How does the Vistor trait allow only partial implementation?

In my limited understanding of Rust, I believed that traits were like interfaces - when you implemented one, you needed to implement all of the methods.

I'm now writing a custom serde::Deserializer.

Looking at https://docs.serde.rs/serde/de/trait.Visitor.html#example

/// A visitor that deserializes a long string - a string containing at least
/// some minimum number of bytes.
struct LongString {
    min: usize,
}

impl<'de> Visitor<'de> for LongString {
    type Value = String;

    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
        write!(formatter, "a string containing at least {} bytes", self.min)
    }

    fn visit_str<E>(self, s: &str) -> Result<Self::Value, E>
    where
        E: de::Error,
    {
        if s.len() >= self.min {
            Ok(s.to_owned())
        } else {
            Err(de::Error::invalid_value(Unexpected::Str(s), &self))
        }
    }
}

and https://serde.rs/impl-deserialize.html

use std::fmt;

use serde::de::{self, Visitor};

struct I32Visitor;

impl<'de> Visitor<'de> for I32Visitor {
    type Value = i32;

    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
        formatter.write_str("an integer between -2^31 and 2^31")
    }

    fn visit_i8<E>(self, value: i8) -> Result<Self::Value, E>
    where
        E: de::Error,
    {
        Ok(i32::from(value))
    }

    fn visit_i32<E>(self, value: i32) -> Result<Self::Value, E>
    where
        E: de::Error,
    {
        Ok(value)
    }

    fn visit_i64<E>(self, value: i64) -> Result<Self::Value, E>
    where
        E: de::Error,
    {
        use std::i32;
        if value >= i64::from(i32::MIN) && value <= i64::from(i32::MAX) {
            Ok(value as i32)
        } else {
            Err(E::custom(format!("i32 out of range: {}", value)))
        }
    }

    // Similar for other methods:
    //   - visit_i16
    //   - visit_u8
    //   - visit_u16
    //   - visit_u32
    //   - visit_u64
}

It seems like the Visitor trait is magic - you just need to implement the methods you feel like implementing and the others default to some form of error.

How does this magic work?