From 2922dd5c8758ae4d351a09118d84b65d06f42734 Mon Sep 17 00:00:00 2001
From: Ted Driggs <ted.driggs@outlook.com>
Date: Wed, 10 Feb 2021 10:46:49 -0800
Subject: [PATCH] Remove use of unreachable for unions

Fixes #85
---
 core/src/ast/data.rs | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/core/src/ast/data.rs b/core/src/ast/data.rs
index 69a075e..5d9fb36 100644
--- a/core/src/ast/data.rs
+++ b/core/src/ast/data.rs
@@ -20,11 +20,14 @@ pub enum Data<V, F> {
 
 impl<V, F> Data<V, F> {
     /// Creates an empty body of the same shape as the passed-in body.
+    ///
+    /// # Panics
+    /// This function will panic if passed `syn::Data::Union`.
     pub fn empty_from(src: &syn::Data) -> Self {
         match *src {
             syn::Data::Enum(_) => Data::Enum(vec![]),
             syn::Data::Struct(ref vd) => Data::Struct(Fields::empty_from(&vd.fields)),
-            syn::Data::Union(_) => unreachable!(),
+            syn::Data::Union(_) => panic!("Unions are not supported"),
         }
     }
 
@@ -120,7 +123,10 @@ impl<V: FromVariant, F: FromField> Data<V, F> {
                 }
             }
             syn::Data::Struct(ref data) => Ok(Data::Struct(Fields::try_from(&data.fields)?)),
-            syn::Data::Union(_) => unreachable!(),
+            // This deliberately doesn't set a span on the error message, as the error is most useful if
+            // applied to the call site of the offending macro. Given that the message is very generic,
+            // putting it on the union keyword ends up being confusing.
+            syn::Data::Union(_) => Err(Error::custom("Unions are not supported")),
         }
     }
 }