Factor attribute-stripping into a fold, similar to libsyntax

[dtolnay]

This stuff:

serde/serde_codegen/src/lib.rs

Lines 199 to 292 in 3a3777a

	fn strip_serde_derives(item: syn::MacroInput) -> (bool, bool, syn::MacroInput) {
	let mut ser = false;
	let mut de = false;
	let item = syn::MacroInput {
	attrs: item.attrs.into_iter().flat_map(|attr| {
	if attr.is_sugared_doc {
	return Some(attr);
	}
	let (name, nested) = match attr.value {
	syn::MetaItem::List(name, nested) => (name, nested),
	_ => return Some(attr)
	};
	if name != "derive" {
	return Some(syn::Attribute {
	value: syn::MetaItem::List(name, nested),
	is_sugared_doc: false,
	});
	}
	let rest: Vec<_> = nested.into_iter().filter(|nested| {
	match *nested {
	syn::MetaItem::Word(ref word) if word == "Serialize" => {
	ser = true;
	false
	}
	syn::MetaItem::Word(ref word) if word == "Deserialize" => {
	de = true;
	false
	}
	_ => true,
	}
	}).collect();
	if rest.is_empty() {
	None
	} else {
	Some(syn::Attribute {
	value: syn::MetaItem::List(name, rest),
	is_sugared_doc: false,
	})
	}
	}).collect(),
	..item
	};
	(ser, de, item)
	}
	fn strip_serde_attrs(item: syn::MacroInput) -> syn::MacroInput {
	syn::MacroInput {
	attrs: strip_serde_from_attrs(item.attrs),
	body: match item.body {
	syn::Body::Enum(variants) => syn::Body::Enum(
	variants.into_iter().map(|variant| {
	syn::Variant {
	ident: variant.ident,
	attrs: strip_serde_from_attrs(variant.attrs),
	data: strip_serde_from_variant_data(variant.data),
	discriminant: variant.discriminant,
	}
	}).collect()
	),
	syn::Body::Struct(variant_data) => syn::Body::Struct(
	strip_serde_from_variant_data(variant_data)
	),
	},
	..item
	}
	}
	fn strip_serde_from_variant_data(data: syn::VariantData) -> syn::VariantData {
	match data {
	syn::VariantData::Struct(fields) => syn::VariantData::Struct(
	fields.into_iter().map(strip_serde_from_field).collect()
	),
	syn::VariantData::Tuple(fields) => syn::VariantData::Tuple(
	fields.into_iter().map(strip_serde_from_field).collect()
	),
	syn::VariantData::Unit => syn::VariantData::Unit,
	}
	}
	fn strip_serde_from_field(field: syn::Field) -> syn::Field {
	syn::Field {
	attrs: strip_serde_from_attrs(field.attrs),
	..field
	}
	}
	fn strip_serde_from_attrs(attrs: Vec<syn::Attribute>) -> Vec<syn::Attribute> {
	attrs.into_iter().filter(|attr| {
	match attr.value {
	syn::MetaItem::List(ref ident, _) => ident != "serde",
	_ => true,
	}
	}).collect()
	}

Here is how libsyntax does it: https://github.com/rust-lang/rust/blob/master/src/libsyntax/fold.rs

[dtolnay]

Half of this code is gone in #592 but I am keeping the ticket open to handle the other half.

[dtolnay]

This code has been replaced by rust-lang/rust#37614.