Merge pull request #1302 from dtolnay/never

Implement traits for `!`

serde/src/de/impls.rs

@@ -49,6 +49,16 @@ impl<'de> Deserialize<'de> for () {
     }
 }
 
+#[cfg(feature = "unstable")]
+impl<'de> Deserialize<'de> for ! {
+    fn deserialize<D>(_deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        Err(Error::custom("cannot deserialize `!`"))
+    }
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 
 struct BoolVisitor;

serde/src/de/mod.rs

@@ -98,6 +98,7 @@
 //!    - PathBuf
 //!    - Range\<T\>
 //!    - num::NonZero*
+//!    - `!` *(unstable)*
 //!  - **Net types**:
 //!    - IpAddr
 //!    - Ipv4Addr

serde/src/de/value.rs

@@ -176,6 +176,48 @@ where
 
 ////////////////////////////////////////////////////////////////////////////////
 
+/// A deserializer that cannot be instantiated.
+#[cfg(feature = "unstable")]
+pub struct NeverDeserializer<E> {
+    never: !,
+    marker: PhantomData<E>,
+}
+
+#[cfg(feature = "unstable")]
+impl<'de, E> IntoDeserializer<'de, E> for !
+where
+    E: de::Error,
+{
+    type Deserializer = NeverDeserializer<E>;
+
+    fn into_deserializer(self) -> Self::Deserializer {
+        self
+    }
+}
+
+#[cfg(feature = "unstable")]
+impl<'de, E> de::Deserializer<'de> for NeverDeserializer<E>
+where
+    E: de::Error,
+{
+    type Error = E;
+
+    fn deserialize_any<V>(self, _visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: de::Visitor<'de>,
+    {
+        self.never
+    }
+
+    forward_to_deserialize_any! {
+        bool i8 i16 i32 i64 i128 u8 u16 u32 u64 u128 f32 f64 char str string
+        bytes byte_buf option unit unit_struct newtype_struct seq tuple
+        tuple_struct map struct enum identifier ignored_any
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
 macro_rules! primitive_deserializer {
     ($ty:ty, $doc:tt, $name:ident, $method:ident $($cast:tt)*) => {
         #[doc = "A deserializer holding"]

serde/src/lib.rs

@@ -89,7 +89,7 @@
 // discussion of these features please refer to this issue:
 //
 //    https://github.com/serde-rs/serde/issues/812
-#![cfg_attr(feature = "unstable", feature(specialization))]
+#![cfg_attr(feature = "unstable", feature(specialization, never_type))]
 #![cfg_attr(feature = "alloc", feature(alloc))]
 #![cfg_attr(feature = "cargo-clippy", deny(clippy, clippy_pedantic))]
 // Whitelisted clippy lints

serde/src/ser/impls.rs

@@ -247,6 +247,16 @@ impl Serialize for () {
     }
 }
 
+#[cfg(feature = "unstable")]
+impl Serialize for ! {
+    fn serialize<S>(&self, _serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        *self
+    }
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 
 macro_rules! tuple_impls {

serde/src/ser/mod.rs

@@ -93,6 +93,7 @@
 //!    - PathBuf
 //!    - Range\<T\>
 //!    - num::NonZero*
+//!    - `!` *(unstable)*
 //!  - **Net types**:
 //!    - IpAddr
 //!    - Ipv4Addr

test_suite/tests/test_de.rs

@@ -7,6 +7,7 @@
 // except according to those terms.
 
 #![cfg_attr(feature = "cargo-clippy", allow(decimal_literal_representation))]
+#![cfg_attr(feature = "unstable", feature(never_type))]
 
 #[macro_use]
 extern crate serde_derive;
@@ -984,6 +985,16 @@ declare_tests! {
     }
 }
 
+#[cfg(feature = "unstable")]
+declare_tests! {
+    test_never_result {
+        Ok::<u8, !>(0) => &[
+            Token::NewtypeVariant { name: "Result", variant: "Ok" },
+            Token::U8(0),
+        ],
+    }
+}
+
 #[cfg(unix)]
 #[test]
 fn test_osstring() {
@@ -1051,6 +1062,20 @@ fn test_cstr_internal_null_end() {
     );
 }
 
+#[cfg(feature = "unstable")]
+#[test]
+fn test_never_type() {
+    assert_de_tokens_error::<!>(&[], "cannot deserialize `!`");
+
+    assert_de_tokens_error::<Result<u8, !>>(
+        &[Token::NewtypeVariant {
+            name: "Result",
+            variant: "Err",
+        }],
+        "cannot deserialize `!`",
+    );
+}
+
 declare_error_tests! {
     test_unknown_field<StructDenyUnknown> {
         &[

test_suite/tests/test_ser.rs

@@ -6,6 +6,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![cfg_attr(feature = "unstable", feature(never_type))]
+
 #[macro_use]
 extern crate serde_derive;
 
@@ -548,6 +550,16 @@ declare_tests! {
     }
 }
 
+#[cfg(feature = "unstable")]
+declare_tests! {
+    test_never_result {
+        Ok::<u8, !>(0) => &[
+            Token::NewtypeVariant { name: "Result", variant: "Ok" },
+            Token::U8(0),
+        ],
+    }
+}
+
 #[test]
 #[cfg(unix)]
 fn test_cannot_serialize_paths() {