#[traverse(skip)] features

[tisonkun]

Currently, #[traverse(skip)] can be added to a struct and its fields, as well as an enum, its variants, and its fields:

traversable/traversable-derive/src/lib.rs

Lines 331 to 336 in 5c6468c

	fn traverse_variant(v: Variant, mutable: bool) -> Result<TokenStream> {
	let mut params = Params::from_attrs(v.attrs, "traverse")?;
	params.validate(&["skip"])?;
	if params.param("skip")?.map(Param::unit).is_some() {
	return Ok(TokenStream::new());
	}

traversable/traversable-derive/src/lib.rs

Lines 371 to 375 in 5c6468c

	Ok(if params.param("skip")?.map(Param::unit).is_some() {
	quote! { #field_name: _ }
	} else {
	field_name.into_token_stream()
	})

traversable/traversable-derive/src/lib.rs

Lines 404 to 409 in 5c6468c

	fn traverse_field(value: &TokenStream, field: Field, mutable: bool) -> Result<TokenStream> {
	let mut params = Params::from_attrs(field.attrs, "traverse")?;
	params.validate(&["skip", "with"])?;
	if params.param("skip")?.map(Param::unit).is_some() {
	return Ok(TokenStream::new());

traversable/traversable-derive/src/lib.rs

Lines 207 to 214 in 5c6468c

	let mut params = Params::from_attrs(input.attrs, "traverse")?;
	params.validate(&["skip"])?;
	let skip_visit_self = params
	.param("skip")?
	.map(Param::unit)
	.transpose()?
	.is_some();

While skip variants and fields work well, "skip visit self" looks weird.

Typically, when you derive a struct as Traversable, you would like to visit it, of course.

Besides, a skip_all parameter to skip all fields of a struct can be useful, but it may be dangerous as well.

We need to decide what #[traverse(skip)] should be possibly noted a struct/enum to skip self, and whether to support #[traverse(skip_all)] before 1.0.