diff --git a/src/items.rs b/src/items.rs
index cdf9fa06eef..053e856b9f1 100644
--- a/src/items.rs
+++ b/src/items.rs
@@ -1118,6 +1118,18 @@ pub(crate) fn format_trait(
     }
 }
 
+struct OpaqueTypeBounds<'a> {
+    generic_bounds: &'a ast::GenericBounds,
+}
+
+impl<'a> Rewrite for OpaqueTypeBounds<'a> {
+    fn rewrite(&self, context: &RewriteContext<'_>, shape: Shape) -> Option<String> {
+        self.generic_bounds
+            .rewrite(context, shape)
+            .map(|s| format!("impl {}", s))
+    }
+}
+
 pub(crate) struct TraitAliasBounds<'a> {
     generic_bounds: &'a ast::GenericBounds,
     generics: &'a ast::Generics,
@@ -1524,13 +1536,14 @@ pub(crate) fn rewrite_opaque_type(
     generics: &ast::Generics,
     vis: &ast::Visibility,
 ) -> Option<String> {
+    let opaque_type_bounds = OpaqueTypeBounds { generic_bounds };
     rewrite_type_item(
         context,
         indent,
         "type",
         " =",
         ident,
-        generic_bounds,
+        &opaque_type_bounds,
         generics,
         vis,
     )
diff --git a/tests/source/existential_type.rs b/tests/source/existential_type.rs
index 92412fca879..33bb9a95159 100644
--- a/tests/source/existential_type.rs
+++ b/tests/source/existential_type.rs
@@ -13,3 +13,11 @@ pub type Adderrr<T> = impl Fn(  T  ) -> T;
 impl Foo for Bar {
 type E  = impl Trait;
 }
+
+pub type Adder_without_impl<F, T>
+where
+    T: Clone,
+    F: Copy
+    = Fn(T) -> T;
+
+pub type Adderrr_without_impl<T> = Fn(  T  ) -> T;
diff --git a/tests/target/existential_type.rs b/tests/target/existential_type.rs
index 869ffda2378..ffc206875b5 100644
--- a/tests/target/existential_type.rs
+++ b/tests/target/existential_type.rs
@@ -6,10 +6,18 @@ pub type Adder<F, T>
 where
     T: Clone,
     F: Copy,
-= Fn(T) -> T;
+= impl Fn(T) -> T;
 
-pub type Adderrr<T> = Fn(T) -> T;
+pub type Adderrr<T> = impl Fn(T) -> T;
 
 impl Foo for Bar {
     type E = impl Trait;
 }
+
+pub type Adder_without_impl<F, T>
+where
+    T: Clone,
+    F: Copy,
+= Fn(T) -> T;
+
+pub type Adderrr_without_impl<T> = Fn(T) -> T;
diff --git a/tests/target/issue-3815.rs b/tests/target/issue-3815.rs
new file mode 100644
index 00000000000..eff27e2de3c
--- /dev/null
+++ b/tests/target/issue-3815.rs
@@ -0,0 +1,4 @@
+pub type Type = impl Deref<Target = i8>;
+
+pub type Type =
+    impl VeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeryLongType<Target = i8>;