From 41068e543530a23bf329ed905c821387a185a462 Mon Sep 17 00:00:00 2001
From: David Tolnay <dtolnay@gmail.com>
Date: Sat, 12 Oct 2019 13:28:04 -0700
Subject: [PATCH] Variants inherit fmt attr from the enum

---
 impl/src/ast.rs       | 20 ++++++++++++++------
 impl/src/attr.rs      |  1 +
 tests/test_display.rs | 10 ++++++++++
 3 files changed, 25 insertions(+), 6 deletions(-)

diff --git a/impl/src/ast.rs b/impl/src/ast.rs
index f68820e..bc4e90a 100644
--- a/impl/src/ast.rs
+++ b/impl/src/ast.rs
@@ -63,15 +63,23 @@ impl<'a> Struct<'a> {
 
 impl<'a> Enum<'a> {
     fn from_syn(node: &'a DeriveInput, data: &'a DataEnum) -> Result<Self> {
+        let attrs = attr::get(&node.attrs)?;
+        let variants = data
+            .variants
+            .iter()
+            .map(|node| {
+                let mut variant = Variant::from_syn(node)?;
+                if let display @ None = &mut variant.attrs.display {
+                    *display = attrs.display.clone();
+                }
+                Ok(variant)
+            })
+            .collect::<Result<_>>()?;
         Ok(Enum {
-            attrs: attr::get(&node.attrs)?,
+            attrs,
             ident: node.ident.clone(),
             generics: &node.generics,
-            variants: data
-                .variants
-                .iter()
-                .map(Variant::from_syn)
-                .collect::<Result<_>>()?,
+            variants,
         })
     }
 }
diff --git a/impl/src/attr.rs b/impl/src/attr.rs
index e741c64..93e95dd 100644
--- a/impl/src/attr.rs
+++ b/impl/src/attr.rs
@@ -12,6 +12,7 @@ pub struct Attrs<'a> {
     pub source: Option<Source<'a>>,
 }
 
+#[derive(Clone)]
 pub struct Display {
     pub fmt: LitStr,
     pub args: TokenStream,
diff --git a/tests/test_display.rs b/tests/test_display.rs
index c7bb857..50a59e2 100644
--- a/tests/test_display.rs
+++ b/tests/test_display.rs
@@ -31,6 +31,14 @@ enum EnumError {
     Unit,
 }
 
+#[derive(Error, Debug)]
+#[error("{0}")]
+enum Inherit {
+    Unit(UnitError),
+    #[error("other error")]
+    Other(UnitError),
+}
+
 #[derive(Error, Debug)]
 #[error("1 + 1 = {}", 1 + 1)]
 struct Arithmetic;
@@ -57,6 +65,8 @@ fn test_display() {
     assert("braced error: 0", EnumError::Braced { id: 0 });
     assert("tuple error: 0", EnumError::Tuple(0));
     assert("unit error", EnumError::Unit);
+    assert("unit error", Inherit::Unit(UnitError));
+    assert("other error", Inherit::Other(UnitError));
     assert("1 + 1 = 2", Arithmetic);
     assert("!bool = false", NestedShorthand(true));
 }