fix panic on failure to format generics in enum

- related issues: #5738, #6137, #6318, #6378
- instead of calling unwrap(), restore original snippet when we fail to format generics in enum
- we need to propagate this rewrite failure later

src/items.rs

@@ -550,9 +550,13 @@ 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;
 

tests/source/crash-on-enum-generics/issue_5738.rs

@@ -0,0 +1,13 @@
+enum Node where P::<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S>>>>>>>>>>>>>>>>>>>>>>>>:  {
+    Cons,
+}
+
+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,
+}

tests/source/crash-on-enum-generics/issue_6137.rs

@@ -0,0 +1,3 @@
+enum MZReaderType<
+    D: DeconvolutedCentroidLike + Default + From<DeconvolutedPeak> + BuildFromArrayMap=DeconvolutedPeak
+> {}

tests/source/crash-on-enum-generics/issue_6318.rs

@@ -0,0 +1,13 @@
+// rustfmt-max_width: 80
+fn my_fn() {
+    enum MyEnum
+    where
+    SomeTypeA___: SomeTrait__<
+            _A,
+            Archived = <SomeTypeB____ as SomeTrait__<
+                Option<[u8; 4]>,
+            >>::Archived,
+        >,
+    {
+    }
+}

tests/target/crash-on-enum-generics/issue_5738.rs

@@ -0,0 +1,13 @@
+enum Node where P::<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S>>>>>>>>>>>>>>>>>>>>>>>>:  {
+    Cons,
+}
+
+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,
+}

tests/target/crash-on-enum-generics/issue_6137.rs

@@ -0,0 +1,3 @@
+enum MZReaderType<
+    D: DeconvolutedCentroidLike + Default + From<DeconvolutedPeak> + BuildFromArrayMap=DeconvolutedPeak
+> {}

tests/target/crash-on-enum-generics/issue_6318.rs

@@ -0,0 +1,12 @@
+// rustfmt-max_width: 80
+fn my_fn() {
+    enum MyEnum
+    where
+    SomeTypeA___: SomeTrait__<
+            _A,
+            Archived = <SomeTypeB____ as SomeTrait__<
+                Option<[u8; 4]>,
+            >>::Archived,
+        >,
+    {}
+}