt_def, substs) = operand_ty.kind()
                     && adt_def.repr().transparent()
diff --git a/tests/mir-opt/combine_transmutes.adt_transmutes.InstSimplify.diff b/tests/mir-opt/combine_transmutes.adt_transmutes.InstSimplify.diff
index c5907e7cf18..15117ea890e 100644
--- a/tests/mir-opt/combine_transmutes.adt_transmutes.InstSimplify.diff
+++ b/tests/mir-opt/combine_transmutes.adt_transmutes.InstSimplify.diff
@@ -4,59 +4,29 @@
   fn adt_transmutes() -> () {
       let mut _0: ();                      // return place in scope 0 at $DIR/combine_transmutes.rs:+0:32: +0:32
       let _1: u8;                          // in scope 0 at $DIR/combine_transmutes.rs:+1:9: +1:11
-      let mut _2: EnumNoRepr;              // in scope 0 at $DIR/combine_transmutes.rs:+1:28: +1:41
-      let mut _4: EnumNoRepr;              // in scope 0 at $DIR/combine_transmutes.rs:+2:28: +2:41
-      let mut _6: EnumReprIsize;           // in scope 0 at $DIR/combine_transmutes.rs:+3:31: +3:47
-      let mut _8: EnumReprIsize;           // in scope 0 at $DIR/combine_transmutes.rs:+4:31: +4:47
-      let mut _10: std::cmp::Ordering;     // in scope 0 at $DIR/combine_transmutes.rs:+5:28: +5:52
-      let mut _12: std::cmp::Ordering;     // in scope 0 at $DIR/combine_transmutes.rs:+6:28: +6:52
-      let mut _14: std::option::Option<std::num::NonZeroU8>; // in scope 0 at $DIR/combine_transmutes.rs:+7:28: +7:58
-      let mut _16: std::num::Wrapping<i16>; // in scope 0 at $DIR/combine_transmutes.rs:+8:29: +8:54
-      let mut _18: std::num::Wrapping<i16>; // in scope 0 at $DIR/combine_transmutes.rs:+9:29: +9:54
-      let mut _20: Union32;                // in scope 0 at $DIR/combine_transmutes.rs:+10:29: +10:47
-      let mut _22: Union32;                // in scope 0 at $DIR/combine_transmutes.rs:+11:29: +11:47
-      let mut _24: std::mem::MaybeUninit<std::string::String>; // in scope 0 at $DIR/combine_transmutes.rs:+12:46: +12:77
+      let mut _2: std::option::Option<std::num::NonZeroU8>; // in scope 0 at $DIR/combine_transmutes.rs:+1:28: +1:58
+      let mut _4: std::num::Wrapping<i16>; // in scope 0 at $DIR/combine_transmutes.rs:+2:29: +2:54
+      let mut _6: std::num::Wrapping<i16>; // in scope 0 at $DIR/combine_transmutes.rs:+3:29: +3:54
+      let mut _8: Union32;                 // in scope 0 at $DIR/combine_transmutes.rs:+4:29: +4:47
+      let mut _10: Union32;                // in scope 0 at $DIR/combine_transmutes.rs:+5:29: +5:47
+      let mut _12: std::mem::MaybeUninit<std::string::String>; // in scope 0 at $DIR/combine_transmutes.rs:+6:46: +6:77
       scope 1 {
           debug _a => _1;                  // in scope 1 at $DIR/combine_transmutes.rs:+1:9: +1:11
-          let _3: i8;                      // in scope 1 at $DIR/combine_transmutes.rs:+2:9: +2:11
+          let _3: i16;                     // in scope 1 at $DIR/combine_transmutes.rs:+2:9: +2:11
           scope 2 {
               debug _a => _3;              // in scope 2 at $DIR/combine_transmutes.rs:+2:9: +2:11
-              let _5: usize;               // in scope 2 at $DIR/combine_transmutes.rs:+3:9: +3:11
+              let _5: u16;                 // in scope 2 at $DIR/combine_transmutes.rs:+3:9: +3:11
               scope 3 {
                   debug _a => _5;          // in scope 3 at $DIR/combine_transmutes.rs:+3:9: +3:11
-                  let _7: isize;           // in scope 3 at $DIR/combine_transmutes.rs:+4:9: +4:11
+                  let _7: u32;             // in scope 3 at $DIR/combine_transmutes.rs:+4:9: +4:11
                   scope 4 {
                       debug _a => _7;      // in scope 4 at $DIR/combine_transmutes.rs:+4:9: +4:11
-                      let _9: u8;          // in scope 4 at $DIR/combine_transmutes.rs:+5:9: +5:11
+                      let _9: i32;         // in scope 4 at $DIR/combine_transmutes.rs:+5:9: +5:11
                       scope 5 {
                           debug _a => _9;  // in scope 5 at $DIR/combine_transmutes.rs:+5:9: +5:11
-                          let _11: i8;     // in scope 5 at $DIR/combine_transmutes.rs:+6:9: +6:11
+                          let _11: std::mem::ManuallyDrop<std::string::String>; // in scope 5 at $DIR/combine_transmutes.rs:+6:9: +6:11
                           scope 6 {
                               debug _a => _11; // in scope 6 at $DIR/combine_transmutes.rs:+6:9: +6:11
-                              let _13: u8; // in scope 6 at $DIR/combine_transmutes.rs:+7:9: +7:11
-                              scope 7 {
-                                  debug _a => _13; // in scope 7 at $DIR/combine_transmutes.rs:+7:9: +7:11
-                                  let _15: i16; // in scope 7 at $DIR/combine_transmutes.rs:+8:9: +8:11
-                                  scope 8 {
-                                      debug _a => _15; // in scope 8 at $DIR/combine_transmutes.rs:+8:9: +8:11
-                                      let _17: u16; // in scope 8 at $DIR/combine_transmutes.rs:+9:9: +9:11
-                                      scope 9 {
-                                          debug _a => _17; // in scope 9 at $DIR/combine_transmutes.rs:+9:9: +9:11
-                                          let _19: u32; // in scope 9 at $DIR/combine_transmutes.rs:+10:9: +10:11
-                                          scope 10 {
-                                              debug _a => _19; // in scope 10 at $DIR/combine_transmutes.rs:+10:9: +10:11
-                                              let _21: i32; // in scope 10 at $DIR/combine_transmutes.rs:+11:9: +11:11
-                                              scope 11 {
-                                                  debug _a => _21; // in scope 11 at $DIR/combine_transmutes.rs:+11:9: +11:11
-                                                  let _23: std::mem::ManuallyDrop<std::string::String>; // in scope 11 at $DIR/combine_transmutes.rs:+12:9: +12:11
-                                                  scope 12 {
-                                                      debug _a => _23; // in scope 12 at $DIR/combine_transmutes.rs:+12:9: +12:11
-                                                  }
-                                              }
-                                          }
-                                      }
-                                  }
-                              }
                           }
                       }
                   }
@@ -66,93 +36,55 @@
   
       bb0: {
           StorageLive(_1);                 // scope 0 at $DIR/combine_transmutes.rs:+1:9: +1:11
-          StorageLive(_2);                 // scope 0 at $DIR/combine_transmutes.rs:+1:28: +1:41
-          _2 = EnumNoRepr::A;              // scope 0 at $DIR/combine_transmutes.rs:+1:28: +1:41
-          _1 = move _2 as u8 (Transmute);  // scope 0 at $DIR/combine_transmutes.rs:+1:18: +1:42
-          StorageDead(_2);                 // scope 0 at $DIR/combine_transmutes.rs:+1:41: +1:42
+          StorageLive(_2);                 // scope 0 at $DIR/combine_transmutes.rs:+1:28: +1:58
+          _2 = Option::<NonZeroU8>::Some(const _); // scope 0 at $DIR/combine_transmutes.rs:+1:28: +1:58
+                                           // mir::Constant
+                                           // + span: $DIR/combine_transmutes.rs:35:33: 35:57
+                                           // + literal: Const { ty: NonZeroU8, val: Unevaluated(NonZeroU8::MAX, [], None) }
+          _1 = move _2 as u8 (Transmute);  // scope 0 at $DIR/combine_transmutes.rs:+1:18: +1:59
+          StorageDead(_2);                 // scope 0 at $DIR/combine_transmutes.rs:+1:58: +1:59
           StorageLive(_3);                 // scope 1 at $DIR/combine_transmutes.rs:+2:9: +2:11
-          StorageLive(_4);                 // scope 1 at $DIR/combine_transmutes.rs:+2:28: +2:41
-          _4 = EnumNoRepr::B;              // scope 1 at $DIR/combine_transmutes.rs:+2:28: +2:41
-          _3 = move _4 as i8 (Transmute);  // scope 1 at $DIR/combine_transmutes.rs:+2:18: +2:42
-          StorageDead(_4);                 // scope 1 at $DIR/combine_transmutes.rs:+2:41: +2:42
+          StorageLive(_4);                 // scope 1 at $DIR/combine_transmutes.rs:+2:29: +2:54
+          _4 = Wrapping::<i16>(const 0_i16); // scope 1 at $DIR/combine_transmutes.rs:+2:29: +2:54
+-         _3 = move _4 as i16 (Transmute); // scope 1 at $DIR/combine_transmutes.rs:+2:19: +2:55
++         _3 = move (_4.0: i16);           // scope 1 at $DIR/combine_transmutes.rs:+2:19: +2:55
+          StorageDead(_4);                 // scope 1 at $DIR/combine_transmutes.rs:+2:54: +2:55
           StorageLive(_5);                 // scope 2 at $DIR/combine_transmutes.rs:+3:9: +3:11
-          StorageLive(_6);                 // scope 2 at $DIR/combine_transmutes.rs:+3:31: +3:47
-          _6 = EnumReprIsize::A;           // scope 2 at $DIR/combine_transmutes.rs:+3:31: +3:47
-          _5 = move _6 as usize (Transmute); // scope 2 at $DIR/combine_transmutes.rs:+3:21: +3:48
-          StorageDead(_6);                 // scope 2 at $DIR/combine_transmutes.rs:+3:47: +3:48
+          StorageLive(_6);                 // scope 2 at $DIR/combine_transmutes.rs:+3:29: +3:54
+          _6 = Wrapping::<i16>(const 0_i16); // scope 2 at $DIR/combine_transmutes.rs:+3:29: +3:54
+          _5 = move _6 as u16 (Transmute); // scope 2 at $DIR/combine_transmutes.rs:+3:19: +3:55
+          StorageDead(_6);                 // scope 2 at $DIR/combine_transmutes.rs:+3:54: +3:55
           StorageLive(_7);                 // scope 3 at $DIR/combine_transmutes.rs:+4:9: +4:11
-          StorageLive(_8);                 // scope 3 at $DIR/combine_transmutes.rs:+4:31: +4:47
-          _8 = EnumReprIsize::B;           // scope 3 at $DIR/combine_transmutes.rs:+4:31: +4:47
--         _7 = move _8 as isize (Transmute); // scope 3 at $DIR/combine_transmutes.rs:+4:21: +4:48
-+         _7 = discriminant(_8);           // scope 3 at $DIR/combine_transmutes.rs:+4:21: +4:48
+          StorageLive(_8);                 // scope 3 at $DIR/combine_transmutes.rs:+4:29: +4:47
+          _8 = Union32 { u32: const 0_i32 }; // scope 3 at $DIR/combine_transmutes.rs:+4:29: +4:47
+          _7 = move _8 as u32 (Transmute); // scope 3 at $DIR/combine_transmutes.rs:+4:19: +4:48
           StorageDead(_8);                 // scope 3 at $DIR/combine_transmutes.rs:+4:47: +4:48
           StorageLive(_9);                 // scope 4 at $DIR/combine_transmutes.rs:+5:9: +5:11
-          StorageLive(_10);                // scope 4 at $DIR/combine_transmutes.rs:+5:28: +5:52
-          _10 = Less;                      // scope 4 at $DIR/combine_transmutes.rs:+5:28: +5:52
-          _9 = move _10 as u8 (Transmute); // scope 4 at $DIR/combine_transmutes.rs:+5:18: +5:53
-          StorageDead(_10);                // scope 4 at $DIR/combine_transmutes.rs:+5:52: +5:53
+          StorageLive(_10);                // scope 4 at $DIR/combine_transmutes.rs:+5:29: +5:47
+          _10 = Union32 { u32: const 0_u32 }; // scope 4 at $DIR/combine_transmutes.rs:+5:29: +5:47
+          _9 = move _10 as i32 (Transmute); // scope 4 at $DIR/combine_transmutes.rs:+5:19: +5:48
+          StorageDead(_10);                // scope 4 at $DIR/combine_transmutes.rs:+5:47: +5:48
           StorageLive(_11);                // scope 5 at $DIR/combine_transmutes.rs:+6:9: +6:11
-          StorageLive(_12);                // scope 5 at $DIR/combine_transmutes.rs:+6:28: +6:52
-          _12 = Less;                      // scope 5 at $DIR/combine_transmutes.rs:+6:28: +6:52
--         _11 = move _12 as i8 (Transmute); // scope 5 at $DIR/combine_transmutes.rs:+6:18: +6:53
-+         _11 = discriminant(_12);         // scope 5 at $DIR/combine_transmutes.rs:+6:18: +6:53
-          StorageDead(_12);                // scope 5 at $DIR/combine_transmutes.rs:+6:52: +6:53
-          StorageLive(_13);                // scope 6 at $DIR/combine_transmutes.rs:+7:9: +7:11
-          StorageLive(_14);                // scope 6 at $DIR/combine_transmutes.rs:+7:28: +7:58
-          _14 = Option::<NonZeroU8>::Some(const _); // scope 6 at $DIR/combine_transmutes.rs:+7:28: +7:58
-                                           // mir::Constant
-                                           // + span: $DIR/combine_transmutes.rs:41:33: 41:57
-                                           // + literal: Const { ty: NonZeroU8, val: Unevaluated(NonZeroU8::MAX, [], None) }
-          _13 = move _14 as u8 (Transmute); // scope 6 at $DIR/combine_transmutes.rs:+7:18: +7:59
-          StorageDead(_14);                // scope 6 at $DIR/combine_transmutes.rs:+7:58: +7:59
-          StorageLive(_15);                // scope 7 at $DIR/combine_transmutes.rs:+8:9: +8:11
-          StorageLive(_16);                // scope 7 at $DIR/combine_transmutes.rs:+8:29: +8:54
-          _16 = Wrapping::<i16>(const 0_i16); // scope 7 at $DIR/combine_transmutes.rs:+8:29: +8:54
--         _15 = move _16 as i16 (Transmute); // scope 7 at $DIR/combine_transmutes.rs:+8:19: +8:55
-+         _15 = move (_16.0: i16);         // scope 7 at $DIR/combine_transmutes.rs:+8:19: +8:55
-          StorageDead(_16);                // scope 7 at $DIR/combine_transmutes.rs:+8:54: +8:55
-          StorageLive(_17);                // scope 8 at $DIR/combine_transmutes.rs:+9:9: +9:11
-          StorageLive(_18);                // scope 8 at $DIR/combine_transmutes.rs:+9:29: +9:54
-          _18 = Wrapping::<i16>(const 0_i16); // scope 8 at $DIR/combine_transmutes.rs:+9:29: +9:54
-          _17 = move _18 as u16 (Transmute); // scope 8 at $DIR/combine_transmutes.rs:+9:19: +9:55
-          StorageDead(_18);                // scope 8 at $DIR/combine_transmutes.rs:+9:54: +9:55
-          StorageLive(_19);                // scope 9 at $DIR/combine_transmutes.rs:+10:9: +10:11
-          StorageLive(_20);                // scope 9 at $DIR/combine_transmutes.rs:+10:29: +10:47
-          _20 = Union32 { u32: const 0_i32 }; // scope 9 at $DIR/combine_transmutes.rs:+10:29: +10:47
-          _19 = move _20 as u32 (Transmute); // scope 9 at $DIR/combine_transmutes.rs:+10:19: +10:48
-          StorageDead(_20);                // scope 9 at $DIR/combine_transmutes.rs:+10:47: +10:48
-          StorageLive(_21);                // scope 10 at $DIR/combine_transmutes.rs:+11:9: +11:11
-          StorageLive(_22);                // scope 10 at $DIR/combine_transmutes.rs:+11:29: +11:47
-          _22 = Union32 { u32: const 0_u32 }; // scope 10 at $DIR/combine_transmutes.rs:+11:29: +11:47
-          _21 = move _22 as i32 (Transmute); // scope 10 at $DIR/combine_transmutes.rs:+11:19: +11:48
-          StorageDead(_22);                // scope 10 at $DIR/combine_transmutes.rs:+11:47: +11:48
-          StorageLive(_23);                // scope 11 at $DIR/combine_transmutes.rs:+12:9: +12:11
-          StorageLive(_24);                // scope 11 at $DIR/combine_transmutes.rs:+12:46: +12:77
-          _24 = MaybeUninit::<String>::uninit() -> [return: bb1, unwind unreachable]; // scope 11 at $DIR/combine_transmutes.rs:+12:46: +12:77
+          StorageLive(_12);                // scope 5 at $DIR/combine_transmutes.rs:+6:46: +6:77
+          _12 = MaybeUninit::<String>::uninit() -> [return: bb1, unwind unreachable]; // scope 5 at $DIR/combine_transmutes.rs:+6:46: +6:77
                                            // mir::Constant
-                                           // + span: $DIR/combine_transmutes.rs:46:46: 46:75
-                                           // + user_ty: UserType(23)
+                                           // + span: $DIR/combine_transmutes.rs:40:46: 40:75
+                                           // + user_ty: UserType(11)
                                            // + literal: Const { ty: fn() -> MaybeUninit<String> {MaybeUninit::<String>::uninit}, val: Value(<ZST>) }
       }
   
       bb1: {
--         _23 = move _24 as std::mem::ManuallyDrop<std::string::String> (Transmute); // scope 11 at $DIR/combine_transmutes.rs:+12:36: +12:78
-+         _23 = move (_24.1: std::mem::ManuallyDrop<std::string::String>); // scope 11 at $DIR/combine_transmutes.rs:+12:36: +12:78
-          StorageDead(_24);                // scope 11 at $DIR/combine_transmutes.rs:+12:77: +12:78
-          _0 = const ();                   // scope 0 at $DIR/combine_transmutes.rs:+0:32: +13:2
-          StorageDead(_23);                // scope 11 at $DIR/combine_transmutes.rs:+13:1: +13:2
-          StorageDead(_21);                // scope 10 at $DIR/combine_transmutes.rs:+13:1: +13:2
-          StorageDead(_19);                // scope 9 at $DIR/combine_transmutes.rs:+13:1: +13:2
-          StorageDead(_17);                // scope 8 at $DIR/combine_transmutes.rs:+13:1: +13:2
-          StorageDead(_15);                // scope 7 at $DIR/combine_transmutes.rs:+13:1: +13:2
-          StorageDead(_13);                // scope 6 at $DIR/combine_transmutes.rs:+13:1: +13:2
-          StorageDead(_11);                // scope 5 at $DIR/combine_transmutes.rs:+13:1: +13:2
-          StorageDead(_9);                 // scope 4 at $DIR/combine_transmutes.rs:+13:1: +13:2
-          StorageDead(_7);                 // scope 3 at $DIR/combine_transmutes.rs:+13:1: +13:2
-          StorageDead(_5);                 // scope 2 at $DIR/combine_transmutes.rs:+13:1: +13:2
-          StorageDead(_3);                 // scope 1 at $DIR/combine_transmutes.rs:+13:1: +13:2
-          StorageDead(_1);                 // scope 0 at $DIR/combine_transmutes.rs:+13:1: +13:2
-          return;                          // scope 0 at $DIR/combine_transmutes.rs:+13:2: +13:2
+-         _11 = move _12 as std::mem::ManuallyDrop<std::string::String> (Transmute); // scope 5 at $DIR/combine_transmutes.rs:+6:36: +6:78
++         _11 = move (_12.1: std::mem::ManuallyDrop<std::string::String>); // scope 5 at $DIR/combine_transmutes.rs:+6:36: +6:78
+          StorageDead(_12);                // scope 5 at $DIR/combine_transmutes.rs:+6:77: +6:78
+          _0 = const ();                   // scope 0 at $DIR/combine_transmutes.rs:+0:32: +7:2
+          StorageDead(_11);                // scope 5 at $DIR/combine_transmutes.rs:+7:1: +7:2
+          StorageDead(_9);                 // scope 4 at $DIR/combine_transmutes.rs:+7:1: +7:2
+          StorageDead(_7);                 // scope 3 at $DIR/combine_transmutes.rs:+7:1: +7:2
+          StorageDead(_5);                 // scope 2 at $DIR/combine_transmutes.rs:+7:1: +7:2
+          StorageDead(_3);                 // scope 1 at $DIR/combine_transmutes.rs:+7:1: +7:2
+          StorageDead(_1);                 // scope 0 at $DIR/combine_transmutes.rs:+7:1: +7:2
+          return;                          // scope 0 at $DIR/combine_transmutes.rs:+7:2: +7:2
       }
   }
   
diff --git a/tests/mir-opt/combine_transmutes.rs b/tests/mir-opt/combine_transmutes.rs
index 7088488c1b8..403f9356ce2 100644
--- a/tests/mir-opt/combine_transmutes.rs
+++ b/tests/mir-opt/combine_transmutes.rs
@@ -32,12 +32,6 @@ pub unsafe fn integer_transmutes() {
 
 // EMIT_MIR combine_transmutes.adt_transmutes.InstSimplify.diff
 pub unsafe fn adt_transmutes() {
-    let _a: u8 = transmute(EnumNoRepr::A);
-    let _a: i8 = transmute(EnumNoRepr::B);
-    let _a: usize = transmute(EnumReprIsize::A);
-    let _a: isize = transmute(EnumReprIsize::B);
-    let _a: u8 = transmute(std::cmp::Ordering::Less);
-    let _a: i8 = transmute(std::cmp::Ordering::Less);
     let _a: u8 = transmute(Some(std::num::NonZeroU8::MAX));
     let _a: i16 = transmute(std::num::Wrapping(0_i16));
     let _a: u16 = transmute(std::num::Wrapping(0_i16));
@@ -46,20 +40,4 @@ pub unsafe fn adt_transmutes() {
     let _a: ManuallyDrop<String> = transmute(MaybeUninit::<String>::uninit());
 }
 
-#[inline(always)]
-#[custom_mir(dialect = "runtime", phase = "initial")]
-const unsafe fn mir_transmute<T, U>(x: T) -> U {
-    mir!{
-        {
-            RET = CastTransmute(x);
-            Return()
-        }
-    }
-}
-
-pub enum EnumNoRepr { A, B, C }
-
-#[repr(isize)]
-pub enum EnumReprIsize { A, B, C }
-
 pub union Union32 { u32: u32, i32: i32 }