Fix MayBe not being shallow

Sadly also a breaking change. Ah well.

3 files changed, 1015 insertions, 11 deletions

diff --git a/Cargo.toml b/Cargo.toml
index 69db765..1c4c931 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "serde-util"
-version = "0.2.0"
+version = "0.3.0"
 authors = ["SoniEx2 <endermoneymod@gmail.com>"]
 license-file = "LICENSE.txt"
 description = "Soni's Serde Utilities"
diff --git a/src/maybe.rs b/src/maybe.rs
index 9b2c419..27f436b 100644
--- a/src/maybe.rs
+++ b/src/maybe.rs
@@ -1,25 +1,37 @@
+#![allow(unused_parens)]
+
+use std::fmt::Formatter;
+
 use serde::Deserialize;
 use serde::Deserializer;
+use serde::de::DeserializeSeed;
+use serde::de::EnumAccess;
+use serde::de::MapAccess;
+use serde::de::SeqAccess;
+use serde::de::VariantAccess;
+use serde::de::Visitor;
 
-/// Something that may be an `T`.
+/// Something that may be of the kind of `T`.
 ///
-/// If a value cannot be deserialized as a `T`, this will *discard* the value
-/// and provide an `MayBe::IsNot`.
+/// If a value doesn't have the kind of `T`, this will *discard* the value and
+/// provide an `MayBe::IsNot`. Nevertheless, if the value does have the kind of
+/// `T` but cannot be deserialized by `T`, this will also fail to deserialize,
+/// making this wrapper "shallow".
+///
+/// For example, `struct`s have the same kind as a JSON object, `{}`, If the
+/// contents of said JSON object don't match what the `struct` is expecting,
+/// like if the `struct Foo { bar: u32 }` is being matched, as `MayBe<Foo>`,
+/// against `{"bar": ""}`, then that is an error. However, matching the same
+/// `MayBe<Foo>` against a JSON array, `[]` would not lead to an error, but
+/// to an `MayBe::IsNot`.
 ///
 /// Can only be used with self-describing formats, like JSON.
-#[derive(Deserialize)]
-#[serde(untagged)]
 #[derive(Copy, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
 pub enum MayBe<T> {
     Is(T),
-    #[serde(deserialize_with = "ignore_any")]
     IsNot,
 }
 
-fn ignore_any<'de, D: Deserializer<'de>>(d: D) -> Result<(), D::Error> {
-    serde::de::IgnoredAny::deserialize(d).map(|_| ())
-}
-
 // FIXME decide on Default
 // This enum is analogous to Result, *not* Option.
 //
@@ -107,3 +119,984 @@ impl<T> From<T> for MayBe<T> {
         MayBe::Is(thing)
     }
 }
+
+// actual deserialize
+
+struct MayBeDer<'a, D>(&'a mut bool, D);
+
+struct MayBeVis<'a, V>(&'a mut bool, V);
+
+// acts as Deserializer, MapAccess, SeqAccess and EnumAccess, depending on T.
+struct DeepDer<'a, T>(&'a mut bool, T);
+
+#[derive(Deserialize)]
+#[serde(untagged)]
+enum MayBeHelper<T> {
+    #[serde(deserialize_with="may_be")]
+    #[serde(bound = "T: Deserialize<'de>")]
+    Value(MayBe<T>),
+}
+
+fn may_be<'de, D: Deserializer<'de>, T: Deserialize<'de>>(
+    d: D,
+) -> Result<MayBe<T>, D::Error> {
+    let mut depth: bool = false;
+    match T::deserialize(MayBeDer(&mut depth, d)) {
+        Ok(t) => Ok(MayBe::Is(t)),
+        Err(_) if !depth => Ok(MayBe::IsNot),
+        Err(e) => Err(e),
+    }
+}
+
+impl<'de, T: Deserialize<'de>> Deserialize<'de> for MayBe<T> {
+    fn deserialize<D: Deserializer<'de>>(d: D) -> Result<Self, D::Error> {
+        MayBeHelper::<T>::deserialize(d).map(|MayBeHelper::Value(v): _| v)
+    }
+}
+
+impl<'a, 'de: 'a, V: Visitor<'de>> Visitor<'de> for MayBeVis<'a, V> {
+    type Value = V::Value;
+
+    fn expecting(&self, formatter: &mut Formatter<'_>) -> std::fmt::Result {
+        let MayBeVis(_depth, inner) = self;
+        inner.expecting(formatter)
+    }
+
+    fn visit_bool<E: serde::de::Error>(
+        self,
+        v: bool,
+    ) -> Result<Self::Value, E> {
+        let MayBeVis(_depth, inner) = self;
+        inner.visit_bool(v)
+    }
+
+    fn visit_i8<E: serde::de::Error>(
+        self,
+        v: i8,
+    ) -> Result<Self::Value, E> {
+        let MayBeVis(_depth, inner) = self;
+        inner.visit_i8(v)
+    }
+
+    fn visit_i16<E: serde::de::Error>(
+        self,
+        v: i16,
+    ) -> Result<Self::Value, E> {
+        let MayBeVis(_depth, inner) = self;
+        inner.visit_i16(v)
+    }
+
+    fn visit_i32<E: serde::de::Error>(
+        self,
+        v: i32,
+    ) -> Result<Self::Value, E> {
+        let MayBeVis(_depth, inner) = self;
+        inner.visit_i32(v)
+    }
+
+    fn visit_i64<E: serde::de::Error>(
+        self,
+        v: i64,
+    ) -> Result<Self::Value, E> {
+        let MayBeVis(_depth, inner) = self;
+        inner.visit_i64(v)
+    }
+
+    fn visit_i128<E: serde::de::Error>(
+        self,
+        v: i128,
+    ) -> Result<Self::Value, E> {
+        let MayBeVis(_depth, inner) = self;
+        inner.visit_i128(v)
+    }
+
+    fn visit_u8<E: serde::de::Error>(
+        self,
+        v: u8,
+    ) -> Result<Self::Value, E> {
+        let MayBeVis(_depth, inner) = self;
+        inner.visit_u8(v)
+    }
+
+    fn visit_u16<E: serde::de::Error>(
+        self,
+        v: u16,
+    ) -> Result<Self::Value, E> {
+        let MayBeVis(_depth, inner) = self;
+        inner.visit_u16(v)
+    }
+
+    fn visit_u32<E: serde::de::Error>(
+        self,
+        v: u32,
+    ) -> Result<Self::Value, E> {
+        let MayBeVis(_depth, inner) = self;
+        inner.visit_u32(v)
+    }
+
+    fn visit_u64<E: serde::de::Error>(
+        self,
+        v: u64,
+    ) -> Result<Self::Value, E> {
+        let MayBeVis(_depth, inner) = self;
+        inner.visit_u64(v)
+    }
+
+    fn visit_u128<E: serde::de::Error>(
+        self,
+        v: u128,
+    ) -> Result<Self::Value, E> {
+        let MayBeVis(_depth, inner) = self;
+        inner.visit_u128(v)
+    }
+
+    fn visit_f32<E: serde::de::Error>(
+        self,
+        v: f32,
+    ) -> Result<Self::Value, E> {
+        let MayBeVis(_depth, inner) = self;
+        inner.visit_f32(v)
+    }
+
+    fn visit_f64<E: serde::de::Error>(
+        self,
+        v: f64,
+    ) -> Result<Self::Value, E> {
+        let MayBeVis(_depth, inner) = self;
+        inner.visit_f64(v)
+    }
+
+    fn visit_char<E: serde::de::Error>(
+        self,
+        v: char,
+    ) -> Result<Self::Value, E> {
+        let MayBeVis(_depth, inner) = self;
+        inner.visit_char(v)
+    }
+
+    fn visit_str<E: serde::de::Error>(
+        self,
+        v: &str,
+    ) -> Result<Self::Value, E> {
+        let MayBeVis(_depth, inner) = self;
+        inner.visit_str(v)
+    }
+
+    fn visit_borrowed_str<E: serde::de::Error>(
+        self,
+        v: &'de str,
+    ) -> Result<Self::Value, E> {
+        let MayBeVis(_depth, inner) = self;
+        inner.visit_borrowed_str(v)
+    }
+
+    fn visit_string<E: serde::de::Error>(
+        self,
+        v: String,
+    ) -> Result<Self::Value, E> {
+        let MayBeVis(_depth, inner) = self;
+        inner.visit_string(v)
+    }
+
+    fn visit_bytes<E: serde::de::Error>(
+        self,
+        v: &[u8],
+    ) -> Result<Self::Value, E> {
+        let MayBeVis(_depth, inner) = self;
+        inner.visit_bytes(v)
+    }
+
+    fn visit_borrowed_bytes<E: serde::de::Error>(
+        self,
+        v: &'de [u8],
+    ) -> Result<Self::Value, E> {
+        let MayBeVis(_depth, inner) = self;
+        inner.visit_borrowed_bytes(v)
+    }
+
+    fn visit_byte_buf<E: serde::de::Error>(
+        self,
+        v: Vec<u8>,
+    ) -> Result<Self::Value, E> {
+        let MayBeVis(_depth, inner) = self;
+        inner.visit_byte_buf(v)
+    }
+
+    fn visit_none<E: serde::de::Error>(
+        self,
+    ) -> Result<Self::Value, E> {
+        let MayBeVis(_depth, inner) = self;
+        inner.visit_none()
+    }
+
+    fn visit_some<D: Deserializer<'de>>(
+        self,
+        deserializer: D,
+    ) -> Result<Self::Value, D::Error> {
+        let MayBeVis(depth, inner) = self;
+        inner.visit_some(DeepDer(depth, deserializer))
+    }
+
+    fn visit_unit<E: serde::de::Error>(
+        self,
+    ) -> Result<Self::Value, E> {
+        let MayBeVis(_depth, inner) = self;
+        inner.visit_unit()
+    }
+
+    fn visit_newtype_struct<D: Deserializer<'de>>(
+        self,
+        deserializer: D,
+    ) -> Result<Self::Value, D::Error> {
+        let MayBeVis(depth, inner) = self;
+        inner.visit_newtype_struct(DeepDer(depth, deserializer))
+    }
+
+    fn visit_seq<A: SeqAccess<'de>>(
+        self,
+        seq: A,
+    ) -> Result<Self::Value, A::Error> {
+        let MayBeVis(depth, inner) = self;
+        inner.visit_seq(DeepDer(depth, seq))
+    }
+
+    fn visit_map<A: MapAccess<'de>>(
+        self,
+        map: A,
+    ) -> Result<Self::Value, A::Error> {
+        let MayBeVis(depth, inner) = self;
+        inner.visit_map(DeepDer(depth, map))
+    }
+
+    fn visit_enum<A: EnumAccess<'de>>(
+        self,
+        data: A,
+    ) -> Result<Self::Value, A::Error> {
+        let MayBeVis(depth, inner) = self;
+        inner.visit_enum(DeepDer(depth, data))
+    }
+}
+
+impl<'a, 'de: 'a, D: Deserializer<'de>> Deserializer<'de> for MayBeDer<'a, D> {
+    type Error = D::Error;
+
+    fn deserialize_any<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let MayBeDer(depth, inner) = self;
+        inner.deserialize_any(MayBeVis(depth, visitor))
+    }
+
+    fn deserialize_bool<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let MayBeDer(depth, inner) = self;
+        inner.deserialize_bool(MayBeVis(depth, visitor))
+    }
+
+    fn deserialize_i8<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let MayBeDer(depth, inner) = self;
+        inner.deserialize_i8(MayBeVis(depth, visitor))
+    }
+
+    fn deserialize_i16<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let MayBeDer(depth, inner) = self;
+        inner.deserialize_i16(MayBeVis(depth, visitor))
+    }
+
+    fn deserialize_i32<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let MayBeDer(depth, inner) = self;
+        inner.deserialize_i32(MayBeVis(depth, visitor))
+    }
+
+    fn deserialize_i64<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let MayBeDer(depth, inner) = self;
+        inner.deserialize_i64(MayBeVis(depth, visitor))
+    }
+
+    fn deserialize_u8<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let MayBeDer(depth, inner) = self;
+        inner.deserialize_u8(MayBeVis(depth, visitor))
+    }
+
+    fn deserialize_u16<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let MayBeDer(depth, inner) = self;
+        inner.deserialize_u16(MayBeVis(depth, visitor))
+    }
+
+    fn deserialize_u32<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let MayBeDer(depth, inner) = self;
+        inner.deserialize_u32(MayBeVis(depth, visitor))
+    }
+
+    fn deserialize_u64<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let MayBeDer(depth, inner) = self;
+        inner.deserialize_u64(MayBeVis(depth, visitor))
+    }
+
+    fn deserialize_f32<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let MayBeDer(depth, inner) = self;
+        inner.deserialize_f32(MayBeVis(depth, visitor))
+    }
+
+    fn deserialize_f64<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let MayBeDer(depth, inner) = self;
+        inner.deserialize_f64(MayBeVis(depth, visitor))
+    }
+
+    fn deserialize_char<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let MayBeDer(depth, inner) = self;
+        inner.deserialize_char(MayBeVis(depth, visitor))
+    }
+
+    fn deserialize_str<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let MayBeDer(depth, inner) = self;
+        inner.deserialize_str(MayBeVis(depth, visitor))
+    }
+
+    fn deserialize_string<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let MayBeDer(depth, inner) = self;
+        inner.deserialize_string(MayBeVis(depth, visitor))
+    }
+
+    fn deserialize_bytes<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let MayBeDer(depth, inner) = self;
+        inner.deserialize_bytes(MayBeVis(depth, visitor))
+    }
+
+    fn deserialize_byte_buf<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let MayBeDer(depth, inner) = self;
+        inner.deserialize_byte_buf(MayBeVis(depth, visitor))
+    }
+
+    fn deserialize_option<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let MayBeDer(depth, inner) = self;
+        inner.deserialize_option(MayBeVis(depth, visitor))
+    }
+
+    fn deserialize_unit<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let MayBeDer(depth, inner) = self;
+        inner.deserialize_unit(MayBeVis(depth, visitor))
+    }
+
+    fn deserialize_unit_struct<V: Visitor<'de>>(
+        self, 
+        name: &'static str, 
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let MayBeDer(depth, inner) = self;
+        inner.deserialize_unit_struct(name, MayBeVis(depth, visitor))
+    }
+
+    fn deserialize_newtype_struct<V: Visitor<'de>>(
+        self, 
+        name: &'static str, 
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let MayBeDer(depth, inner) = self;
+        inner.deserialize_newtype_struct(name, MayBeVis(depth, visitor))
+    }
+
+    fn deserialize_seq<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let MayBeDer(depth, inner) = self;
+        inner.deserialize_seq(MayBeVis(depth, visitor))
+    }
+
+    fn deserialize_tuple<V: Visitor<'de>>(
+        self, 
+        len: usize, 
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let MayBeDer(depth, inner) = self;
+        inner.deserialize_tuple(len, MayBeVis(depth, visitor))
+    }
+
+    fn deserialize_tuple_struct<V: Visitor<'de>>(
+        self, 
+        name: &'static str, 
+        len: usize, 
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let MayBeDer(depth, inner) = self;
+        inner.deserialize_tuple_struct(name, len, MayBeVis(depth, visitor))
+    }
+
+    fn deserialize_map<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let MayBeDer(depth, inner) = self;
+        inner.deserialize_map(MayBeVis(depth, visitor))
+    }
+
+    fn deserialize_struct<V: Visitor<'de>>(
+        self, 
+        name: &'static str, 
+        fields: &'static [&'static str], 
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let MayBeDer(depth, inner) = self;
+        inner.deserialize_struct(name, fields, MayBeVis(depth, visitor))
+    }
+
+    fn deserialize_enum<V: Visitor<'de>>(
+        self, 
+        name: &'static str, 
+        variants: &'static [&'static str], 
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let MayBeDer(depth, inner) = self;
+        inner.deserialize_enum(name, variants, MayBeVis(depth, visitor))
+    }
+
+    fn deserialize_identifier<V: Visitor<'de>>(
+        self, 
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let MayBeDer(depth, inner) = self;
+        inner.deserialize_identifier(MayBeVis(depth, visitor))
+    }
+
+    fn deserialize_ignored_any<V: Visitor<'de>>(
+        self, 
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let MayBeDer(depth, inner) = self;
+        inner.deserialize_ignored_any(MayBeVis(depth, visitor))
+    }
+
+    fn deserialize_i128<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let MayBeDer(depth, inner) = self;
+        inner.deserialize_i128(MayBeVis(depth, visitor))
+    }
+
+    fn deserialize_u128<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let MayBeDer(depth, inner) = self;
+        inner.deserialize_u128(MayBeVis(depth, visitor))
+    }
+
+    fn is_human_readable(&self) -> bool {
+        let MayBeDer(_depth, inner) = self;
+        inner.is_human_readable()
+    }
+}
+
+impl<'a, 'de: 'a, A: VariantAccess<'de>> VariantAccess<'de> for DeepDer<'a, A> {
+    type Error = A::Error;
+
+    fn unit_variant(self) -> Result<(), Self::Error> {
+        let DeepDer(depth, inner) = self;
+        *depth = true;
+        inner.unit_variant()
+    }
+
+    fn newtype_variant_seed<T: DeserializeSeed<'de>>(
+        self,
+        seed: T,
+    ) -> Result<T::Value, Self::Error> {
+        let DeepDer(depth, inner) = self;
+        *depth = true;
+        inner.newtype_variant_seed(seed)
+    }
+
+    fn tuple_variant<V: Visitor<'de>>(
+        self, 
+        len: usize, 
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let DeepDer(depth, inner) = self;
+        *depth = true;
+        inner.tuple_variant(len, visitor)
+    }
+
+    fn struct_variant<V: Visitor<'de>>(
+        self, 
+        fields: &'static [&'static str], 
+        visitor: V
+    ) -> Result<V::Value, Self::Error> {
+        let DeepDer(depth, inner) = self;
+        *depth = true;
+        inner.struct_variant(fields, visitor)
+    }
+
+    fn newtype_variant<T: Deserialize<'de>>(
+        self,
+    ) -> Result<T, Self::Error> {
+        let DeepDer(depth, inner) = self;
+        *depth = true;
+        inner.newtype_variant()
+    }
+}
+
+impl<'a, 'de: 'a, A: EnumAccess<'de>> EnumAccess<'de> for DeepDer<'a, A> {
+    type Error = A::Error;
+
+    type Variant = DeepDer<'a, A::Variant>;
+
+    fn variant_seed<V: DeserializeSeed<'de>>(
+        self, 
+        seed: V,
+    ) -> Result<(V::Value, Self::Variant), Self::Error> {
+        let DeepDer(depth, inner) = self;
+        *depth = true;
+        inner.variant_seed(seed).map(move |(a, b)| {
+            (a, DeepDer(depth, b))
+        })
+    }
+
+    fn variant<V: Deserialize<'de>>(
+        self,
+    ) -> Result<(V, Self::Variant), Self::Error> {
+        let DeepDer(depth, inner) = self;
+        *depth = true;
+        inner.variant().map(move |(a, b)| {
+            (a, DeepDer(depth, b))
+        })
+    }
+}
+
+impl<'a, 'de: 'a, A: SeqAccess<'de>> SeqAccess<'de> for DeepDer<'a, A> {
+    type Error = A::Error;
+
+    fn next_element_seed<T: DeserializeSeed<'de>>(
+        &mut self, 
+        seed: T,
+    ) -> Result<Option<T::Value>, Self::Error> {
+        let DeepDer(ref mut depth, inner) = self;
+        **depth = true;
+        inner.next_element_seed(seed)
+    }
+
+    fn next_element<T: Deserialize<'de>>(
+        &mut self, 
+    ) -> Result<Option<T>, Self::Error> {
+        let DeepDer(ref mut depth, inner) = self;
+        **depth = true;
+        inner.next_element()
+    }
+
+    fn size_hint(&self) -> Option<usize> {
+        let DeepDer(_depth, inner) = self;
+        inner.size_hint()
+    }
+}
+
+impl<'a, 'de: 'a, A: MapAccess<'de>> MapAccess<'de> for DeepDer<'a, A> {
+    type Error = A::Error;
+
+    fn next_key_seed<K: DeserializeSeed<'de>>(
+        &mut self, 
+        seed: K,
+    ) -> Result<Option<K::Value>, Self::Error> {
+        let DeepDer(ref mut depth, inner) = self;
+        **depth = true;
+        inner.next_key_seed(seed)
+    }
+
+    fn next_value_seed<V: DeserializeSeed<'de>>(
+        &mut self,
+        seed: V,
+    ) -> Result<V::Value, Self::Error> {
+        let DeepDer(ref mut depth, inner) = self;
+        **depth = true;
+        inner.next_value_seed(seed)
+    }
+
+    fn next_entry_seed<K: DeserializeSeed<'de>, V: DeserializeSeed<'de>>(
+        &mut self, 
+        kseed: K, 
+        vseed: V,
+    ) -> Result<Option<(K::Value, V::Value)>, Self::Error> {
+        let DeepDer(ref mut depth, inner) = self;
+        **depth = true;
+        inner.next_entry_seed(kseed, vseed)
+    }
+
+    fn next_key<K: Deserialize<'de>>(
+        &mut self,
+    ) -> Result<Option<K>, Self::Error> {
+        let DeepDer(ref mut depth, inner) = self;
+        **depth = true;
+        inner.next_key()
+    }
+
+    fn next_value<V: Deserialize<'de>>(
+        &mut self,
+    ) -> Result<V, Self::Error> {
+        let DeepDer(ref mut depth, inner) = self;
+        **depth = true;
+        inner.next_value()
+    }
+
+    fn next_entry<K: Deserialize<'de>, V: Deserialize<'de>>(
+        &mut self,
+    ) -> Result<Option<(K, V)>, Self::Error> {
+        let DeepDer(ref mut depth, inner) = self;
+        **depth = true;
+        inner.next_entry()
+    }
+
+    fn size_hint(&self) -> Option<usize> {
+        let DeepDer(_depth, inner) = self;
+        inner.size_hint()
+    }
+}
+
+impl<'a, 'de: 'a, D: Deserializer<'de>> Deserializer<'de> for DeepDer<'a, D> {
+    type Error = D::Error;
+
+    fn deserialize_any<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let DeepDer(depth, inner) = self;
+        *depth = true;
+        inner.deserialize_any((visitor))
+    }
+
+    fn deserialize_bool<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let DeepDer(depth, inner) = self;
+        *depth = true;
+        inner.deserialize_bool((visitor))
+    }
+
+    fn deserialize_i8<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let DeepDer(depth, inner) = self;
+        *depth = true;
+        inner.deserialize_i8((visitor))
+    }
+
+    fn deserialize_i16<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let DeepDer(depth, inner) = self;
+        *depth = true;
+        inner.deserialize_i16((visitor))
+    }
+
+    fn deserialize_i32<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let DeepDer(depth, inner) = self;
+        *depth = true;
+        inner.deserialize_i32((visitor))
+    }
+
+    fn deserialize_i64<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let DeepDer(depth, inner) = self;
+        *depth = true;
+        inner.deserialize_i64((visitor))
+    }
+
+    fn deserialize_u8<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let DeepDer(depth, inner) = self;
+        *depth = true;
+        inner.deserialize_u8((visitor))
+    }
+
+    fn deserialize_u16<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let DeepDer(depth, inner) = self;
+        *depth = true;
+        inner.deserialize_u16((visitor))
+    }
+
+    fn deserialize_u32<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let DeepDer(depth, inner) = self;
+        *depth = true;
+        inner.deserialize_u32((visitor))
+    }
+
+    fn deserialize_u64<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let DeepDer(depth, inner) = self;
+        *depth = true;
+        inner.deserialize_u64((visitor))
+    }
+
+    fn deserialize_f32<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let DeepDer(depth, inner) = self;
+        *depth = true;
+        inner.deserialize_f32((visitor))
+    }
+
+    fn deserialize_f64<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let DeepDer(depth, inner) = self;
+        *depth = true;
+        inner.deserialize_f64((visitor))
+    }
+
+    fn deserialize_char<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let DeepDer(depth, inner) = self;
+        *depth = true;
+        inner.deserialize_char((visitor))
+    }
+
+    fn deserialize_str<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let DeepDer(depth, inner) = self;
+        *depth = true;
+        inner.deserialize_str((visitor))
+    }
+
+    fn deserialize_string<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let DeepDer(depth, inner) = self;
+        *depth = true;
+        inner.deserialize_string((visitor))
+    }
+
+    fn deserialize_bytes<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let DeepDer(depth, inner) = self;
+        *depth = true;
+        inner.deserialize_bytes((visitor))
+    }
+
+    fn deserialize_byte_buf<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let DeepDer(depth, inner) = self;
+        *depth = true;
+        inner.deserialize_byte_buf((visitor))
+    }
+
+    fn deserialize_option<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let DeepDer(depth, inner) = self;
+        *depth = true;
+        inner.deserialize_option((visitor))
+    }
+
+    fn deserialize_unit<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let DeepDer(depth, inner) = self;
+        *depth = true;
+        inner.deserialize_unit((visitor))
+    }
+
+    fn deserialize_unit_struct<V: Visitor<'de>>(
+        self, 
+        name: &'static str, 
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let DeepDer(depth, inner) = self;
+        *depth = true;
+        inner.deserialize_unit_struct(name, (visitor))
+    }
+
+    fn deserialize_newtype_struct<V: Visitor<'de>>(
+        self, 
+        name: &'static str, 
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let DeepDer(depth, inner) = self;
+        *depth = true;
+        inner.deserialize_newtype_struct(name, (visitor))
+    }
+
+    fn deserialize_seq<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let DeepDer(depth, inner) = self;
+        *depth = true;
+        inner.deserialize_seq((visitor))
+    }
+
+    fn deserialize_tuple<V: Visitor<'de>>(
+        self, 
+        len: usize, 
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let DeepDer(depth, inner) = self;
+        *depth = true;
+        inner.deserialize_tuple(len, (visitor))
+    }
+
+    fn deserialize_tuple_struct<V: Visitor<'de>>(
+        self, 
+        name: &'static str, 
+        len: usize, 
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let DeepDer(depth, inner) = self;
+        *depth = true;
+        inner.deserialize_tuple_struct(name, len, (visitor))
+    }
+
+    fn deserialize_map<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let DeepDer(depth, inner) = self;
+        *depth = true;
+        inner.deserialize_map((visitor))
+    }
+
+    fn deserialize_struct<V: Visitor<'de>>(
+        self, 
+        name: &'static str, 
+        fields: &'static [&'static str], 
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let DeepDer(depth, inner) = self;
+        *depth = true;
+        inner.deserialize_struct(name, fields, (visitor))
+    }
+
+    fn deserialize_enum<V: Visitor<'de>>(
+        self, 
+        name: &'static str, 
+        variants: &'static [&'static str], 
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let DeepDer(depth, inner) = self;
+        *depth = true;
+        inner.deserialize_enum(name, variants, (visitor))
+    }
+
+    fn deserialize_identifier<V: Visitor<'de>>(
+        self, 
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let DeepDer(depth, inner) = self;
+        *depth = true;
+        inner.deserialize_identifier((visitor))
+    }
+
+    fn deserialize_ignored_any<V: Visitor<'de>>(
+        self, 
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let DeepDer(depth, inner) = self;
+        *depth = true;
+        inner.deserialize_ignored_any((visitor))
+    }
+
+    fn deserialize_i128<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let DeepDer(depth, inner) = self;
+        *depth = true;
+        inner.deserialize_i128((visitor))
+    }
+
+    fn deserialize_u128<V: Visitor<'de>>(
+        self,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error> {
+        let DeepDer(depth, inner) = self;
+        *depth = true;
+        inner.deserialize_u128((visitor))
+    }
+
+    fn is_human_readable(&self) -> bool {
+        let DeepDer(_depth, inner) = self;
+        inner.is_human_readable()
+    }
+}
diff --git a/tests/maybe.rs b/tests/maybe.rs
index 1437195..702f0c5 100644
--- a/tests/maybe.rs
+++ b/tests/maybe.rs
@@ -47,3 +47,14 @@ fn test_not_t_in_struct() {
     let json = "{\"bar\": []}";
     assert!(from_str::<Foo>(json).unwrap().bar.is_not());
 }
+
+#[test]
+fn test_shallow() {
+    #[derive(Deserialize)]
+    struct Foo {
+        #[serde(rename = "bar")]
+        _bar: f64,
+    }
+    let json = "{\"bar\": []}";
+    assert!(from_str::<MayBe<Foo>>(json).is_err());
+}