Do not project to uninhabited variant in JumpThreading, const printing

Author: compiler-errors

compiler/rustc_const_eval/src/const_eval/mod.rs

@@ -89,6 +89,9 @@ pub(crate) fn try_destructure_mir_constant_for_user_output<'tcx>(
         }
         ty::Adt(def, _) => {
             let variant = ecx.read_discriminant(&op).ok()?;
+            if op.layout.for_variant(&ecx, variant).abi.is_uninhabited() {
+                return None;
+            }
             let down = ecx.project_downcast(&op, variant).ok()?;
             (def.variants()[variant].fields.len(), Some(variant), down)
         }

compiler/rustc_mir_transform/src/jump_threading.rs

@@ -370,7 +370,12 @@ impl<'tcx, 'a> TOFinder<'tcx, 'a> {
             constant,
             &mut |elem, op| match elem {
                 TrackElem::Field(idx) => self.ecx.project_field(op, idx.as_usize()).ok(),
-                TrackElem::Variant(idx) => self.ecx.project_downcast(op, idx).ok(),
+                TrackElem::Variant(idx) => {
+                    if op.layout.for_variant(&self.ecx, idx).abi.is_uninhabited() {
+                        return None;
+                    }
+                    self.ecx.project_downcast(op, idx).ok()
+                }
                 TrackElem::Discriminant => {
                     let variant = self.ecx.read_discriminant(op).ok()?;
                     let discr_value =

tests/mir-opt/jump_threading_uninhabited.from.JumpThreading.panic-abort.diff

@@ -0,0 +1,13 @@
+- // MIR for `from` before JumpThreading
++ // MIR for `from` after JumpThreading
+  
+  fn from(_1: Wrap<HiddenType>) -> HiddenType {
+      debug _it => const ZeroSized: Wrap<HiddenType>;
+      let mut _0: HiddenType;
+      let mut _2: !;
+  
+      bb0: {
+          _2 = core::panicking::panic(const "not yet implemented") -> unwind unreachable;
+      }
+  }
+  

tests/mir-opt/jump_threading_uninhabited.from.JumpThreading.panic-unwind.diff

@@ -0,0 +1,13 @@
+- // MIR for `from` before JumpThreading
++ // MIR for `from` after JumpThreading
+  
+  fn from(_1: Wrap<HiddenType>) -> HiddenType {
+      debug _it => const ZeroSized: Wrap<HiddenType>;
+      let mut _0: HiddenType;
+      let mut _2: !;
+  
+      bb0: {
+          _2 = core::panicking::panic(const "not yet implemented") -> unwind continue;
+      }
+  }
+  

tests/mir-opt/jump_threading_uninhabited.rs

@@ -0,0 +1,15 @@
+// unit-test: JumpThreading
+// compile-flags: -Zmir-opt-level=1
+// EMIT_MIR_FOR_EACH_PANIC_STRATEGY
+// skip-filecheck
+
+pub enum HiddenType {}
+
+pub struct Wrap<T>(T);
+
+// EMIT_MIR jump_threading_uninhabited.from.JumpThreading.diff
+pub fn from(_it: Wrap<HiddenType>) -> HiddenType {
+    todo!()
+}
+
+fn main() {}