Fix ICE in read_discriminant for enums with non-contiguous discriminants

Author: enthropy7

compiler/rustc_const_eval/src/interpret/discriminant.rs

@@ -121,20 +121,20 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
                 // discriminants are int-like.
                 let discr_val = self.int_to_int_or_float(&tag_val, discr_layout).unwrap();
                 let discr_bits = discr_val.to_scalar().to_bits(discr_layout.size)?;
-                // Convert discriminant to variant index. Since we validated the tag against the
-                // layout range above, this cannot fail.
+                // Convert discriminant to variant index. The tag may pass the layout range
+                // check above but still not match any actual variant discriminant (e.g.,
+                // non-contiguous discriminants with a wrapping valid_range).
                 let index = match *ty.kind() {
                     ty::Adt(adt, _) => {
-                        adt.discriminants(*self.tcx).find(|(_, var)| var.val == discr_bits).unwrap()
+                        adt.discriminants(*self.tcx).find(|(_, var)| var.val == discr_bits)
                     }
                     ty::Coroutine(def_id, args) => {
                         let args = args.as_coroutine();
-                        args.discriminants(def_id, *self.tcx)
-                            .find(|(_, var)| var.val == discr_bits)
-                            .unwrap()
+                        args.discriminants(def_id, *self.tcx).find(|(_, var)| var.val == discr_bits)
                     }
                     _ => span_bug!(self.cur_span(), "tagged layout for non-adt non-coroutine"),
-                };
+                }
+                .ok_or_else(|| err_ub!(InvalidTag(Scalar::from_uint(tag_bits, tag_layout.size))))?;
                 // Return the cast value, and the index.
                 index.0
             }

tests/ui/consts/const-eval/ub-enum.rs

@@ -108,4 +108,17 @@ const TEST_ICE_89765: () = {
     };
 };
 
+// # Regression test for https://github.com/rust-lang/rust/issues/153758
+// Discriminants at i64::MIN and i64::MAX produce a wrapping valid_range that covers
+// all values. A value like 0 passes the range check but doesn't match any variant.
+#[repr(i64)]
+#[derive(Copy, Clone)]
+enum WideRangeDiscriminants {
+    A = i64::MIN,
+    B = i64::MAX,
+}
+
+const BAD_WIDE_RANGE_ENUM: WideRangeDiscriminants = unsafe { mem::transmute(0_i64) };
+//~^ ERROR expected a valid enum tag
+
 fn main() {}

tests/ui/consts/const-eval/ub-enum.stderr

@@ -129,6 +129,17 @@ LL |         std::mem::discriminant(&*(&() as *const () as *const Never));
 note: inside `discriminant::<Never>`
   --> $SRC_DIR/core/src/mem/mod.rs:LL:COL
 
-error: aborting due to 14 previous errors
+error[E0080]: constructing invalid value of type WideRangeDiscriminants: at .<enum-tag>, encountered 0x0, but expected a valid enum tag
+  --> $DIR/ub-enum.rs:121:1
+   |
+LL | const BAD_WIDE_RANGE_ENUM: WideRangeDiscriminants = unsafe { mem::transmute(0_i64) };
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ it is undefined behavior to use this value
+   |
+   = note: the rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
+   = note: the raw bytes of the constant (size: $SIZE, align: $ALIGN) {
+               HEX_DUMP
+           }
+
+error: aborting due to 15 previous errors
 
 For more information about this error, try `rustc --explain E0080`.