interpret: extend comment on the inhabitedness check in downcast

Author: RalfJung

compiler/rustc_const_eval/src/interpret/projection.rs

@@ -208,6 +208,24 @@ where
         if layout.abi.is_uninhabited() {
             // `read_discriminant` should have excluded uninhabited variants... but ConstProp calls
             // us on dead code.
+            // In the future we might want to allow this to permit code like this:
+            // (this is a Rust/MIR pseudocode mix)
+            // ```
+            // enum Option2 {
+            //   Some(i32, !),
+            //   None,
+            // }
+            //
+            // fn panic() -> ! { panic!() }
+            //
+            // let x: Option2;
+            // x.Some.0 = 42;
+            // x.Some.1 = panic();
+            // SetDiscriminant(x, Some);
+            // ```
+            // However, for now we don't generate such MIR, and this check here *has* found real
+            // bugs (see https://github.com/rust-lang/rust/issues/115145), so we will keep rejecting
+            // it.
             throw_inval!(ConstPropNonsense)
         }
         // This cannot be `transmute` as variants *can* have a smaller size than the entire enum.