diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 71a7fade5..4a4b09c0c 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -21,7 +21,7 @@ jobs:
 
         include:
           # Test MSRV
-          - rust: 1.36.0
+          - rust: 1.40.0
             TARGET: x86_64-unknown-linux-gnu
 
           # Test nightly but don't fail
diff --git a/src/de/enum_.rs b/src/de/enum_.rs
index 7e516878e..9012873a5 100644
--- a/src/de/enum_.rs
+++ b/src/de/enum_.rs
@@ -53,3 +53,56 @@ impl<'de, 'a> de::VariantAccess<'de> for UnitVariantAccess<'a, 'de> {
         Err(Error::InvalidType)
     }
 }
+
+pub(crate) struct VariantAccess<'a, 'b> {
+    de: &'a mut Deserializer<'b>,
+}
+
+impl<'a, 'b> VariantAccess<'a, 'b> {
+    pub(crate) fn new(de: &'a mut Deserializer<'b>) -> Self {
+        VariantAccess { de }
+    }
+}
+
+impl<'a, 'de> de::EnumAccess<'de> for VariantAccess<'a, 'de> {
+    type Error = Error;
+    type Variant = Self;
+
+    fn variant_seed<V>(self, seed: V) -> Result<(V::Value, Self)>
+    where
+        V: de::DeserializeSeed<'de>,
+    {
+        let variant = seed.deserialize(&mut *self.de)?;
+        self.de.parse_object_colon()?;
+        Ok((variant, self))
+    }
+}
+
+impl<'de, 'a> de::VariantAccess<'de> for VariantAccess<'a, 'de> {
+    type Error = Error;
+
+    fn unit_variant(self) -> Result<()> {
+        de::Deserialize::deserialize(self.de)
+    }
+
+    fn newtype_variant_seed<T>(self, seed: T) -> Result<T::Value>
+    where
+        T: de::DeserializeSeed<'de>,
+    {
+        seed.deserialize(self.de)
+    }
+
+    fn tuple_variant<V>(self, _len: usize, visitor: V) -> Result<V::Value>
+    where
+        V: de::Visitor<'de>,
+    {
+        de::Deserializer::deserialize_seq(self.de, visitor)
+    }
+
+    fn struct_variant<V>(self, fields: &'static [&'static str], visitor: V) -> Result<V::Value>
+    where
+        V: de::Visitor<'de>,
+    {
+        de::Deserializer::deserialize_struct(self.de, "", fields, visitor)
+    }
+}
diff --git a/src/de/mod.rs b/src/de/mod.rs
index da89308b0..5bed2dcfc 100644
--- a/src/de/mod.rs
+++ b/src/de/mod.rs
@@ -5,7 +5,7 @@ use core::{fmt, str};
 
 use serde::de::{self, Visitor};
 
-use self::enum_::UnitVariantAccess;
+use self::enum_::{UnitVariantAccess, VariantAccess};
 use self::map::MapAccess;
 use self::seq::SeqAccess;
 
@@ -18,6 +18,7 @@ pub type Result<T> = core::result::Result<T, Error>;
 
 /// This type represents all possible errors that can occur when deserializing JSON data
 #[derive(Debug, PartialEq)]
+#[non_exhaustive]
 pub enum Error {
     /// EOF while parsing a list.
     EofWhileParsingList,
@@ -73,9 +74,6 @@ pub enum Error {
     /// Error with a custom message that was preserved.
     #[cfg(feature = "custom-error-messages")]
     CustomErrorWithMessage(heapless::String<heapless::consts::U64>),
-
-    #[doc(hidden)]
-    __Extensible,
 }
 
 #[cfg(feature = "std")]
@@ -498,28 +496,40 @@ impl<'a, 'de> de::Deserializer<'de> for &'a mut Deserializer<'de> {
         }
     }
 
-    /// Unsupported. Use a more specific deserialize_* method
-    fn deserialize_unit<V>(self, _visitor: V) -> Result<V::Value>
+    fn deserialize_unit<V>(self, visitor: V) -> Result<V::Value>
     where
         V: Visitor<'de>,
     {
-        unreachable!()
+        let peek = match self.parse_whitespace() {
+            Some(b) => b,
+            None => {
+                return Err(Error::EofWhileParsingValue);
+            }
+        };
+
+        match peek {
+            b'n' => {
+                self.eat_char();
+                self.parse_ident(b"ull")?;
+                visitor.visit_unit()
+            }
+            _ => Err(Error::InvalidType),
+        }
     }
 
-    /// Unsupported. Use a more specific deserialize_* method
-    fn deserialize_unit_struct<V>(self, _name: &'static str, _visitor: V) -> Result<V::Value>
+    fn deserialize_unit_struct<V>(self, _name: &'static str, visitor: V) -> Result<V::Value>
     where
         V: Visitor<'de>,
     {
-        unreachable!()
+        self.deserialize_unit(visitor)
     }
 
     /// Unsupported. We can’t parse newtypes because we don’t know the underlying type.
-    fn deserialize_newtype_struct<V>(self, _name: &'static str, _visitor: V) -> Result<V::Value>
+    fn deserialize_newtype_struct<V>(self, _name: &'static str, visitor: V) -> Result<V::Value>
     where
         V: Visitor<'de>,
     {
-        unreachable!()
+        visitor.visit_newtype_struct(self)
     }
 
     fn deserialize_seq<V>(self, visitor: V) -> Result<V::Value>
@@ -600,6 +610,17 @@ impl<'a, 'de> de::Deserializer<'de> for &'a mut Deserializer<'de> {
     {
         match self.parse_whitespace().ok_or(Error::EofWhileParsingValue)? {
             b'"' => visitor.visit_enum(UnitVariantAccess::new(self)),
+            b'{' => {
+                self.eat_char();
+                let value = visitor.visit_enum(VariantAccess::new(self))?;
+                match self.parse_whitespace().ok_or(Error::EofWhileParsingValue)? {
+                    b'}' => {
+                        self.eat_char();
+                        Ok(value)
+                    }
+                    _ => Err(Error::ExpectedSomeValue),
+                }
+            }
             _ => Err(Error::ExpectedSomeValue),
         }
     }
@@ -961,6 +982,41 @@ mod tests {
         assert!(crate::from_str::<Temperature>(r#"{ "temperature": -1 }"#).is_err());
     }
 
+    #[test]
+    fn test_unit() {
+        assert_eq!(crate::from_str::<()>(r#"null"#), Ok(((), 4)));
+    }
+
+    #[test]
+    fn newtype_struct() {
+        #[derive(Deserialize, Debug, PartialEq)]
+        struct A(pub u32);
+
+        assert_eq!(crate::from_str::<A>(r#"54"#), Ok((A(54), 2)));
+    }
+
+    #[test]
+    fn test_newtype_variant() {
+        #[derive(Deserialize, Debug, PartialEq)]
+        enum A {
+            A(u32),
+        }
+        let a = A::A(54);
+        let x = crate::from_str::<A>(r#"{"A":54}"#);
+        assert_eq!(x, Ok((a, 8)));
+    }
+
+    #[test]
+    fn test_struct_variant() {
+        #[derive(Deserialize, Debug, PartialEq)]
+        enum A {
+            A { x: u32, y: u16 },
+        }
+        let a = A::A { x: 54, y: 720 };
+        let x = crate::from_str::<A>(r#"{"A": {"x":54,"y":720 } }"#);
+        assert_eq!(x, Ok((a, 25)));
+    }
+
     #[test]
     #[cfg(not(feature = "custom-error-messages"))]
     fn struct_tuple() {
diff --git a/src/ser/mod.rs b/src/ser/mod.rs
index c19138b06..a1e2ea148 100644
--- a/src/ser/mod.rs
+++ b/src/ser/mod.rs
@@ -3,12 +3,13 @@
 use core::{fmt, fmt::Write};
 
 use serde::ser;
+use serde::ser::SerializeStruct as _;
 
 use heapless::{consts::*, String, Vec};
 
 use self::map::SerializeMap;
 use self::seq::SerializeSeq;
-use self::struct_::SerializeStruct;
+use self::struct_::{SerializeStruct, SerializeStructVariant};
 
 mod map;
 mod seq;
@@ -19,11 +20,10 @@ pub type Result<T> = ::core::result::Result<T, Error>;
 
 /// This type represents all possible errors that can occur when serializing JSON data
 #[derive(Debug)]
+#[non_exhaustive]
 pub enum Error {
     /// Buffer is full
     BufferFull,
-    #[doc(hidden)]
-    __Extensible,
 }
 
 impl From<()> for Error {
@@ -84,7 +84,7 @@ impl<'a> Serializer<'a> {
             Err(Error::BufferFull)
         } else {
             for c in other {
-                unsafe { self.push_unchecked(c.clone()) };
+                unsafe { self.push_unchecked(*c) };
             }
             Ok(())
         }
@@ -178,7 +178,7 @@ impl<'a, 'b: 'a> ser::Serializer for &'a mut Serializer<'b> {
     type SerializeTupleVariant = Unreachable;
     type SerializeMap = SerializeMap<'a, 'b>;
     type SerializeStruct = SerializeStruct<'a, 'b>;
-    type SerializeStructVariant = Unreachable;
+    type SerializeStructVariant = SerializeStructVariant<'a, 'b>;
 
     fn serialize_bool(self, v: bool) -> Result<Self::Ok> {
         if v {
@@ -320,11 +320,11 @@ impl<'a, 'b: 'a> ser::Serializer for &'a mut Serializer<'b> {
     }
 
     fn serialize_unit(self) -> Result<Self::Ok> {
-        unreachable!()
+        self.serialize_none()
     }
 
     fn serialize_unit_struct(self, _name: &'static str) -> Result<Self::Ok> {
-        unreachable!()
+        self.serialize_unit()
     }
 
     fn serialize_unit_variant(
@@ -336,28 +336,28 @@ impl<'a, 'b: 'a> ser::Serializer for &'a mut Serializer<'b> {
         self.serialize_str(variant)
     }
 
-    fn serialize_newtype_struct<T: ?Sized>(
-        self,
-        _name: &'static str,
-        _value: &T,
-    ) -> Result<Self::Ok>
+    fn serialize_newtype_struct<T: ?Sized>(self, _name: &'static str, value: &T) -> Result<Self::Ok>
     where
         T: ser::Serialize,
     {
-        unreachable!()
+        value.serialize(self)
     }
 
     fn serialize_newtype_variant<T: ?Sized>(
-        self,
+        mut self,
         _name: &'static str,
         _variant_index: u32,
-        _variant: &'static str,
-        _value: &T,
+        variant: &'static str,
+        value: &T,
     ) -> Result<Self::Ok>
     where
         T: ser::Serialize,
     {
-        unreachable!()
+        self.push(b'{')?;
+        let mut s = SerializeStruct::new(&mut self);
+        s.serialize_field(variant, value)?;
+        s.end()?;
+        Ok(())
     }
 
     fn serialize_seq(self, _len: Option<usize>) -> Result<Self::SerializeSeq> {
@@ -404,10 +404,14 @@ impl<'a, 'b: 'a> ser::Serializer for &'a mut Serializer<'b> {
         self,
         _name: &'static str,
         _variant_index: u32,
-        _variant: &'static str,
+        variant: &'static str,
         _len: usize,
     ) -> Result<Self::SerializeStructVariant> {
-        unreachable!()
+        self.extend_from_slice(b"{\"")?;
+        self.extend_from_slice(variant.as_bytes())?;
+        self.extend_from_slice(b"\":{")?;
+
+        Ok(SerializeStructVariant::new(self))
     }
 
     fn collect_str<T: ?Sized>(self, _value: &T) -> Result<Self::Ok>
@@ -510,22 +514,6 @@ impl ser::SerializeMap for Unreachable {
     }
 }
 
-impl ser::SerializeStructVariant for Unreachable {
-    type Ok = ();
-    type Error = Error;
-
-    fn serialize_field<T: ?Sized>(&mut self, _key: &'static str, _value: &T) -> Result<()>
-    where
-        T: ser::Serialize,
-    {
-        unreachable!()
-    }
-
-    fn end(self) -> Result<Self::Ok> {
-        unreachable!()
-    }
-}
-
 #[cfg(test)]
 mod tests {
     use serde_derive::Serialize;
@@ -768,4 +756,43 @@ mod tests {
             r#"{"a":true,"b":false}"#
         );
     }
+
+    #[test]
+    fn test_unit() {
+        let a = ();
+        assert_eq!(&*crate::to_string::<N, _>(&a).unwrap(), r#"null"#);
+    }
+
+    #[test]
+    fn test_newtype_struct() {
+        #[derive(Serialize)]
+        struct A(pub u32);
+        let a = A(54);
+        assert_eq!(&*crate::to_string::<N, _>(&a).unwrap(), r#"54"#);
+    }
+
+    #[test]
+    fn test_newtype_variant() {
+        #[derive(Serialize)]
+        enum A {
+            A(u32),
+        }
+        let a = A::A(54);
+
+        assert_eq!(&*crate::to_string::<N, _>(&a).unwrap(), r#"{"A":54}"#);
+    }
+
+    #[test]
+    fn test_struct_variant() {
+        #[derive(Serialize)]
+        enum A {
+            A { x: u32, y: u16 },
+        }
+        let a = A::A { x: 54, y: 720 };
+
+        assert_eq!(
+            &*crate::to_string::<N, _>(&a).unwrap(),
+            r#"{"A":{"x":54,"y":720}}"#
+        );
+    }
 }
diff --git a/src/ser/struct_.rs b/src/ser/struct_.rs
index 71558f667..a6c31a8ca 100644
--- a/src/ser/struct_.rs
+++ b/src/ser/struct_.rs
@@ -41,3 +41,43 @@ impl<'a, 'b: 'a> ser::SerializeStruct for SerializeStruct<'a, 'b> {
         Ok(())
     }
 }
+
+pub struct SerializeStructVariant<'a, 'b> {
+    ser: &'a mut Serializer<'b>,
+    first: bool,
+}
+
+impl<'a, 'b: 'a> SerializeStructVariant<'a, 'b> {
+    pub(crate) fn new(ser: &'a mut Serializer<'b>) -> Self {
+        SerializeStructVariant { ser, first: true }
+    }
+}
+
+impl<'a, 'b: 'a> ser::SerializeStructVariant for SerializeStructVariant<'a, 'b> {
+    type Ok = ();
+    type Error = Error;
+
+    fn serialize_field<T: ?Sized>(&mut self, key: &'static str, value: &T) -> Result<()>
+    where
+        T: ser::Serialize,
+    {
+        // XXX if `value` is `None` we not produce any output for this field
+        if !self.first {
+            self.ser.push(b',')?;
+        }
+        self.first = false;
+
+        self.ser.push(b'"')?;
+        self.ser.extend_from_slice(key.as_bytes())?;
+        self.ser.extend_from_slice(b"\":")?;
+
+        value.serialize(&mut *self.ser)?;
+
+        Ok(())
+    }
+
+    fn end(self) -> Result<Self::Ok> {
+        self.ser.extend_from_slice(b"}}")?;
+        Ok(())
+    }
+}