Do not generate Field structure for each variant

serde/src/de/identifier.rs

@@ -486,3 +486,202 @@ impl<'de, 'a, 'b> DeserializeSeed<'de> for &'b mut FieldFlattenSeed<'a> {
         deserializer.deserialize_identifier(self)
     }
 }
+
+////////////////////////////////////////////////////////////////////////////////
+
+/// Creates a field deserialization seed that wrapped array with all possible
+/// field names and their aliases.
+///
+/// # Example
+///
+/// ```
+/// # use serde::__private::de::VariantSeed;
+/// let seed = VariantSeed::new(
+///     &[
+///         // First field with two aliases
+///         &["a", "alias 1", "alias 2"],
+///         // Second field with one alias
+///         &["b", "alias 3"],
+///         // Third field without aliases
+///         &["c"],
+///     ],
+///     &[
+///         // First field with two aliases
+///         "a", "alias 1", "alias 2",
+///         // Second field with one alias
+///         "b", "alias 3",
+///         // Third field without aliases
+///         "c",
+///     ],
+/// );
+/// ```
+#[derive(Debug)]
+pub struct VariantSeed<'a> {
+    aliases: &'a [&'a [&'a str]],
+    variants: &'static [&'static str],
+}
+
+impl<'a> VariantSeed<'a> {
+    #[allow(missing_docs)]
+    pub const fn new(aliases: &'a [&'a [&'a str]], variants: &'static [&'static str]) -> Self {
+        Self { aliases, variants }
+    }
+
+    fn matches(&self, value: &[u8]) -> Option<usize> {
+        self.aliases
+            .iter()
+            .position(|variant| variant.iter().any(|v| v.as_bytes() == value))
+    }
+}
+
+impl<'de, 'a> Visitor<'de> for VariantSeed<'a> {
+    type Value = usize;
+
+    fn expecting(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.write_str("variant identifier")
+    }
+
+    fn visit_u64<E>(self, value: u64) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        if value < self.aliases.len() as u64 {
+            Ok(value as usize)
+        } else {
+            // length of string "variant index 0 <= i < " and u64::MAX.to_string()
+            let mut buf = [0u8; 23 + 20];
+            let mut writer = format::Buf::new(&mut buf);
+            fmt::Write::write_fmt(
+                &mut writer,
+                format_args!("variant index 0 <= i < {}", value),
+            )
+            .unwrap();
+            Err(Error::invalid_value(
+                Unexpected::Unsigned(value),
+                &writer.as_str(),
+            ))
+        }
+    }
+
+    fn visit_str<E>(self, value: &str) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        match self.matches(value.as_bytes()) {
+            Some(index) => Ok(index),
+            None => Err(Error::unknown_variant(value, self.variants)),
+        }
+    }
+
+    fn visit_bytes<E>(self, value: &[u8]) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        match self.matches(value) {
+            Some(index) => Ok(index),
+            None => Err(Error::unknown_variant(
+                &from_utf8_lossy(value),
+                self.variants,
+            )),
+        }
+    }
+}
+
+impl<'de, 'a> DeserializeSeed<'de> for VariantSeed<'a> {
+    type Value = usize;
+
+    #[inline]
+    fn deserialize<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        deserializer.deserialize_identifier(self)
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+/// Creates a field deserialization seed that wrapped array with all possible
+/// field names and their aliases.
+///
+/// # Example
+///
+/// ```
+/// # use serde::__private::de::VariantOtherSeed;
+/// let seed = VariantOtherSeed::new(
+///     &[
+///         // First field with two aliases
+///         &["a", "alias 1", "alias 2"],
+///         // Second field with one alias
+///         &["b", "alias 3"],
+///         // Third field without aliases
+///         &["c"],
+///     ],
+///     2,
+/// );
+/// ```
+#[derive(Debug)]
+pub struct VariantOtherSeed<'a> {
+    aliases: &'a [&'a [&'a str]],
+    other: usize,
+}
+
+impl<'a> VariantOtherSeed<'a> {
+    #[allow(missing_docs)]
+    pub const fn new(aliases: &'a [&'a [&'a str]], other: usize) -> Self {
+        assert!(other < aliases.len());
+        Self { aliases, other }
+    }
+
+    fn matches(&self, value: &[u8]) -> usize {
+        self.aliases
+            .iter()
+            .position(|variant| variant.iter().any(|v| v.as_bytes() == value))
+            .unwrap_or(self.other)
+    }
+}
+
+impl<'de, 'a> Visitor<'de> for VariantOtherSeed<'a> {
+    type Value = usize;
+
+    fn expecting(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.write_str("variant identifier")
+    }
+
+    fn visit_u64<E>(self, value: u64) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        if value < self.aliases.len() as u64 {
+            Ok(value as usize)
+        } else {
+            Ok(self.other)
+        }
+    }
+
+    fn visit_str<E>(self, value: &str) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        Ok(self.matches(value.as_bytes()))
+    }
+
+    fn visit_bytes<E>(self, value: &[u8]) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        Ok(self.matches(value))
+    }
+}
+
+impl<'de, 'a> DeserializeSeed<'de> for VariantOtherSeed<'a> {
+    type Value = usize;
+
+    #[inline]
+    fn deserialize<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        deserializer.deserialize_identifier(self)
+    }
+}

serde/src/private/de.rs

@@ -16,7 +16,9 @@ pub use self::content::{
     TagOrContentField, TagOrContentFieldVisitor, TaggedContentVisitor, UntaggedUnitVisitor,
 };
 
-pub use crate::de::identifier::{Field, FieldSeed, FieldStrong, FieldStrongSeed};
+pub use crate::de::identifier::{
+    Field, FieldSeed, FieldStrong, FieldStrongSeed, VariantOtherSeed, VariantSeed,
+};
 #[cfg(any(feature = "std", feature = "alloc"))]
 pub use crate::de::identifier::{FieldFlatten, FieldFlattenSeed};
 pub use crate::seed::InPlaceSeed;

serde_derive/src/de.rs

@@ -1263,50 +1263,50 @@ fn deserialize_externally_tagged_enum(
     let expecting = format!("enum {}", params.type_name());
     let expecting = cattrs.expecting().unwrap_or(&expecting);
 
-    let (variants_stmt, variant_visitor) = prepare_enum_variant_enum(variants, cattrs);
-
     // Match arms to extract a variant from a string
     let variant_arms = variants
         .iter()
+        .filter(|variant| !variant.attrs.skip_deserializing())
         .enumerate()
-        .filter(|&(_, variant)| !variant.attrs.skip_deserializing())
         .map(|(i, variant)| {
-            let variant_name = field_i(i);
-
             let block = Match(deserialize_externally_tagged_variant(
                 params, variant, cattrs,
             ));
 
             quote! {
-                (__Field::#variant_name, __variant) => #block
+                (#i, __variant) => #block
             }
         });
 
-    let all_skipped = variants
+    let seed = match variants
         .iter()
-        .all(|variant| variant.attrs.skip_deserializing());
-    let match_variant = if all_skipped {
-        // This is an empty enum like `enum Impossible {}` or an enum in which
-        // all variants have `#[serde(skip_deserializing)]`.
-        quote! {
-            // FIXME: Once feature(exhaustive_patterns) is stable:
-            // let _serde::__private::Err(__err) = _serde::de::EnumAccess::variant::<__Field>(__data);
-            // _serde::__private::Err(__err)
-            _serde::__private::Result::map(
-                _serde::de::EnumAccess::variant::<__Field>(__data),
-                |(__impossible, _)| match __impossible {})
-        }
-    } else {
-        quote! {
-            match _serde::de::EnumAccess::variant(__data)? {
-                #(#variant_arms)*
-            }
+        .filter(|variant| !variant.attrs.skip_deserializing())
+        .position(|variant| variant.attrs.other())
+    {
+        Some(other) => {
+            quote!(_serde::__private::de::VariantOtherSeed::new(VARIANT_ALIASES, #other))
         }
+        None => quote!(_serde::__private::de::VariantSeed::new(
+            VARIANT_ALIASES,
+            VARIANTS
+        )),
     };
 
-    quote_block! {
-        #variant_visitor
+    let variant_names = variants
+        .iter()
+        .filter(|variant| !variant.attrs.skip_deserializing())
+        .map(|variant| variant.attrs.name().deserialize_name());
 
+    let aliases = variants.iter().filter_map(|variant| {
+        if variant.attrs.skip_deserializing() {
+            None
+        } else {
+            let aliases = variant.attrs.aliases();
+            Some(quote!(&[ #(#aliases),* ]))
+        }
+    });
+
+    quote_block! {
         #[doc(hidden)]
         struct __Visitor #de_impl_generics #where_clause {
             marker: _serde::__private::PhantomData<#this_type #ty_generics>,
@@ -1324,11 +1324,17 @@ fn deserialize_externally_tagged_enum(
             where
                 __A: _serde::de::EnumAccess<#delife>,
             {
-                #match_variant
+                match _serde::de::EnumAccess::variant_seed(__data, #seed)? {
+                    #(#variant_arms)*
+                    _ => unreachable!(),
+                }
             }
         }
 
-        #variants_stmt
+        #[doc(hidden)]
+        const VARIANTS: &'static [&'static str] = &[ #(#variant_names),* ];
+        #[doc(hidden)]
+        const VARIANT_ALIASES: &[&[&str]] = &[ #(#aliases),* ];
 
         _serde::Deserializer::deserialize_enum(
             __deserializer,