#[serde(default)] behavior for tuples is inconsistent

[Mingun]

Currently, you can define default values for any fields of tuple structs, but at the same time cannot define #[serde(default)] on the type itself, which is inconsistent:

#[derive(Deserialize)]
// #[serde(default)] // Impossible!
struct Newtype(#[serde(default)] u8);

#[derive(Deserialize)]
// #[serde(default)] // Impossible!
struct Tuple(
    #[serde(default)] u8,
    u8,
    #[serde(default)] i8,
);

Actually, the #[serde(default)] on u8 is useless here, because this derive generates the following code:

#[inline]
fn visit_seq<A>(self, mut seq: A) -> Result<Self::Value, A::Error>
where
    A: SeqAccess<'de>,
{
    let field0 = match seq.next_element::<u8>()? {
        Some(value) => value,
        None => Default::default(),
    };
    let field1 = match seq.next_element::<u8>()? {
        Some(value) => value,
        None => return Err(Error::invalid_length(1, &"tuple struct Tuple with 3 elements")),
    };
    let field2 = match seq.next_element::<i8>()? {
        Some(value) => value,
        None => Default::default(),
    };
    Ok(Tuple(field0, field1, field2))
}

If you need to use default value for the field, you should also use it for all subsequent fields. So I propose:

• allow #[serde(default)] on tuples. Currently an error #[serde(default)] can only be used on structs with named fields is produced
• forbid fields without #[serde(default)] if before them there is a field with #[serde(default)]
  • except when type itself is marked with #[serde(default)]
• change the error message, because if that would be implemented, then tuples with up to X elements would be allowed

[Mingun]

Fixed by #2553