pretty/mir: const value enums with no variants

This commit modifies the pretty printer and const eval in the MIR so
that `destructure_const` (used in `pretty_print_const_value`) can handle
enums with no variants (or types containing enums with no variants).

Signed-off-by: David Wood <david@davidtw.co>

Author: davidtwco

src/librustc_middle/mir/query.rs

@@ -244,6 +244,6 @@ pub enum ClosureOutlivesSubject<'tcx> {
 /// The constituent parts of an ADT or array.
 #[derive(Copy, Clone, Debug, HashStable)]
 pub struct DestructuredConst<'tcx> {
-    pub variant: VariantIdx,
+    pub variant: Option<VariantIdx>,
     pub fields: &'tcx [&'tcx ty::Const<'tcx>],
 }

src/librustc_middle/ty/layout.rs

@@ -2001,6 +2001,8 @@ where
                 }
 
                 let fields = match this.ty.kind {
+                    ty::Adt(def, _) if def.variants.is_empty() =>
+                        bug!("for_variant called on zero-variant enum"),
                     ty::Adt(def, _) => def.variants[variant_index].fields.len(),
                     _ => bug!(),
                 };

src/librustc_middle/ty/mod.rs

@@ -2339,6 +2339,7 @@ impl<'tcx> AdtDef {
     /// Alternatively, if there is no explicit discriminant, returns the
     /// inferred discriminant directly.
     pub fn discriminant_def_for_variant(&self, variant_index: VariantIdx) -> (Option<DefId>, u32) {
+        assert!(!self.variants.is_empty());
         let mut explicit_index = variant_index.as_u32();
         let expr_did;
         loop {

src/librustc_middle/ty/print/pretty.rs

@@ -1177,8 +1177,13 @@ pub trait PrettyPrinter<'tcx>:
                         }
                         p!(write(")"));
                     }
+                    ty::Adt(def, substs) if def.variants.is_empty() => {
+                        p!(print_value_path(def.did, substs));
+                    }
                     ty::Adt(def, substs) => {
-                        let variant_def = &def.variants[contents.variant];
+                        let variant_id =
+                            contents.variant.expect("destructed const of adt without variant id");
+                        let variant_def = &def.variants[variant_id];
                         p!(print_value_path(variant_def.def_id, substs));
 
                         match variant_def.ctor_kind {

src/librustc_middle/ty/sty.rs

@@ -2099,6 +2099,9 @@ impl<'tcx> TyS<'tcx> {
         variant_index: VariantIdx,
     ) -> Option<Discr<'tcx>> {
         match self.kind {
+            TyKind::Adt(adt, _) if adt.variants.is_empty() => {
+                bug!("discriminant_for_variant called on zero variant enum");
+            }
             TyKind::Adt(adt, _) if adt.is_enum() => {
                 Some(adt.discriminant_for_variant(tcx, variant_index))
             }

src/librustc_mir/const_eval/mod.rs

@@ -30,8 +30,10 @@ pub(crate) fn const_caller_location(
     ConstValue::Scalar(loc_place.ptr)
 }
 
-// this function uses `unwrap` copiously, because an already validated constant
-// must have valid fields and can thus never fail outside of compiler bugs
+/// This function uses `unwrap` copiously, because an already validated constant
+/// must have valid fields and can thus never fail outside of compiler bugs. However, it is
+/// invoked from the pretty printer, where it can receive enums with no variants and e.g.
+/// `read_discriminant` needs to be able to handle that.
 pub(crate) fn destructure_const<'tcx>(
     tcx: TyCtxt<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
@@ -41,17 +43,21 @@ pub(crate) fn destructure_const<'tcx>(
     let ecx = mk_eval_cx(tcx, DUMMY_SP, param_env, false);
     let op = ecx.eval_const_to_op(val, None).unwrap();
 
-    let variant = ecx.read_discriminant(op).unwrap().1;
-
     // We go to `usize` as we cannot allocate anything bigger anyway.
-    let field_count = match val.ty.kind {
-        ty::Array(_, len) => usize::try_from(len.eval_usize(tcx, param_env)).unwrap(),
-        ty::Adt(def, _) => def.variants[variant].fields.len(),
-        ty::Tuple(substs) => substs.len(),
+    let (field_count, variant, down) = match val.ty.kind {
+        ty::Array(_, len) => (usize::try_from(len.eval_usize(tcx, param_env)).unwrap(), None, op),
+        ty::Adt(def, _) if def.variants.is_empty() => {
+            return mir::DestructuredConst { variant: None, fields: tcx.arena.alloc_slice(&[]) };
+        }
+        ty::Adt(def, _) => {
+            let variant = ecx.read_discriminant(op).unwrap().1;
+            let down = ecx.operand_downcast(op, variant).unwrap();
+            (def.variants[variant].fields.len(), Some(variant), down)
+        }
+        ty::Tuple(substs) => (substs.len(), None, op),
         _ => bug!("cannot destructure constant {:?}", val),
     };
 
-    let down = ecx.operand_downcast(op, variant).unwrap();
     let fields_iter = (0..field_count).map(|i| {
         let field_op = ecx.operand_field(down, i).unwrap();
         let val = op_to_const(&ecx, field_op);

src/librustc_mir_build/hair/pattern/_match.rs

@@ -800,7 +800,11 @@ impl<'tcx> Constructor<'tcx> {
                 assert!(!adt.is_enum());
                 VariantIdx::new(0)
             }
-            ConstantValue(c) => cx.tcx.destructure_const(cx.param_env.and(c)).variant,
+            ConstantValue(c) => cx
+                .tcx
+                .destructure_const(cx.param_env.and(c))
+                .variant
+                .expect("destructed const of adt without variant id"),
             _ => bug!("bad constructor {:?} for adt {:?}", self, adt),
         }
     }

src/librustc_mir_build/hair/pattern/const_to_pat.rs

@@ -275,7 +275,9 @@ impl<'a, 'tcx> ConstToPat<'a, 'tcx> {
                 PatKind::Variant {
                     adt_def,
                     substs,
-                    variant_index: destructured.variant,
+                    variant_index: destructured
+                        .variant
+                        .expect("destructed const of adt without variant id"),
                     subpatterns: field_pats(destructured.fields),
                 }
             }

src/test/mir-opt/issue-72181-1.rs

@@ -0,0 +1,21 @@
+// compile-flags: -Z mir-opt-level=1
+// Regression test for #72181, this ICE requires `-Z mir-opt-level=1` flags.
+
+#![feature(never_type)]
+#![allow(unused, invalid_value)]
+
+enum Void {}
+
+// EMIT_MIR rustc.f.mir_map.0.mir
+fn f(v: Void) -> ! {
+    match v {}
+}
+
+// EMIT_MIR rustc.main.mir_map.0.mir
+fn main() {
+    let v: Void = unsafe {
+        std::mem::transmute::<(), Void>(())
+    };
+
+    f(v);
+}

src/test/mir-opt/issue-72181-1/rustc.f.mir_map.0.mir

@@ -0,0 +1,37 @@
+// MIR for `f` 0 mir_map
+
+fn f(_1: Void) -> ! {
+    debug v => _1;                       // in scope 0 at $DIR/issue-72181-1.rs:10:6: 10:7
+    let mut _0: !;                       // return place in scope 0 at $DIR/issue-72181-1.rs:10:18: 10:19
+    let mut _2: !;                       // in scope 0 at $DIR/issue-72181-1.rs:10:20: 12:2
+    let mut _3: !;                       // in scope 0 at $DIR/issue-72181-1.rs:11:5: 11:15
+
+    bb0: {
+        StorageLive(_2);                 // scope 0 at $DIR/issue-72181-1.rs:10:20: 12:2
+        StorageLive(_3);                 // scope 0 at $DIR/issue-72181-1.rs:11:5: 11:15
+        FakeRead(ForMatchedPlace, _1);   // scope 0 at $DIR/issue-72181-1.rs:11:11: 11:12
+        unreachable;                     // scope 0 at $DIR/issue-72181-1.rs:11:11: 11:12
+    }
+
+    bb1 (cleanup): {
+        resume;                          // scope 0 at $DIR/issue-72181-1.rs:10:1: 12:2
+    }
+
+    bb2: {
+        unreachable;                     // scope 0 at $DIR/issue-72181-1.rs:11:5: 11:15
+    }
+
+    bb3: {
+        StorageDead(_3);                 // scope 0 at $DIR/issue-72181-1.rs:11:14: 11:15
+        unreachable;                     // scope 0 at $DIR/issue-72181-1.rs:10:20: 12:2
+    }
+
+    bb4: {
+        StorageDead(_2);                 // scope 0 at $DIR/issue-72181-1.rs:12:1: 12:2
+        goto -> bb5;                     // scope 0 at $DIR/issue-72181-1.rs:12:2: 12:2
+    }
+
+    bb5: {
+        return;                          // scope 0 at $DIR/issue-72181-1.rs:12:2: 12:2
+    }
+}