Trait UnsizedVisitor

Source

pub trait UnsizedVisitor<'de, C, T>: Sizedwhere
    C: Context<Error = Self::Error, Allocator = Self::Allocator>,
    T: ?Sized + ToOwned,{
    type Ok;
    type Error;
    type Allocator: Allocator;

    // Required method
    fn expecting(&self, f: &mut Formatter<'_>) -> Result;

    // Provided methods
    fn visit_owned(
        self,
        cx: C,
        value: T::Owned<Self::Allocator>,
    ) -> Result<Self::Ok, Self::Error> { ... }
    fn visit_borrowed(
        self,
        cx: C,
        value: &'de T,
    ) -> Result<Self::Ok, Self::Error> { ... }
    fn visit_ref(self, cx: C, value: &T) -> Result<Self::Ok, Self::Error> { ... }
}

Expand description

A visitor for data where we might need to borrow without copying from the underlying Decoder.

When implementing this trait you must use the #[musli::trait_defaults] attribute macro.

A visitor is needed with Decoder::decode_bytes and Decoder::decode_string because the caller doesn’t know if the encoding format is capable of producing references to the underlying data directly or if it needs to be processed first.

If all you want is to decode a value by reference, use the Decoder::decode_unsized method.

By requiring a visitor we ensure that the caller has to handle both scenarios, even if one involves erroring. A type like Cow is an example of a type which can comfortably handle both.

§Examples

use std::fmt;

use musli::Context;
use musli::de::UnsizedVisitor;

struct Visitor;

#[musli::trait_defaults]
impl<'de, C> UnsizedVisitor<'de, C, [u8]> for Visitor
where
    C: Context,
{
    type Ok = ();

    #[inline]
    fn expecting(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(
            f,
            "a reference of bytes"
        )
    }
}