fix panic on failure to format generics in enum

- instead of calling unwrap(), restore original snippet when we fail to format generics in enum
- we need to propagate this rewrite failure later

Author: ding-young

Diff for: src/items.rs

@@ -558,9 +558,13 @@ impl<'a> FmtVisitor<'a> {
             // make a span that starts right after `enum Foo`
             mk_sp(ident.span.hi(), body_start),
             last_line_width(&enum_header),
-        )
-        .unwrap();
-        self.push_str(&generics_str);
+        );
+
+        if let Some(generics_str) = generics_str {
+            self.push_str(&generics_str);
+        } else {
+            self.push_str(self.snippet(mk_sp(ident.span.hi(), body_start)));
+        }
 
         self.last_pos = body_start;
 

Diff for: tests/source/issue-5738/enum_with_generics.rs

@@ -0,0 +1,40 @@
+enum En4<'x1, 'x2, T: Tr1<As1:  >> {
+    V0,
+    V1,
+}
+
+enum _En5<'x1, 'x2, T: Tr1<As1:  >> {
+    _V0,
+    _V1,
+}
+
+enum En6
+where
+    T: Tr1<En2<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S>>>>>>>>>>>>>>>>>>>>>>>>,
+{
+    V0,
+    V1,
+    V2,
+    V3,
+}
+
+enum _En7 
+where
+    T: ,
+{
+    V0,
+    V1,
+}
+
+fn _make_en7()
+where
+    T: ,
+{
+    
+}
+
+enum EnSelf<T> where Self: Tr1<As1: > {
+    V0(T),
+    V1,
+    V2,
+}

Diff for: tests/target/issue-5738/enum_with_generics.rs

@@ -0,0 +1,42 @@
+enum En4<'x1, 'x2, T: Tr1<As1: >> {
+    V0,
+    V1,
+}
+
+enum _En5<'x1, 'x2, T: Tr1<As1: >> {
+    _V0,
+    _V1,
+}
+
+enum En6
+where
+    T: Tr1<En2<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S>>>>>>>>>>>>>>>>>>>>>>>>,
+{
+    V0,
+    V1,
+    V2,
+    V3,
+}
+
+enum _En7
+where
+    T:,
+{
+    V0,
+    V1,
+}
+
+fn _make_en7()
+where
+    T:,
+{
+}
+
+enum EnSelf<T>
+where
+    Self: Tr1<As1: >,
+{
+    V0(T),
+    V1,
+    V2,
+}