Rollup merge of #73442 - davidtwco:issue-72181-pretty-print-const-val-enum-no-variants, r=oli-obk

pretty/mir: const value enums with no variants

Fixes #72181.

This PR 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).

I'm not convinced that this is the correct approach, folks more familiar with `destructure_const` would be able to say - happy to adjust the PR. Looking through `destructure_const` and the functions that it invokes, it didn't seem like it was written to handle zero-variant-enums - I assume that case is handled earlier in some way so `destructure_const` doesn't need to under normal circumstances. It didn't seem like it would be straightforward to make `destructure_const` handle this case in a first-class-feeling way (e.g. adding a `Variants::None` variant), so this PR makes some minimal changes to avoid ICEs.

4 files changed, 12 insertions, 1 deletions

diff --git a/src/librustc_middle/ty/layout.rs b/src/librustc_middle/ty/layout.rs
index 80f919d0c03..68af22569e3 100644
--- a/src/librustc_middle/ty/layout.rs
+++ b/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!(),
                 };
diff --git a/src/librustc_middle/ty/mod.rs b/src/librustc_middle/ty/mod.rs
index e26e09ae6a9..ca0a1ac71c6 100644
--- a/src/librustc_middle/ty/mod.rs
+++ b/src/librustc_middle/ty/mod.rs
@@ -2352,6 +2352,7 @@ 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 {
diff --git a/src/librustc_middle/ty/print/pretty.rs b/src/librustc_middle/ty/print/pretty.rs
index 17203fcce5e..1a08639a533 100644
--- a/src/librustc_middle/ty/print/pretty.rs
+++ b/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 {
diff --git a/src/librustc_middle/ty/sty.rs b/src/librustc_middle/ty/sty.rs
index cf11adb0285..b0addcb2bb6 100644
--- a/src/librustc_middle/ty/sty.rs
+++ b/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))
             }