Get all variants to eliminate the default branching if we cannot get the layout of type

Author: dianqk

compiler/rustc_mir_transform/src/uninhabited_enum_branching.rs

@@ -95,6 +95,12 @@ impl<'tcx> MirPass<'tcx> for UninhabitedEnumBranching {
 
             let mut allowed_variants = if let Ok(layout) = layout {
                 variant_discriminants(&layout, discriminant_ty, tcx)
+            } else if let Some(variant_range) = discriminant_ty.variant_range(tcx) {
+                variant_range
+                    .map(|variant| {
+                        discriminant_ty.discriminant_for_variant(tcx, variant).unwrap().val
+                    })
+                    .collect()
             } else {
                 continue;
             };

tests/mir-opt/uninhabited_enum_branching.otherwise_t5_uninhabited_default.UninhabitedEnumBranching.diff

@@ -5,7 +5,7 @@
       let mut _0: ();
       let _1: &str;
       let mut _2: Test5<T>;
-      let mut _3: isize;
+      let mut _3: i8;
       let _4: &str;
       let _5: &str;
       let _6: &str;
@@ -15,7 +15,8 @@
           StorageLive(_2);
           _2 = Test5::<T>::C;
           _3 = discriminant(_2);
-          switchInt(move _3) -> [0: bb2, 1: bb3, 2: bb4, otherwise: bb1];
+-         switchInt(move _3) -> [255: bb2, 0: bb3, 5: bb4, otherwise: bb1];
++         switchInt(move _3) -> [255: bb2, 0: bb3, 5: bb4, 3: bb1, otherwise: bb8];
       }
 
       bb1: {
@@ -60,6 +61,10 @@
 
       bb7 (cleanup): {
           resume;
++     }
++ 
++     bb8: {
++         unreachable;
       }
   }
 

tests/mir-opt/uninhabited_enum_branching.rs

@@ -30,11 +30,12 @@ enum Test4 {
     D,
 }
 
+#[repr(i8)]
 enum Test5<T> {
-    A(T),
-    B(T),
-    C,
-    D,
+    A(T) = -1,
+    B(T) = 0,
+    C = 5,
+    D = 3,
 }
 
 struct Plop {
@@ -163,10 +164,9 @@ fn otherwise_t4() {
 fn otherwise_t5_uninhabited_default<T>() {
     // CHECK-LABEL: fn otherwise_t5_uninhabited_default(
     // CHECK: [[discr:_.*]] = discriminant(
-    // CHECK: switchInt(move [[discr]]) -> [0: bb2, 1: bb3, 2: bb4, otherwise: bb1];
-    // CHECK: bb1: {
-    // CHECK-NOT: unreachable;
-    // CHECK: }
+    // CHECK: switchInt(move [[discr]]) -> [255: bb2, 0: bb3, 5: bb4, 3: bb1, otherwise: [[unreachable:bb.*]]];
+    // CHECK: [[unreachable]]: {
+    // CHECK-NEXT: unreachable;
     match Test5::<T>::C {
         Test5::A(_) => "A(T)",
         Test5::B(_) => "B(T)",