为MessagePack实现Serde的序列序列化程序时无法正确获得生存期

分类：编程问答

如何解决为MessagePack实现Serde的序列序列化程序时无法正确获得生存期

我想在Rust中编写一个无标准兼容的MessagePack序列化器/反序列化器。我有大部分的序列化工作，但是我在努力使用serde序列化结构，序列和图。我从serde_json_core复制了很多内容，但是序列序列化程序的生存期不正确。

playground 

extern crate generic_array_0_13_2; // 0.13.2
extern crate serde; 

use serde::ser;

pub struct Serializer<'a> {
    buf: &'a mut [u8],pos: usize,}
pub struct SerializeSeq<'a> {
    ser: &'a mut Serializer<'a>,}

impl<'a> ser::SerializeSeq for SerializeSeq<'a> {
    type Ok = ();
    type Error = Error;

    fn serialize_element<T: ?Sized>(&mut self,value: &T) -> Result<Self::Ok,Self::Error>
    where
        T: ser::Serialize,{
        // -------- This does not compile
        value.serialize(&mut *self.ser)?;
        Ok(())
    }
    fn end(self) -> Result<Self::Ok,Self::Error> {
        Ok(())
    }
}

impl<'a> ser::Serializer for &'a mut Serializer<'a> {
    type Ok = ();
    type Error = Error;
    type SerializeSeq = SerializeSeq<'a>;
    type SerializeTuple = SerializeSeq<'a>;
    type SerializeTupleStruct = Unreachable;
    type SerializeTupleVariant = Unreachable;
    type SerializeMap = SerializeMap<'a>;
    type SerializeStruct = SerializeStruct<'a>;
    type SerializeStructVariant = Unreachable;
    // ...

    fn serialize_seq(self,_len: Option<usize>) -> Result<Self::SerializeSeq,Self::Error> {
        Ok(SerializeSeq::new(self))
    }
    fn serialize_tuple(self,len: usize) -> Result<Self::SerializeTuple,Self::Error> {
        self.serialize_seq(Some(len))
    }

    fn serialize_bool(self,_: bool) -> Result<Self::Ok,Self::Error> {
        todo!()
    }
    fn serialize_i8(self,_: i8) -> Result<Self::Ok,Self::Error> {
        todo!()
    }
    fn serialize_i16(self,_: i16) -> Result<Self::Ok,Self::Error> {
        todo!()
    }
    fn serialize_i32(self,_: i32) -> Result<Self::Ok,Self::Error> {
        todo!()
    }
    fn serialize_i64(self,_: i64) -> Result<Self::Ok,Self::Error> {
        todo!()
    }
    fn serialize_u8(self,_: u8) -> Result<Self::Ok,Self::Error> {
        todo!()
    }
    fn serialize_u16(self,_: u16) -> Result<Self::Ok,Self::Error> {
        todo!()
    }
    fn serialize_u32(self,_: u32) -> Result<Self::Ok,Self::Error> {
        todo!()
    }
    fn serialize_u64(self,_: u64) -> Result<Self::Ok,Self::Error> {
        todo!()
    }
    fn serialize_f32(self,_: f32) -> Result<Self::Ok,Self::Error> {
        todo!()
    }
    fn serialize_f64(self,_: f64) -> Result<Self::Ok,Self::Error> {
        todo!()
    }
    fn serialize_char(self,_: char) -> Result<Self::Ok,Self::Error> {
        todo!()
    }
    fn serialize_str(self,_: &str) -> Result<Self::Ok,Self::Error> {
        todo!()
    }
    fn serialize_bytes(self,_: &[u8]) -> Result<Self::Ok,Self::Error> {
        todo!()
    }
    fn serialize_none(self) -> Result<Self::Ok,Self::Error> {
        todo!()
    }
    fn serialize_some<T: ?Sized>(self,_: &T) -> Result<Self::Ok,{
        todo!()
    }
    fn serialize_unit(self) -> Result<Self::Ok,Self::Error> {
        todo!()
    }
    fn serialize_unit_struct(self,_: &'static str) -> Result<Self::Ok,Self::Error> {
        todo!()
    }
    fn serialize_unit_variant(
        self,_: &'static str,_: u32,) -> Result<Self::Ok,Self::Error> {
        todo!()
    }
    fn serialize_newtype_struct<T: ?Sized>(
        self,_: &T,Self::Error>
    where
        T: serde::Serialize,{
        todo!()
    }
    fn serialize_newtype_variant<T: ?Sized>(
        self,{
        todo!()
    }
    fn serialize_tuple_struct(
        self,_: usize,) -> std::result::Result<
        <Self as serde::Serializer>::SerializeTupleStruct,<Self as serde::Serializer>::Error,> {
        todo!()
    }
    fn serialize_tuple_variant(
        self,) -> std::result::Result<
        <Self as serde::Serializer>::SerializeTupleVariant,> {
        todo!()
    }
    fn serialize_map(
        self,_: std::option::Option<usize>,) -> std::result::Result<
        <Self as serde::Serializer>::SerializeMap,> {
        todo!()
    }
    fn serialize_struct(
        self,) -> std::result::Result<
        <Self as serde::Serializer>::SerializeStruct,> {
        todo!()
    }
    fn serialize_struct_variant(
        self,) -> std::result::Result<
        <Self as serde::Serializer>::SerializeStructVariant,> {
        todo!()
    }
}

// ---- The remaining part is just to make the compiler stop complaining about missing implementations

impl<'a> SerializeSeq<'a> {
    pub fn new(ser: &'a mut Serializer<'a>) -> Self {
        SerializeSeq { ser }
    }
}

pub struct SerializeMap<'a> {
    ser: &'a mut Serializer<'a>,}
pub struct SerializeStruct<'a> {
    ser: &'a mut Serializer<'a>,}

impl<'a> ser::SerializeTuple for SerializeSeq<'a> {
    type Ok = ();
    type Error = Error;

    fn serialize_element<T: ?Sized>(&mut self,{
        ser::SerializeSeq::serialize_element(self,value)
    }

    fn end(self) -> Result<Self::Ok,Self::Error> {
        ser::SerializeSeq::end(self)
    }
}

impl<'a> ser::SerializeStruct for SerializeStruct<'a> {
    type Ok = ();
    type Error = Error;

    fn serialize_field<T: ?Sized>(
        &mut self,key: &'static str,value: &T,{
        //value.serialize(&mut *self.ser)?;
        Ok(())
    }

    fn end(self) -> Result<Self::Ok,Self::Error> {
        Ok(())
    }
}

impl<'a> ser::SerializeMap for SerializeMap<'a> {
    type Ok = ();
    type Error = Error;

    fn end(self) -> Result<Self::Ok,Self::Error> {
        Ok(())
    }

    fn serialize_key<T: ?Sized>(&mut self,key: &T) -> Result<Self::Ok,{
        //key.serialize(&mut *self.ser)?;
        Ok(())
    }

    fn serialize_value<T: ?Sized>(&mut self,{
        //value.serialize(&mut *self.ser)?;
        Ok(())
    }
}

#[derive(Debug)]
enum Error {}
impl std::fmt::Display for Error {
    fn fmt(&self,f: &mut std::fmt::Formatter) -> std::fmt::Result {
        todo!()
    }
}
impl std::error::Error for Error {}
impl serde::ser::Error for Error {
    fn custom<T>(_: T) -> Self
    where
        T: std::fmt::Display,{
        todo!()
    }
}
pub enum Unreachable {}

impl ser::SerializeTupleStruct for Unreachable {
    type Ok = ();
    type Error = Error;

    fn serialize_field<T: ?Sized>(&mut self,_value: &T) -> Result<Self::Ok,Self::Error> {
        unreachable!()
    }

    fn end(self) -> Result<Self::Ok,Self::Error> {
        unreachable!()
    }
}

impl ser::SerializeTupleVariant for Unreachable {
    type Ok = ();
    type Error = Error;

    fn serialize_field<T: ?Sized>(&mut self,Self::Error> {
        unreachable!()
    }
}

impl ser::SerializeMap for Unreachable {
    type Ok = ();
    type Error = Error;

    fn serialize_key<T: ?Sized>(&mut self,_key: &T) -> Result<Self::Ok,{
        unreachable!()
    }

    fn serialize_value<T: ?Sized>(&mut self,{
        unreachable!()
    }

    fn end(self) -> Result<Self::Ok,Self::Error> {
        unreachable!()
    }
}

impl ser::SerializeStructVariant for Unreachable {
    type Ok = ();
    type Error = Error;

    fn serialize_field<T: ?Sized>(
        &mut self,_key: &'static str,_value: &T,Self::Error> {
        unreachable!()
    }
}

这是我得到的错误：

error[E0495]: cannot infer an appropriate lifetime for borrow expression due to conflicting requirements
  --> src/lib.rs:23:25
   |
23 |         value.serialize(&mut *self.ser)?;
   |                         ^^^^^^^^^^^^^^
   |
note: first,the lifetime cannot outlive the anonymous lifetime #1 defined on the method body at 18:5...
  --> src/lib.rs:18:5
   |
18 | /     fn serialize_element<T: ?Sized>(&mut self,Self::Error>
19 | |     where
20 | |         T: ser::Serialize,21 | |     {
...  |
24 | |         Ok(())
25 | |     }
   | |_____^
note: ...so that reference does not outlive borrowed content
  --> src/lib.rs:23:25
   |
23 |         value.serialize(&mut *self.ser)?;
   |                         ^^^^^^^^^^^^^^
note: but,the lifetime must be valid for the lifetime `'a` as defined on the impl at 14:6...
  --> src/lib.rs:14:6
   |
14 | impl<'a> ser::SerializeSeq for SerializeSeq<'a> {
   |      ^^
note: ...so that the types are compatible
  --> src/lib.rs:23:15
   |
23 |         value.serialize(&mut *self.ser)?;
   |               ^^^^^^^^^
   = note: expected `serde::Serializer`
              found `serde::Serializer`

这与Serializer中字节片的生存期和&mut Serializer冲突的生存期有关，但是我一生都无法解决。我尝试添加更多的生命周期注释，并引入了特征绑定以摆脱第二生命周期，但没有任何效果...

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