Adding upstream version 115.7.0esr.upstream/115.7.0esrupstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 1952 insertions, 0 deletions

diff --git a/third_party/rust/serde_derive/src/internals/attr.rs b/third_party/rust/serde_derive/src/internals/attr.rs
new file mode 100644
index 0000000000..dc532484d2
--- /dev/null
+++ b/third_party/rust/serde_derive/src/internals/attr.rs
@@ -0,0 +1,1952 @@
+use internals::respan::respan;
+use internals::symbol::*;
+use internals::{ungroup, Ctxt};
+use proc_macro2::{Spacing, Span, TokenStream, TokenTree};
+use quote::ToTokens;
+use std::borrow::Cow;
+use std::collections::BTreeSet;
+use syn;
+use syn::parse::{self, Parse, ParseStream};
+use syn::punctuated::Punctuated;
+use syn::Ident;
+use syn::Meta::{List, NameValue, Path};
+use syn::NestedMeta::{Lit, Meta};
+
+// This module handles parsing of `#[serde(...)]` attributes. The entrypoints
+// are `attr::Container::from_ast`, `attr::Variant::from_ast`, and
+// `attr::Field::from_ast`. Each returns an instance of the corresponding
+// struct. Note that none of them return a Result. Unrecognized, malformed, or
+// duplicated attributes result in a span_err but otherwise are ignored. The
+// user will see errors simultaneously for all bad attributes in the crate
+// rather than just the first.
+
+pub use internals::case::RenameRule;
+
+struct Attr<'c, T> {
+    cx: &'c Ctxt,
+    name: Symbol,
+    tokens: TokenStream,
+    value: Option<T>,
+}
+
+impl<'c, T> Attr<'c, T> {
+    fn none(cx: &'c Ctxt, name: Symbol) -> Self {
+        Attr {
+            cx,
+            name,
+            tokens: TokenStream::new(),
+            value: None,
+        }
+    }
+
+    fn set<A: ToTokens>(&mut self, obj: A, value: T) {
+        let tokens = obj.into_token_stream();
+
+        if self.value.is_some() {
+            self.cx
+                .error_spanned_by(tokens, format!("duplicate serde attribute `{}`", self.name));
+        } else {
+            self.tokens = tokens;
+            self.value = Some(value);
+        }
+    }
+
+    fn set_opt<A: ToTokens>(&mut self, obj: A, value: Option<T>) {
+        if let Some(value) = value {
+            self.set(obj, value);
+        }
+    }
+
+    fn set_if_none(&mut self, value: T) {
+        if self.value.is_none() {
+            self.value = Some(value);
+        }
+    }
+
+    fn get(self) -> Option<T> {
+        self.value
+    }
+
+    fn get_with_tokens(self) -> Option<(TokenStream, T)> {
+        match self.value {
+            Some(v) => Some((self.tokens, v)),
+            None => None,
+        }
+    }
+}
+
+struct BoolAttr<'c>(Attr<'c, ()>);
+
+impl<'c> BoolAttr<'c> {
+    fn none(cx: &'c Ctxt, name: Symbol) -> Self {
+        BoolAttr(Attr::none(cx, name))
+    }
+
+    fn set_true<A: ToTokens>(&mut self, obj: A) {
+        self.0.set(obj, ());
+    }
+
+    fn get(&self) -> bool {
+        self.0.value.is_some()
+    }
+}
+
+struct VecAttr<'c, T> {
+    cx: &'c Ctxt,
+    name: Symbol,
+    first_dup_tokens: TokenStream,
+    values: Vec<T>,
+}
+
+impl<'c, T> VecAttr<'c, T> {
+    fn none(cx: &'c Ctxt, name: Symbol) -> Self {
+        VecAttr {
+            cx,
+            name,
+            first_dup_tokens: TokenStream::new(),
+            values: Vec::new(),
+        }
+    }
+
+    fn insert<A: ToTokens>(&mut self, obj: A, value: T) {
+        if self.values.len() == 1 {
+            self.first_dup_tokens = obj.into_token_stream();
+        }
+        self.values.push(value);
+    }
+
+    fn at_most_one(mut self) -> Result<Option<T>, ()> {
+        if self.values.len() > 1 {
+            let dup_token = self.first_dup_tokens;
+            self.cx.error_spanned_by(
+                dup_token,
+                format!("duplicate serde attribute `{}`", self.name),
+            );
+            Err(())
+        } else {
+            Ok(self.values.pop())
+        }
+    }
+
+    fn get(self) -> Vec<T> {
+        self.values
+    }
+}
+
+pub struct Name {
+    serialize: String,
+    serialize_renamed: bool,
+    deserialize: String,
+    deserialize_renamed: bool,
+    deserialize_aliases: Vec<String>,
+}
+
+#[allow(deprecated)]
+fn unraw(ident: &Ident) -> String {
+    // str::trim_start_matches was added in 1.30, trim_left_matches deprecated
+    // in 1.33. We currently support rustc back to 1.15 so we need to continue
+    // to use the deprecated one.
+    ident.to_string().trim_left_matches("r#").to_owned()
+}
+
+impl Name {
+    fn from_attrs(
+        source_name: String,
+        ser_name: Attr<String>,
+        de_name: Attr<String>,
+        de_aliases: Option<VecAttr<String>>,
+    ) -> Name {
+        let deserialize_aliases = match de_aliases {
+            Some(de_aliases) => {
+                let mut alias_list = BTreeSet::new();
+                for alias_name in de_aliases.get() {
+                    alias_list.insert(alias_name);
+                }
+                alias_list.into_iter().collect()
+            }
+            None => Vec::new(),
+        };
+
+        let ser_name = ser_name.get();
+        let ser_renamed = ser_name.is_some();
+        let de_name = de_name.get();
+        let de_renamed = de_name.is_some();
+        Name {
+            serialize: ser_name.unwrap_or_else(|| source_name.clone()),
+            serialize_renamed: ser_renamed,
+            deserialize: de_name.unwrap_or(source_name),
+            deserialize_renamed: de_renamed,
+            deserialize_aliases,
+        }
+    }
+
+    /// Return the container name for the container when serializing.
+    pub fn serialize_name(&self) -> String {
+        self.serialize.clone()
+    }
+
+    /// Return the container name for the container when deserializing.
+    pub fn deserialize_name(&self) -> String {
+        self.deserialize.clone()
+    }
+
+    fn deserialize_aliases(&self) -> Vec<String> {
+        let mut aliases = self.deserialize_aliases.clone();
+        let main_name = self.deserialize_name();
+        if !aliases.contains(&main_name) {
+            aliases.push(main_name);
+        }
+        aliases
+    }
+}
+
+pub struct RenameAllRules {
+    serialize: RenameRule,
+    deserialize: RenameRule,
+}
+
+/// Represents struct or enum attribute information.
+pub struct Container {
+    name: Name,
+    transparent: bool,
+    deny_unknown_fields: bool,
+    default: Default,
+    rename_all_rules: RenameAllRules,
+    ser_bound: Option<Vec<syn::WherePredicate>>,
+    de_bound: Option<Vec<syn::WherePredicate>>,
+    tag: TagType,
+    type_from: Option<syn::Type>,
+    type_try_from: Option<syn::Type>,
+    type_into: Option<syn::Type>,
+    remote: Option<syn::Path>,
+    identifier: Identifier,
+    has_flatten: bool,
+    serde_path: Option<syn::Path>,
+    is_packed: bool,
+    /// Error message generated when type can't be deserialized
+    expecting: Option<String>,
+}
+
+/// Styles of representing an enum.
+pub enum TagType {
+    /// The default.
+    ///
+    /// ```json
+    /// {"variant1": {"key1": "value1", "key2": "value2"}}
+    /// ```
+    External,
+
+    /// `#[serde(tag = "type")]`
+    ///
+    /// ```json
+    /// {"type": "variant1", "key1": "value1", "key2": "value2"}
+    /// ```
+    Internal { tag: String },
+
+    /// `#[serde(tag = "t", content = "c")]`
+    ///
+    /// ```json
+    /// {"t": "variant1", "c": {"key1": "value1", "key2": "value2"}}
+    /// ```
+    Adjacent { tag: String, content: String },
+
+    /// `#[serde(untagged)]`
+    ///
+    /// ```json
+    /// {"key1": "value1", "key2": "value2"}
+    /// ```
+    None,
+}
+
+/// Whether this enum represents the fields of a struct or the variants of an
+/// enum.
+#[derive(Copy, Clone)]
+pub enum Identifier {
+    /// It does not.
+    No,
+
+    /// This enum represents the fields of a struct. All of the variants must be
+    /// unit variants, except possibly one which is annotated with
+    /// `#[serde(other)]` and is a newtype variant.
+    Field,
+
+    /// This enum represents the variants of an enum. All of the variants must
+    /// be unit variants.
+    Variant,
+}
+
+impl Identifier {
+    #[cfg(feature = "deserialize_in_place")]
+    pub fn is_some(self) -> bool {
+        match self {
+            Identifier::No => false,
+            Identifier::Field | Identifier::Variant => true,
+        }
+    }
+}
+
+impl Container {
+    /// Extract out the `#[serde(...)]` attributes from an item.
+    pub fn from_ast(cx: &Ctxt, item: &syn::DeriveInput) -> Self {
+        let mut ser_name = Attr::none(cx, RENAME);
+        let mut de_name = Attr::none(cx, RENAME);
+        let mut transparent = BoolAttr::none(cx, TRANSPARENT);
+        let mut deny_unknown_fields = BoolAttr::none(cx, DENY_UNKNOWN_FIELDS);
+        let mut default = Attr::none(cx, DEFAULT);
+        let mut rename_all_ser_rule = Attr::none(cx, RENAME_ALL);
+        let mut rename_all_de_rule = Attr::none(cx, RENAME_ALL);
+        let mut ser_bound = Attr::none(cx, BOUND);
+        let mut de_bound = Attr::none(cx, BOUND);
+        let mut untagged = BoolAttr::none(cx, UNTAGGED);
+        let mut internal_tag = Attr::none(cx, TAG);
+        let mut content = Attr::none(cx, CONTENT);
+        let mut type_from = Attr::none(cx, FROM);
+        let mut type_try_from = Attr::none(cx, TRY_FROM);
+        let mut type_into = Attr::none(cx, INTO);
+        let mut remote = Attr::none(cx, REMOTE);
+        let mut field_identifier = BoolAttr::none(cx, FIELD_IDENTIFIER);
+        let mut variant_identifier = BoolAttr::none(cx, VARIANT_IDENTIFIER);
+        let mut serde_path = Attr::none(cx, CRATE);
+        let mut expecting = Attr::none(cx, EXPECTING);
+
+        for meta_item in item
+            .attrs
+            .iter()
+            .flat_map(|attr| get_serde_meta_items(cx, attr))
+            .flatten()
+        {
+            match &meta_item {
+                // Parse `#[serde(rename = "foo")]`
+                Meta(NameValue(m)) if m.path == RENAME => {
+                    if let Ok(s) = get_lit_str(cx, RENAME, &m.lit) {
+                        ser_name.set(&m.path, s.value());
+                        de_name.set(&m.path, s.value());
+                    }
+                }
+
+                // Parse `#[serde(rename(serialize = "foo", deserialize = "bar"))]`
+                Meta(List(m)) if m.path == RENAME => {
+                    if let Ok((ser, de)) = get_renames(cx, &m.nested) {
+                        ser_name.set_opt(&m.path, ser.map(syn::LitStr::value));
+                        de_name.set_opt(&m.path, de.map(syn::LitStr::value));
+                    }
+                }
+
+                // Parse `#[serde(rename_all = "foo")]`
+                Meta(NameValue(m)) if m.path == RENAME_ALL => {
+                    if let Ok(s) = get_lit_str(cx, RENAME_ALL, &m.lit) {
+                        match RenameRule::from_str(&s.value()) {
+                            Ok(rename_rule) => {
+                                rename_all_ser_rule.set(&m.path, rename_rule);
+                                rename_all_de_rule.set(&m.path, rename_rule);
+                            }
+                            Err(err) => cx.error_spanned_by(s, err),
+                        }
+                    }
+                }
+
+                // Parse `#[serde(rename_all(serialize = "foo", deserialize = "bar"))]`
+                Meta(List(m)) if m.path == RENAME_ALL => {
+                    if let Ok((ser, de)) = get_renames(cx, &m.nested) {
+                        if let Some(ser) = ser {
+                            match RenameRule::from_str(&ser.value()) {
+                                Ok(rename_rule) => rename_all_ser_rule.set(&m.path, rename_rule),
+                                Err(err) => cx.error_spanned_by(ser, err),
+                            }
+                        }
+                        if let Some(de) = de {
+                            match RenameRule::from_str(&de.value()) {
+                                Ok(rename_rule) => rename_all_de_rule.set(&m.path, rename_rule),
+                                Err(err) => cx.error_spanned_by(de, err),
+                            }
+                        }
+                    }
+                }
+
+                // Parse `#[serde(transparent)]`
+                Meta(Path(word)) if word == TRANSPARENT => {
+                    transparent.set_true(word);
+                }
+
+                // Parse `#[serde(deny_unknown_fields)]`
+                Meta(Path(word)) if word == DENY_UNKNOWN_FIELDS => {
+                    deny_unknown_fields.set_true(word);
+                }
+
+                // Parse `#[serde(default)]`
+                Meta(Path(word)) if word == DEFAULT => match &item.data {
+                    syn::Data::Struct(syn::DataStruct { fields, .. }) => match fields {
+                        syn::Fields::Named(_) => {
+                            default.set(word, Default::Default);
+                        }
+                        syn::Fields::Unnamed(_) | syn::Fields::Unit => cx.error_spanned_by(
+                            fields,
+                            "#[serde(default)] can only be used on structs with named fields",
+                        ),
+                    },
+                    syn::Data::Enum(syn::DataEnum { enum_token, .. }) => cx.error_spanned_by(
+                        enum_token,
+                        "#[serde(default)] can only be used on structs with named fields",
+                    ),
+                    syn::Data::Union(syn::DataUnion { union_token, .. }) => cx.error_spanned_by(
+                        union_token,
+                        "#[serde(default)] can only be used on structs with named fields",
+                    ),
+                },
+
+                // Parse `#[serde(default = "...")]`
+                Meta(NameValue(m)) if m.path == DEFAULT => {
+                    if let Ok(path) = parse_lit_into_expr_path(cx, DEFAULT, &m.lit) {
+                        match &item.data {
+                            syn::Data::Struct(syn::DataStruct { fields, .. }) => {
+                                match fields {
+                                    syn::Fields::Named(_) => {
+                                        default.set(&m.path, Default::Path(path));
+                                    }
+                                    syn::Fields::Unnamed(_) | syn::Fields::Unit => cx
+                                        .error_spanned_by(
+                                            fields,
+                                            "#[serde(default = \"...\")] can only be used on structs with named fields",
+                                        ),
+                                }
+                            }
+                            syn::Data::Enum(syn::DataEnum { enum_token, .. }) => cx
+                                .error_spanned_by(
+                                    enum_token,
+                                    "#[serde(default = \"...\")] can only be used on structs with named fields",
+                                ),
+                            syn::Data::Union(syn::DataUnion {
+                                union_token, ..
+                            }) => cx.error_spanned_by(
+                                union_token,
+                                "#[serde(default = \"...\")] can only be used on structs with named fields",
+                            ),
+                        }
+                    }
+                }
+
+                // Parse `#[serde(bound = "T: SomeBound")]`
+                Meta(NameValue(m)) if m.path == BOUND => {
+                    if let Ok(where_predicates) = parse_lit_into_where(cx, BOUND, BOUND, &m.lit) {
+                        ser_bound.set(&m.path, where_predicates.clone());
+                        de_bound.set(&m.path, where_predicates);
+                    }
+                }
+
+                // Parse `#[serde(bound(serialize = "...", deserialize = "..."))]`
+                Meta(List(m)) if m.path == BOUND => {
+                    if let Ok((ser, de)) = get_where_predicates(cx, &m.nested) {
+                        ser_bound.set_opt(&m.path, ser);
+                        de_bound.set_opt(&m.path, de);
+                    }
+                }
+
+                // Parse `#[serde(untagged)]`
+                Meta(Path(word)) if word == UNTAGGED => match item.data {
+                    syn::Data::Enum(_) => {
+                        untagged.set_true(word);
+                    }
+                    syn::Data::Struct(syn::DataStruct { struct_token, .. }) => {
+                        cx.error_spanned_by(
+                            struct_token,
+                            "#[serde(untagged)] can only be used on enums",
+                        );
+                    }
+                    syn::Data::Union(syn::DataUnion { union_token, .. }) => {
+                        cx.error_spanned_by(
+                            union_token,
+                            "#[serde(untagged)] can only be used on enums",
+                        );
+                    }
+                },
+
+                // Parse `#[serde(tag = "type")]`
+                Meta(NameValue(m)) if m.path == TAG => {
+                    if let Ok(s) = get_lit_str(cx, TAG, &m.lit) {
+                        match &item.data {
+                            syn::Data::Enum(_) => {
+                                internal_tag.set(&m.path, s.value());
+                            }
+                            syn::Data::Struct(syn::DataStruct { fields, .. }) => match fields {
+                                syn::Fields::Named(_) => {
+                                    internal_tag.set(&m.path, s.value());
+                                }
+                                syn::Fields::Unnamed(_) | syn::Fields::Unit => {
+                                    cx.error_spanned_by(
+                                            fields,
+                                            "#[serde(tag = \"...\")] can only be used on enums and structs with named fields",
+                                        );
+                                }
+                            },
+                            syn::Data::Union(syn::DataUnion { union_token, .. }) => {
+                                cx.error_spanned_by(
+                                    union_token,
+                                    "#[serde(tag = \"...\")] can only be used on enums and structs with named fields",
+                                );
+                            }
+                        }
+                    }
+                }
+
+                // Parse `#[serde(content = "c")]`
+                Meta(NameValue(m)) if m.path == CONTENT => {
+                    if let Ok(s) = get_lit_str(cx, CONTENT, &m.lit) {
+                        match &item.data {
+                            syn::Data::Enum(_) => {
+                                content.set(&m.path, s.value());
+                            }
+                            syn::Data::Struct(syn::DataStruct { struct_token, .. }) => {
+                                cx.error_spanned_by(
+                                    struct_token,
+                                    "#[serde(content = \"...\")] can only be used on enums",
+                                );
+                            }
+                            syn::Data::Union(syn::DataUnion { union_token, .. }) => {
+                                cx.error_spanned_by(
+                                    union_token,
+                                    "#[serde(content = \"...\")] can only be used on enums",
+                                );
+                            }
+                        }
+                    }
+                }
+
+                // Parse `#[serde(from = "Type")]
+                Meta(NameValue(m)) if m.path == FROM => {
+                    if let Ok(from_ty) = parse_lit_into_ty(cx, FROM, &m.lit) {
+                        type_from.set_opt(&m.path, Some(from_ty));
+                    }
+                }
+
+                // Parse `#[serde(try_from = "Type")]
+                Meta(NameValue(m)) if m.path == TRY_FROM => {
+                    if let Ok(try_from_ty) = parse_lit_into_ty(cx, TRY_FROM, &m.lit) {
+                        type_try_from.set_opt(&m.path, Some(try_from_ty));
+                    }
+                }
+
+                // Parse `#[serde(into = "Type")]
+                Meta(NameValue(m)) if m.path == INTO => {
+                    if let Ok(into_ty) = parse_lit_into_ty(cx, INTO, &m.lit) {
+                        type_into.set_opt(&m.path, Some(into_ty));
+                    }
+                }
+
+                // Parse `#[serde(remote = "...")]`
+                Meta(NameValue(m)) if m.path == REMOTE => {
+                    if let Ok(path) = parse_lit_into_path(cx, REMOTE, &m.lit) {
+                        if is_primitive_path(&path, "Self") {
+                            remote.set(&m.path, item.ident.clone().into());
+                        } else {
+                            remote.set(&m.path, path);
+                        }
+                    }
+                }
+
+                // Parse `#[serde(field_identifier)]`
+                Meta(Path(word)) if word == FIELD_IDENTIFIER => {
+                    field_identifier.set_true(word);
+                }
+
+                // Parse `#[serde(variant_identifier)]`
+                Meta(Path(word)) if word == VARIANT_IDENTIFIER => {
+                    variant_identifier.set_true(word);
+                }
+
+                // Parse `#[serde(crate = "foo")]`
+                Meta(NameValue(m)) if m.path == CRATE => {
+                    if let Ok(path) = parse_lit_into_path(cx, CRATE, &m.lit) {
+                        serde_path.set(&m.path, path);
+                    }
+                }
+
+                // Parse `#[serde(expecting = "a message")]`
+                Meta(NameValue(m)) if m.path == EXPECTING => {
+                    if let Ok(s) = get_lit_str(cx, EXPECTING, &m.lit) {
+                        expecting.set(&m.path, s.value());
+                    }
+                }
+
+                Meta(meta_item) => {
+                    let path = meta_item
+                        .path()
+                        .into_token_stream()
+                        .to_string()
+                        .replace(' ', "");
+                    cx.error_spanned_by(
+                        meta_item.path(),
+                        format!("unknown serde container attribute `{}`", path),
+                    );
+                }
+
+                Lit(lit) => {
+                    cx.error_spanned_by(lit, "unexpected literal in serde container attribute");
+                }
+            }
+        }
+
+        let mut is_packed = false;
+        for attr in &item.attrs {
+            if attr.path.is_ident("repr") {
+                let _ = attr.parse_args_with(|input: ParseStream| {
+                    while let Some(token) = input.parse()? {
+                        if let TokenTree::Ident(ident) = token {
+                            is_packed |= ident == "packed";
+                        }
+                    }
+                    Ok(())
+                });
+            }
+        }
+
+        Container {
+            name: Name::from_attrs(unraw(&item.ident), ser_name, de_name, None),
+            transparent: transparent.get(),
+            deny_unknown_fields: deny_unknown_fields.get(),
+            default: default.get().unwrap_or(Default::None),
+            rename_all_rules: RenameAllRules {
+                serialize: rename_all_ser_rule.get().unwrap_or(RenameRule::None),
+                deserialize: rename_all_de_rule.get().unwrap_or(RenameRule::None),
+            },
+            ser_bound: ser_bound.get(),
+            de_bound: de_bound.get(),
+            tag: decide_tag(cx, item, untagged, internal_tag, content),
+            type_from: type_from.get(),
+            type_try_from: type_try_from.get(),
+            type_into: type_into.get(),
+            remote: remote.get(),
+            identifier: decide_identifier(cx, item, field_identifier, variant_identifier),
+            has_flatten: false,
+            serde_path: serde_path.get(),
+            is_packed,
+            expecting: expecting.get(),
+        }
+    }
+
+    pub fn name(&self) -> &Name {
+        &self.name
+    }
+
+    pub fn rename_all_rules(&self) -> &RenameAllRules {
+        &self.rename_all_rules
+    }
+
+    pub fn transparent(&self) -> bool {
+        self.transparent
+    }
+
+    pub fn deny_unknown_fields(&self) -> bool {
+        self.deny_unknown_fields
+    }
+
+    pub fn default(&self) -> &Default {
+        &self.default
+    }
+
+    pub fn ser_bound(&self) -> Option<&[syn::WherePredicate]> {
+        self.ser_bound.as_ref().map(|vec| &vec[..])
+    }
+
+    pub fn de_bound(&self) -> Option<&[syn::WherePredicate]> {
+        self.de_bound.as_ref().map(|vec| &vec[..])
+    }
+
+    pub fn tag(&self) -> &TagType {
+        &self.tag
+    }
+
+    pub fn type_from(&self) -> Option<&syn::Type> {
+        self.type_from.as_ref()
+    }
+
+    pub fn type_try_from(&self) -> Option<&syn::Type> {
+        self.type_try_from.as_ref()
+    }
+
+    pub fn type_into(&self) -> Option<&syn::Type> {
+        self.type_into.as_ref()
+    }
+
+    pub fn remote(&self) -> Option<&syn::Path> {
+        self.remote.as_ref()
+    }
+
+    pub fn is_packed(&self) -> bool {
+        self.is_packed
+    }
+
+    pub fn identifier(&self) -> Identifier {
+        self.identifier
+    }
+
+    pub fn has_flatten(&self) -> bool {
+        self.has_flatten
+    }
+
+    pub fn mark_has_flatten(&mut self) {
+        self.has_flatten = true;
+    }
+
+    pub fn custom_serde_path(&self) -> Option<&syn::Path> {
+        self.serde_path.as_ref()
+    }
+
+    pub fn serde_path(&self) -> Cow<syn::Path> {
+        self.custom_serde_path()
+            .map_or_else(|| Cow::Owned(parse_quote!(_serde)), Cow::Borrowed)
+    }
+
+    /// Error message generated when type can't be deserialized.
+    /// If `None`, default message will be used
+    pub fn expecting(&self) -> Option<&str> {
+        self.expecting.as_ref().map(String::as_ref)
+    }
+}
+
+fn decide_tag(
+    cx: &Ctxt,
+    item: &syn::DeriveInput,
+    untagged: BoolAttr,
+    internal_tag: Attr<String>,
+    content: Attr<String>,
+) -> TagType {
+    match (
+        untagged.0.get_with_tokens(),
+        internal_tag.get_with_tokens(),
+        content.get_with_tokens(),
+    ) {
+        (None, None, None) => TagType::External,
+        (Some(_), None, None) => TagType::None,
+        (None, Some((_, tag)), None) => {
+            // Check that there are no tuple variants.
+            if let syn::Data::Enum(data) = &item.data {
+                for variant in &data.variants {
+                    match &variant.fields {
+                        syn::Fields::Named(_) | syn::Fields::Unit => {}
+                        syn::Fields::Unnamed(fields) => {
+                            if fields.unnamed.len() != 1 {
+                                cx.error_spanned_by(
+                                    variant,
+                                    "#[serde(tag = \"...\")] cannot be used with tuple variants",
+                                );
+                                break;
+                            }
+                        }
+                    }
+                }
+            }
+            TagType::Internal { tag }
+        }
+        (Some((untagged_tokens, _)), Some((tag_tokens, _)), None) => {
+            cx.error_spanned_by(
+                untagged_tokens,
+                "enum cannot be both untagged and internally tagged",
+            );
+            cx.error_spanned_by(
+                tag_tokens,
+                "enum cannot be both untagged and internally tagged",
+            );
+            TagType::External // doesn't matter, will error
+        }
+        (None, None, Some((content_tokens, _))) => {
+            cx.error_spanned_by(
+                content_tokens,
+                "#[serde(tag = \"...\", content = \"...\")] must be used together",
+            );
+            TagType::External
+        }
+        (Some((untagged_tokens, _)), None, Some((content_tokens, _))) => {
+            cx.error_spanned_by(
+                untagged_tokens,
+                "untagged enum cannot have #[serde(content = \"...\")]",
+            );
+            cx.error_spanned_by(
+                content_tokens,
+                "untagged enum cannot have #[serde(content = \"...\")]",
+            );
+            TagType::External
+        }
+        (None, Some((_, tag)), Some((_, content))) => TagType::Adjacent { tag, content },
+        (Some((untagged_tokens, _)), Some((tag_tokens, _)), Some((content_tokens, _))) => {
+            cx.error_spanned_by(
+                untagged_tokens,
+                "untagged enum cannot have #[serde(tag = \"...\", content = \"...\")]",
+            );
+            cx.error_spanned_by(
+                tag_tokens,
+                "untagged enum cannot have #[serde(tag = \"...\", content = \"...\")]",
+            );
+            cx.error_spanned_by(
+                content_tokens,
+                "untagged enum cannot have #[serde(tag = \"...\", content = \"...\")]",
+            );
+            TagType::External
+        }
+    }
+}
+
+fn decide_identifier(
+    cx: &Ctxt,
+    item: &syn::DeriveInput,
+    field_identifier: BoolAttr,
+    variant_identifier: BoolAttr,
+) -> Identifier {
+    match (
+        &item.data,
+        field_identifier.0.get_with_tokens(),
+        variant_identifier.0.get_with_tokens(),
+    ) {
+        (_, None, None) => Identifier::No,
+        (_, Some((field_identifier_tokens, _)), Some((variant_identifier_tokens, _))) => {
+            cx.error_spanned_by(
+                field_identifier_tokens,
+                "#[serde(field_identifier)] and #[serde(variant_identifier)] cannot both be set",
+            );
+            cx.error_spanned_by(
+                variant_identifier_tokens,
+                "#[serde(field_identifier)] and #[serde(variant_identifier)] cannot both be set",
+            );
+            Identifier::No
+        }
+        (syn::Data::Enum(_), Some(_), None) => Identifier::Field,
+        (syn::Data::Enum(_), None, Some(_)) => Identifier::Variant,
+        (syn::Data::Struct(syn::DataStruct { struct_token, .. }), Some(_), None) => {
+            cx.error_spanned_by(
+                struct_token,
+                "#[serde(field_identifier)] can only be used on an enum",
+            );
+            Identifier::No
+        }
+        (syn::Data::Union(syn::DataUnion { union_token, .. }), Some(_), None) => {
+            cx.error_spanned_by(
+                union_token,
+                "#[serde(field_identifier)] can only be used on an enum",
+            );
+            Identifier::No
+        }
+        (syn::Data::Struct(syn::DataStruct { struct_token, .. }), None, Some(_)) => {
+            cx.error_spanned_by(
+                struct_token,
+                "#[serde(variant_identifier)] can only be used on an enum",
+            );
+            Identifier::No
+        }
+        (syn::Data::Union(syn::DataUnion { union_token, .. }), None, Some(_)) => {
+            cx.error_spanned_by(
+                union_token,
+                "#[serde(variant_identifier)] can only be used on an enum",
+            );
+            Identifier::No
+        }
+    }
+}
+
+/// Represents variant attribute information
+pub struct Variant {
+    name: Name,
+    rename_all_rules: RenameAllRules,
+    ser_bound: Option<Vec<syn::WherePredicate>>,
+    de_bound: Option<Vec<syn::WherePredicate>>,
+    skip_deserializing: bool,
+    skip_serializing: bool,
+    other: bool,
+    serialize_with: Option<syn::ExprPath>,
+    deserialize_with: Option<syn::ExprPath>,
+    borrow: Option<syn::Meta>,
+}
+
+impl Variant {
+    pub fn from_ast(cx: &Ctxt, variant: &syn::Variant) -> Self {
+        let mut ser_name = Attr::none(cx, RENAME);
+        let mut de_name = Attr::none(cx, RENAME);
+        let mut de_aliases = VecAttr::none(cx, RENAME);
+        let mut skip_deserializing = BoolAttr::none(cx, SKIP_DESERIALIZING);
+        let mut skip_serializing = BoolAttr::none(cx, SKIP_SERIALIZING);
+        let mut rename_all_ser_rule = Attr::none(cx, RENAME_ALL);
+        let mut rename_all_de_rule = Attr::none(cx, RENAME_ALL);
+        let mut ser_bound = Attr::none(cx, BOUND);
+        let mut de_bound = Attr::none(cx, BOUND);
+        let mut other = BoolAttr::none(cx, OTHER);
+        let mut serialize_with = Attr::none(cx, SERIALIZE_WITH);
+        let mut deserialize_with = Attr::none(cx, DESERIALIZE_WITH);
+        let mut borrow = Attr::none(cx, BORROW);
+
+        for meta_item in variant
+            .attrs
+            .iter()
+            .flat_map(|attr| get_serde_meta_items(cx, attr))
+            .flatten()
+        {
+            match &meta_item {
+                // Parse `#[serde(rename = "foo")]`
+                Meta(NameValue(m)) if m.path == RENAME => {
+                    if let Ok(s) = get_lit_str(cx, RENAME, &m.lit) {
+                        ser_name.set(&m.path, s.value());
+                        de_name.set_if_none(s.value());
+                        de_aliases.insert(&m.path, s.value());
+                    }
+                }
+
+                // Parse `#[serde(rename(serialize = "foo", deserialize = "bar"))]`
+                Meta(List(m)) if m.path == RENAME => {
+                    if let Ok((ser, de)) = get_multiple_renames(cx, &m.nested) {
+                        ser_name.set_opt(&m.path, ser.map(syn::LitStr::value));
+                        for de_value in de {
+                            de_name.set_if_none(de_value.value());
+                            de_aliases.insert(&m.path, de_value.value());
+                        }
+                    }
+                }
+
+                // Parse `#[serde(alias = "foo")]`
+                Meta(NameValue(m)) if m.path == ALIAS => {
+                    if let Ok(s) = get_lit_str(cx, ALIAS, &m.lit) {
+                        de_aliases.insert(&m.path, s.value());
+                    }
+                }
+
+                // Parse `#[serde(rename_all = "foo")]`
+                Meta(NameValue(m)) if m.path == RENAME_ALL => {
+                    if let Ok(s) = get_lit_str(cx, RENAME_ALL, &m.lit) {
+                        match RenameRule::from_str(&s.value()) {
+                            Ok(rename_rule) => {
+                                rename_all_ser_rule.set(&m.path, rename_rule);
+                                rename_all_de_rule.set(&m.path, rename_rule);
+                            }
+                            Err(err) => cx.error_spanned_by(s, err),
+                        }
+                    }
+                }
+
+                // Parse `#[serde(rename_all(serialize = "foo", deserialize = "bar"))]`
+                Meta(List(m)) if m.path == RENAME_ALL => {
+                    if let Ok((ser, de)) = get_renames(cx, &m.nested) {
+                        if let Some(ser) = ser {
+                            match RenameRule::from_str(&ser.value()) {
+                                Ok(rename_rule) => rename_all_ser_rule.set(&m.path, rename_rule),
+                                Err(err) => cx.error_spanned_by(ser, err),
+                            }
+                        }
+                        if let Some(de) = de {
+                            match RenameRule::from_str(&de.value()) {
+                                Ok(rename_rule) => rename_all_de_rule.set(&m.path, rename_rule),
+                                Err(err) => cx.error_spanned_by(de, err),
+                            }
+                        }
+                    }
+                }
+
+                // Parse `#[serde(skip)]`
+                Meta(Path(word)) if word == SKIP => {
+                    skip_serializing.set_true(word);
+                    skip_deserializing.set_true(word);
+                }
+
+                // Parse `#[serde(skip_deserializing)]`
+                Meta(Path(word)) if word == SKIP_DESERIALIZING => {
+                    skip_deserializing.set_true(word);
+                }
+
+                // Parse `#[serde(skip_serializing)]`
+                Meta(Path(word)) if word == SKIP_SERIALIZING => {
+                    skip_serializing.set_true(word);
+                }
+
+                // Parse `#[serde(other)]`
+                Meta(Path(word)) if word == OTHER => {
+                    other.set_true(word);
+                }
+
+                // Parse `#[serde(bound = "T: SomeBound")]`
+                Meta(NameValue(m)) if m.path == BOUND => {
+                    if let Ok(where_predicates) = parse_lit_into_where(cx, BOUND, BOUND, &m.lit) {
+                        ser_bound.set(&m.path, where_predicates.clone());
+                        de_bound.set(&m.path, where_predicates);
+                    }
+                }
+
+                // Parse `#[serde(bound(serialize = "...", deserialize = "..."))]`
+                Meta(List(m)) if m.path == BOUND => {
+                    if let Ok((ser, de)) = get_where_predicates(cx, &m.nested) {
+                        ser_bound.set_opt(&m.path, ser);
+                        de_bound.set_opt(&m.path, de);
+                    }
+                }
+
+                // Parse `#[serde(with = "...")]`
+                Meta(NameValue(m)) if m.path == WITH => {
+                    if let Ok(path) = parse_lit_into_expr_path(cx, WITH, &m.lit) {
+                        let mut ser_path = path.clone();
+                        ser_path
+                            .path
+                            .segments
+                            .push(Ident::new("serialize", Span::call_site()).into());
+                        serialize_with.set(&m.path, ser_path);
+                        let mut de_path = path;
+                        de_path
+                            .path
+                            .segments
+                            .push(Ident::new("deserialize", Span::call_site()).into());
+                        deserialize_with.set(&m.path, de_path);
+                    }
+                }
+
+                // Parse `#[serde(serialize_with = "...")]`
+                Meta(NameValue(m)) if m.path == SERIALIZE_WITH => {
+                    if let Ok(path) = parse_lit_into_expr_path(cx, SERIALIZE_WITH, &m.lit) {
+                        serialize_with.set(&m.path, path);
+                    }
+                }
+
+                // Parse `#[serde(deserialize_with = "...")]`
+                Meta(NameValue(m)) if m.path == DESERIALIZE_WITH => {
+                    if let Ok(path) = parse_lit_into_expr_path(cx, DESERIALIZE_WITH, &m.lit) {
+                        deserialize_with.set(&m.path, path);
+                    }
+                }
+
+                // Defer `#[serde(borrow)]` and `#[serde(borrow = "'a + 'b")]`
+                Meta(m) if m.path() == BORROW => match &variant.fields {
+                    syn::Fields::Unnamed(fields) if fields.unnamed.len() == 1 => {
+                        borrow.set(m.path(), m.clone());
+                    }
+                    _ => {
+                        cx.error_spanned_by(
+                            variant,
+                            "#[serde(borrow)] may only be used on newtype variants",
+                        );
+                    }
+                },
+
+                Meta(meta_item) => {
+                    let path = meta_item
+                        .path()
+                        .into_token_stream()
+                        .to_string()
+                        .replace(' ', "");
+                    cx.error_spanned_by(
+                        meta_item.path(),
+                        format!("unknown serde variant attribute `{}`", path),
+                    );
+                }
+
+                Lit(lit) => {
+                    cx.error_spanned_by(lit, "unexpected literal in serde variant attribute");
+                }
+            }
+        }
+
+        Variant {
+            name: Name::from_attrs(unraw(&variant.ident), ser_name, de_name, Some(de_aliases)),
+            rename_all_rules: RenameAllRules {
+                serialize: rename_all_ser_rule.get().unwrap_or(RenameRule::None),
+                deserialize: rename_all_de_rule.get().unwrap_or(RenameRule::None),
+            },
+            ser_bound: ser_bound.get(),
+            de_bound: de_bound.get(),
+            skip_deserializing: skip_deserializing.get(),
+            skip_serializing: skip_serializing.get(),
+            other: other.get(),
+            serialize_with: serialize_with.get(),
+            deserialize_with: deserialize_with.get(),
+            borrow: borrow.get(),
+        }
+    }
+
+    pub fn name(&self) -> &Name {
+        &self.name
+    }
+
+    pub fn aliases(&self) -> Vec<String> {
+        self.name.deserialize_aliases()
+    }
+
+    pub fn rename_by_rules(&mut self, rules: &RenameAllRules) {
+        if !self.name.serialize_renamed {
+            self.name.serialize = rules.serialize.apply_to_variant(&self.name.serialize);
+        }
+        if !self.name.deserialize_renamed {
+            self.name.deserialize = rules.deserialize.apply_to_variant(&self.name.deserialize);
+        }
+    }
+
+    pub fn rename_all_rules(&self) -> &RenameAllRules {
+        &self.rename_all_rules
+    }
+
+    pub fn ser_bound(&self) -> Option<&[syn::WherePredicate]> {
+        self.ser_bound.as_ref().map(|vec| &vec[..])
+    }
+
+    pub fn de_bound(&self) -> Option<&[syn::WherePredicate]> {
+        self.de_bound.as_ref().map(|vec| &vec[..])
+    }
+
+    pub fn skip_deserializing(&self) -> bool {
+        self.skip_deserializing
+    }
+
+    pub fn skip_serializing(&self) -> bool {
+        self.skip_serializing
+    }
+
+    pub fn other(&self) -> bool {
+        self.other
+    }
+
+    pub fn serialize_with(&self) -> Option<&syn::ExprPath> {
+        self.serialize_with.as_ref()
+    }
+
+    pub fn deserialize_with(&self) -> Option<&syn::ExprPath> {
+        self.deserialize_with.as_ref()
+    }
+}
+
+/// Represents field attribute information
+pub struct Field {
+    name: Name,
+    skip_serializing: bool,
+    skip_deserializing: bool,
+    skip_serializing_if: Option<syn::ExprPath>,
+    default: Default,
+    serialize_with: Option<syn::ExprPath>,
+    deserialize_with: Option<syn::ExprPath>,
+    ser_bound: Option<Vec<syn::WherePredicate>>,
+    de_bound: Option<Vec<syn::WherePredicate>>,
+    borrowed_lifetimes: BTreeSet<syn::Lifetime>,
+    getter: Option<syn::ExprPath>,
+    flatten: bool,
+    transparent: bool,
+}
+
+/// Represents the default to use for a field when deserializing.
+pub enum Default {
+    /// Field must always be specified because it does not have a default.
+    None,
+    /// The default is given by `std::default::Default::default()`.
+    Default,
+    /// The default is given by this function.
+    Path(syn::ExprPath),
+}
+
+impl Default {
+    pub fn is_none(&self) -> bool {
+        match self {
+            Default::None => true,
+            Default::Default | Default::Path(_) => false,
+        }
+    }
+}
+
+impl Field {
+    /// Extract out the `#[serde(...)]` attributes from a struct field.
+    pub fn from_ast(
+        cx: &Ctxt,
+        index: usize,
+        field: &syn::Field,
+        attrs: Option<&Variant>,
+        container_default: &Default,
+    ) -> Self {
+        let mut ser_name = Attr::none(cx, RENAME);
+        let mut de_name = Attr::none(cx, RENAME);
+        let mut de_aliases = VecAttr::none(cx, RENAME);
+        let mut skip_serializing = BoolAttr::none(cx, SKIP_SERIALIZING);
+        let mut skip_deserializing = BoolAttr::none(cx, SKIP_DESERIALIZING);
+        let mut skip_serializing_if = Attr::none(cx, SKIP_SERIALIZING_IF);
+        let mut default = Attr::none(cx, DEFAULT);
+        let mut serialize_with = Attr::none(cx, SERIALIZE_WITH);
+        let mut deserialize_with = Attr::none(cx, DESERIALIZE_WITH);
+        let mut ser_bound = Attr::none(cx, BOUND);
+        let mut de_bound = Attr::none(cx, BOUND);
+        let mut borrowed_lifetimes = Attr::none(cx, BORROW);
+        let mut getter = Attr::none(cx, GETTER);
+        let mut flatten = BoolAttr::none(cx, FLATTEN);
+
+        let ident = match &field.ident {
+            Some(ident) => unraw(ident),
+            None => index.to_string(),
+        };
+
+        let variant_borrow = attrs
+            .and_then(|variant| variant.borrow.as_ref())
+            .map(|borrow| Meta(borrow.clone()));
+
+        for meta_item in field
+            .attrs
+            .iter()
+            .flat_map(|attr| get_serde_meta_items(cx, attr))
+            .flatten()
+            .chain(variant_borrow)
+        {
+            match &meta_item {
+                // Parse `#[serde(rename = "foo")]`
+                Meta(NameValue(m)) if m.path == RENAME => {
+                    if let Ok(s) = get_lit_str(cx, RENAME, &m.lit) {
+                        ser_name.set(&m.path, s.value());
+                        de_name.set_if_none(s.value());
+                        de_aliases.insert(&m.path, s.value());
+                    }
+                }
+
+                // Parse `#[serde(rename(serialize = "foo", deserialize = "bar"))]`
+                Meta(List(m)) if m.path == RENAME => {
+                    if let Ok((ser, de)) = get_multiple_renames(cx, &m.nested) {
+                        ser_name.set_opt(&m.path, ser.map(syn::LitStr::value));
+                        for de_value in de {
+                            de_name.set_if_none(de_value.value());
+                            de_aliases.insert(&m.path, de_value.value());
+                        }
+                    }
+                }
+
+                // Parse `#[serde(alias = "foo")]`
+                Meta(NameValue(m)) if m.path == ALIAS => {
+                    if let Ok(s) = get_lit_str(cx, ALIAS, &m.lit) {
+                        de_aliases.insert(&m.path, s.value());
+                    }
+                }
+
+                // Parse `#[serde(default)]`
+                Meta(Path(word)) if word == DEFAULT => {
+                    default.set(word, Default::Default);
+                }
+
+                // Parse `#[serde(default = "...")]`
+                Meta(NameValue(m)) if m.path == DEFAULT => {
+                    if let Ok(path) = parse_lit_into_expr_path(cx, DEFAULT, &m.lit) {
+                        default.set(&m.path, Default::Path(path));
+                    }
+                }
+
+                // Parse `#[serde(skip_serializing)]`
+                Meta(Path(word)) if word == SKIP_SERIALIZING => {
+                    skip_serializing.set_true(word);
+                }
+
+                // Parse `#[serde(skip_deserializing)]`
+                Meta(Path(word)) if word == SKIP_DESERIALIZING => {
+                    skip_deserializing.set_true(word);
+                }
+
+                // Parse `#[serde(skip)]`
+                Meta(Path(word)) if word == SKIP => {
+                    skip_serializing.set_true(word);
+                    skip_deserializing.set_true(word);
+                }
+
+                // Parse `#[serde(skip_serializing_if = "...")]`
+                Meta(NameValue(m)) if m.path == SKIP_SERIALIZING_IF => {
+                    if let Ok(path) = parse_lit_into_expr_path(cx, SKIP_SERIALIZING_IF, &m.lit) {
+                        skip_serializing_if.set(&m.path, path);
+                    }
+                }
+
+                // Parse `#[serde(serialize_with = "...")]`
+                Meta(NameValue(m)) if m.path == SERIALIZE_WITH => {
+                    if let Ok(path) = parse_lit_into_expr_path(cx, SERIALIZE_WITH, &m.lit) {
+                        serialize_with.set(&m.path, path);
+                    }
+                }
+
+                // Parse `#[serde(deserialize_with = "...")]`
+                Meta(NameValue(m)) if m.path == DESERIALIZE_WITH => {
+                    if let Ok(path) = parse_lit_into_expr_path(cx, DESERIALIZE_WITH, &m.lit) {
+                        deserialize_with.set(&m.path, path);
+                    }
+                }
+
+                // Parse `#[serde(with = "...")]`
+                Meta(NameValue(m)) if m.path == WITH => {
+                    if let Ok(path) = parse_lit_into_expr_path(cx, WITH, &m.lit) {
+                        let mut ser_path = path.clone();
+                        ser_path
+                            .path
+                            .segments
+                            .push(Ident::new("serialize", Span::call_site()).into());
+                        serialize_with.set(&m.path, ser_path);
+                        let mut de_path = path;
+                        de_path
+                            .path
+                            .segments
+                            .push(Ident::new("deserialize", Span::call_site()).into());
+                        deserialize_with.set(&m.path, de_path);
+                    }
+                }
+
+                // Parse `#[serde(bound = "T: SomeBound")]`
+                Meta(NameValue(m)) if m.path == BOUND => {
+                    if let Ok(where_predicates) = parse_lit_into_where(cx, BOUND, BOUND, &m.lit) {
+                        ser_bound.set(&m.path, where_predicates.clone());
+                        de_bound.set(&m.path, where_predicates);
+                    }
+                }
+
+                // Parse `#[serde(bound(serialize = "...", deserialize = "..."))]`
+                Meta(List(m)) if m.path == BOUND => {
+                    if let Ok((ser, de)) = get_where_predicates(cx, &m.nested) {
+                        ser_bound.set_opt(&m.path, ser);
+                        de_bound.set_opt(&m.path, de);
+                    }
+                }
+
+                // Parse `#[serde(borrow)]`
+                Meta(Path(word)) if word == BORROW => {
+                    if let Ok(borrowable) = borrowable_lifetimes(cx, &ident, field) {
+                        borrowed_lifetimes.set(word, borrowable);
+                    }
+                }
+
+                // Parse `#[serde(borrow = "'a + 'b")]`
+                Meta(NameValue(m)) if m.path == BORROW => {
+                    if let Ok(lifetimes) = parse_lit_into_lifetimes(cx, BORROW, &m.lit) {
+                        if let Ok(borrowable) = borrowable_lifetimes(cx, &ident, field) {
+                            for lifetime in &lifetimes {
+                                if !borrowable.contains(lifetime) {
+                                    cx.error_spanned_by(
+                                        field,
+                                        format!(
+                                            "field `{}` does not have lifetime {}",
+                                            ident, lifetime
+                                        ),
+                                    );
+                                }
+                            }
+                            borrowed_lifetimes.set(&m.path, lifetimes);
+                        }
+                    }
+                }
+
+                // Parse `#[serde(getter = "...")]`
+                Meta(NameValue(m)) if m.path == GETTER => {
+                    if let Ok(path) = parse_lit_into_expr_path(cx, GETTER, &m.lit) {
+                        getter.set(&m.path, path);
+                    }
+                }
+
+                // Parse `#[serde(flatten)]`
+                Meta(Path(word)) if word == FLATTEN => {
+                    flatten.set_true(word);
+                }
+
+                Meta(meta_item) => {
+                    let path = meta_item
+                        .path()
+                        .into_token_stream()
+                        .to_string()
+                        .replace(' ', "");
+                    cx.error_spanned_by(
+                        meta_item.path(),
+                        format!("unknown serde field attribute `{}`", path),
+                    );
+                }
+
+                Lit(lit) => {
+                    cx.error_spanned_by(lit, "unexpected literal in serde field attribute");
+                }
+            }
+        }
+
+        // Is skip_deserializing, initialize the field to Default::default() unless a
+        // different default is specified by `#[serde(default = "...")]` on
+        // ourselves or our container (e.g. the struct we are in).
+        if let Default::None = *container_default {
+            if skip_deserializing.0.value.is_some() {
+                default.set_if_none(Default::Default);
+            }
+        }
+
+        let mut borrowed_lifetimes = borrowed_lifetimes.get().unwrap_or_default();
+        if !borrowed_lifetimes.is_empty() {
+            // Cow<str> and Cow<[u8]> never borrow by default:
+            //
+            //     impl<'de, 'a, T: ?Sized> Deserialize<'de> for Cow<'a, T>
+            //
+            // A #[serde(borrow)] attribute enables borrowing that corresponds
+            // roughly to these impls:
+            //
+            //     impl<'de: 'a, 'a> Deserialize<'de> for Cow<'a, str>
+            //     impl<'de: 'a, 'a> Deserialize<'de> for Cow<'a, [u8]>
+            if is_cow(&field.ty, is_str) {
+                let mut path = syn::Path {
+                    leading_colon: None,
+                    segments: Punctuated::new(),
+                };
+                let span = Span::call_site();
+                path.segments.push(Ident::new("_serde", span).into());
+                path.segments.push(Ident::new("__private", span).into());
+                path.segments.push(Ident::new("de", span).into());
+                path.segments
+                    .push(Ident::new("borrow_cow_str", span).into());
+                let expr = syn::ExprPath {
+                    attrs: Vec::new(),
+                    qself: None,
+                    path,
+                };
+                deserialize_with.set_if_none(expr);
+            } else if is_cow(&field.ty, is_slice_u8) {
+                let mut path = syn::Path {
+                    leading_colon: None,
+                    segments: Punctuated::new(),
+                };
+                let span = Span::call_site();
+                path.segments.push(Ident::new("_serde", span).into());
+                path.segments.push(Ident::new("__private", span).into());
+                path.segments.push(Ident::new("de", span).into());
+                path.segments
+                    .push(Ident::new("borrow_cow_bytes", span).into());
+                let expr = syn::ExprPath {
+                    attrs: Vec::new(),
+                    qself: None,
+                    path,
+                };
+                deserialize_with.set_if_none(expr);
+            }
+        } else if is_implicitly_borrowed(&field.ty) {
+            // Types &str and &[u8] are always implicitly borrowed. No need for
+            // a #[serde(borrow)].
+            collect_lifetimes(&field.ty, &mut borrowed_lifetimes);
+        }
+
+        Field {
+            name: Name::from_attrs(ident, ser_name, de_name, Some(de_aliases)),
+            skip_serializing: skip_serializing.get(),
+            skip_deserializing: skip_deserializing.get(),
+            skip_serializing_if: skip_serializing_if.get(),
+            default: default.get().unwrap_or(Default::None),
+            serialize_with: serialize_with.get(),
+            deserialize_with: deserialize_with.get(),
+            ser_bound: ser_bound.get(),
+            de_bound: de_bound.get(),
+            borrowed_lifetimes,
+            getter: getter.get(),
+            flatten: flatten.get(),
+            transparent: false,
+        }
+    }
+
+    pub fn name(&self) -> &Name {
+        &self.name
+    }
+
+    pub fn aliases(&self) -> Vec<String> {
+        self.name.deserialize_aliases()
+    }
+
+    pub fn rename_by_rules(&mut self, rules: &RenameAllRules) {
+        if !self.name.serialize_renamed {
+            self.name.serialize = rules.serialize.apply_to_field(&self.name.serialize);
+        }
+        if !self.name.deserialize_renamed {
+            self.name.deserialize = rules.deserialize.apply_to_field(&self.name.deserialize);
+        }
+    }
+
+    pub fn skip_serializing(&self) -> bool {
+        self.skip_serializing
+    }
+
+    pub fn skip_deserializing(&self) -> bool {
+        self.skip_deserializing
+    }
+
+    pub fn skip_serializing_if(&self) -> Option<&syn::ExprPath> {
+        self.skip_serializing_if.as_ref()
+    }
+
+    pub fn default(&self) -> &Default {
+        &self.default
+    }
+
+    pub fn serialize_with(&self) -> Option<&syn::ExprPath> {
+        self.serialize_with.as_ref()
+    }
+
+    pub fn deserialize_with(&self) -> Option<&syn::ExprPath> {
+        self.deserialize_with.as_ref()
+    }
+
+    pub fn ser_bound(&self) -> Option<&[syn::WherePredicate]> {
+        self.ser_bound.as_ref().map(|vec| &vec[..])
+    }
+
+    pub fn de_bound(&self) -> Option<&[syn::WherePredicate]> {
+        self.de_bound.as_ref().map(|vec| &vec[..])
+    }
+
+    pub fn borrowed_lifetimes(&self) -> &BTreeSet<syn::Lifetime> {
+        &self.borrowed_lifetimes
+    }
+
+    pub fn getter(&self) -> Option<&syn::ExprPath> {
+        self.getter.as_ref()
+    }
+
+    pub fn flatten(&self) -> bool {
+        self.flatten
+    }
+
+    pub fn transparent(&self) -> bool {
+        self.transparent
+    }
+
+    pub fn mark_transparent(&mut self) {
+        self.transparent = true;
+    }
+}
+
+type SerAndDe<T> = (Option<T>, Option<T>);
+
+fn get_ser_and_de<'a, 'b, T, F>(
+    cx: &'b Ctxt,
+    attr_name: Symbol,
+    metas: &'a Punctuated<syn::NestedMeta, Token![,]>,
+    f: F,
+) -> Result<(VecAttr<'b, T>, VecAttr<'b, T>), ()>
+where
+    T: 'a,
+    F: Fn(&Ctxt, Symbol, Symbol, &'a syn::Lit) -> Result<T, ()>,
+{
+    let mut ser_meta = VecAttr::none(cx, attr_name);
+    let mut de_meta = VecAttr::none(cx, attr_name);
+
+    for meta in metas {
+        match meta {
+            Meta(NameValue(meta)) if meta.path == SERIALIZE => {
+                if let Ok(v) = f(cx, attr_name, SERIALIZE, &meta.lit) {
+                    ser_meta.insert(&meta.path, v);
+                }
+            }
+
+            Meta(NameValue(meta)) if meta.path == DESERIALIZE => {
+                if let Ok(v) = f(cx, attr_name, DESERIALIZE, &meta.lit) {
+                    de_meta.insert(&meta.path, v);
+                }
+            }
+
+            _ => {
+                cx.error_spanned_by(
+                    meta,
+                    format!(
+                        "malformed {0} attribute, expected `{0}(serialize = ..., deserialize = ...)`",
+                        attr_name
+                    ),
+                );
+                return Err(());
+            }
+        }
+    }
+
+    Ok((ser_meta, de_meta))
+}
+
+fn get_renames<'a>(
+    cx: &Ctxt,
+    items: &'a Punctuated<syn::NestedMeta, Token![,]>,
+) -> Result<SerAndDe<&'a syn::LitStr>, ()> {
+    let (ser, de) = get_ser_and_de(cx, RENAME, items, get_lit_str2)?;
+    Ok((ser.at_most_one()?, de.at_most_one()?))
+}
+
+fn get_multiple_renames<'a>(
+    cx: &Ctxt,
+    items: &'a Punctuated<syn::NestedMeta, Token![,]>,
+) -> Result<(Option<&'a syn::LitStr>, Vec<&'a syn::LitStr>), ()> {
+    let (ser, de) = get_ser_and_de(cx, RENAME, items, get_lit_str2)?;
+    Ok((ser.at_most_one()?, de.get()))
+}
+
+fn get_where_predicates(
+    cx: &Ctxt,
+    items: &Punctuated<syn::NestedMeta, Token![,]>,
+) -> Result<SerAndDe<Vec<syn::WherePredicate>>, ()> {
+    let (ser, de) = get_ser_and_de(cx, BOUND, items, parse_lit_into_where)?;
+    Ok((ser.at_most_one()?, de.at_most_one()?))
+}
+
+pub fn get_serde_meta_items(cx: &Ctxt, attr: &syn::Attribute) -> Result<Vec<syn::NestedMeta>, ()> {
+    if attr.path != SERDE {
+        return Ok(Vec::new());
+    }
+
+    match attr.parse_meta() {
+        Ok(List(meta)) => Ok(meta.nested.into_iter().collect()),
+        Ok(other) => {
+            cx.error_spanned_by(other, "expected #[serde(...)]");
+            Err(())
+        }
+        Err(err) => {
+            cx.syn_error(err);
+            Err(())
+        }
+    }
+}
+
+fn get_lit_str<'a>(cx: &Ctxt, attr_name: Symbol, lit: &'a syn::Lit) -> Result<&'a syn::LitStr, ()> {
+    get_lit_str2(cx, attr_name, attr_name, lit)
+}
+
+fn get_lit_str2<'a>(
+    cx: &Ctxt,
+    attr_name: Symbol,
+    meta_item_name: Symbol,
+    lit: &'a syn::Lit,
+) -> Result<&'a syn::LitStr, ()> {
+    if let syn::Lit::Str(lit) = lit {
+        Ok(lit)
+    } else {
+        cx.error_spanned_by(
+            lit,
+            format!(
+                "expected serde {} attribute to be a string: `{} = \"...\"`",
+                attr_name, meta_item_name
+            ),
+        );
+        Err(())
+    }
+}
+
+fn parse_lit_into_path(cx: &Ctxt, attr_name: Symbol, lit: &syn::Lit) -> Result<syn::Path, ()> {
+    let string = get_lit_str(cx, attr_name, lit)?;
+    parse_lit_str(string).map_err(|_| {
+        cx.error_spanned_by(lit, format!("failed to parse path: {:?}", string.value()));
+    })
+}
+
+fn parse_lit_into_expr_path(
+    cx: &Ctxt,
+    attr_name: Symbol,
+    lit: &syn::Lit,
+) -> Result<syn::ExprPath, ()> {
+    let string = get_lit_str(cx, attr_name, lit)?;
+    parse_lit_str(string).map_err(|_| {
+        cx.error_spanned_by(lit, format!("failed to parse path: {:?}", string.value()));
+    })
+}
+
+fn parse_lit_into_where(
+    cx: &Ctxt,
+    attr_name: Symbol,
+    meta_item_name: Symbol,
+    lit: &syn::Lit,
+) -> Result<Vec<syn::WherePredicate>, ()> {
+    let string = get_lit_str2(cx, attr_name, meta_item_name, lit)?;
+    if string.value().is_empty() {
+        return Ok(Vec::new());
+    }
+
+    let where_string = syn::LitStr::new(&format!("where {}", string.value()), string.span());
+
+    parse_lit_str::<syn::WhereClause>(&where_string)
+        .map(|wh| wh.predicates.into_iter().collect())
+        .map_err(|err| cx.error_spanned_by(lit, err))
+}
+
+fn parse_lit_into_ty(cx: &Ctxt, attr_name: Symbol, lit: &syn::Lit) -> Result<syn::Type, ()> {
+    let string = get_lit_str(cx, attr_name, lit)?;
+
+    parse_lit_str(string).map_err(|_| {
+        cx.error_spanned_by(
+            lit,
+            format!("failed to parse type: {} = {:?}", attr_name, string.value()),
+        );
+    })
+}
+
+// Parses a string literal like "'a + 'b + 'c" containing a nonempty list of
+// lifetimes separated by `+`.
+fn parse_lit_into_lifetimes(
+    cx: &Ctxt,
+    attr_name: Symbol,
+    lit: &syn::Lit,
+) -> Result<BTreeSet<syn::Lifetime>, ()> {
+    let string = get_lit_str(cx, attr_name, lit)?;
+    if string.value().is_empty() {
+        cx.error_spanned_by(lit, "at least one lifetime must be borrowed");
+        return Err(());
+    }
+
+    struct BorrowedLifetimes(Punctuated<syn::Lifetime, Token![+]>);
+
+    impl Parse for BorrowedLifetimes {
+        fn parse(input: ParseStream) -> parse::Result<Self> {
+            Punctuated::parse_separated_nonempty(input).map(BorrowedLifetimes)
+        }
+    }
+
+    if let Ok(BorrowedLifetimes(lifetimes)) = parse_lit_str(string) {
+        let mut set = BTreeSet::new();
+        for lifetime in lifetimes {
+            if !set.insert(lifetime.clone()) {
+                cx.error_spanned_by(lit, format!("duplicate borrowed lifetime `{}`", lifetime));
+            }
+        }
+        return Ok(set);
+    }
+
+    cx.error_spanned_by(
+        lit,
+        format!("failed to parse borrowed lifetimes: {:?}", string.value()),
+    );
+    Err(())
+}
+
+fn is_implicitly_borrowed(ty: &syn::Type) -> bool {
+    is_implicitly_borrowed_reference(ty) || is_option(ty, is_implicitly_borrowed_reference)
+}
+
+fn is_implicitly_borrowed_reference(ty: &syn::Type) -> bool {
+    is_reference(ty, is_str) || is_reference(ty, is_slice_u8)
+}
+
+// Whether the type looks like it might be `std::borrow::Cow<T>` where elem="T".
+// This can have false negatives and false positives.
+//
+// False negative:
+//
+//     use std::borrow::Cow as Pig;
+//
+//     #[derive(Deserialize)]
+//     struct S<'a> {
+//         #[serde(borrow)]
+//         pig: Pig<'a, str>,
+//     }
+//
+// False positive:
+//
+//     type str = [i16];
+//
+//     #[derive(Deserialize)]
+//     struct S<'a> {
+//         #[serde(borrow)]
+//         cow: Cow<'a, str>,
+//     }
+fn is_cow(ty: &syn::Type, elem: fn(&syn::Type) -> bool) -> bool {
+    let path = match ungroup(ty) {
+        syn::Type::Path(ty) => &ty.path,
+        _ => {
+            return false;
+        }
+    };
+    let seg = match path.segments.last() {
+        Some(seg) => seg,
+        None => {
+            return false;
+        }
+    };
+    let args = match &seg.arguments {
+        syn::PathArguments::AngleBracketed(bracketed) => &bracketed.args,
+        _ => {
+            return false;
+        }
+    };
+    seg.ident == "Cow"
+        && args.len() == 2
+        && match (&args[0], &args[1]) {
+            (syn::GenericArgument::Lifetime(_), syn::GenericArgument::Type(arg)) => elem(arg),
+            _ => false,
+        }
+}
+
+fn is_option(ty: &syn::Type, elem: fn(&syn::Type) -> bool) -> bool {
+    let path = match ungroup(ty) {
+        syn::Type::Path(ty) => &ty.path,
+        _ => {
+            return false;
+        }
+    };
+    let seg = match path.segments.last() {
+        Some(seg) => seg,
+        None => {
+            return false;
+        }
+    };
+    let args = match &seg.arguments {
+        syn::PathArguments::AngleBracketed(bracketed) => &bracketed.args,
+        _ => {
+            return false;
+        }
+    };
+    seg.ident == "Option"
+        && args.len() == 1
+        && match &args[0] {
+            syn::GenericArgument::Type(arg) => elem(arg),
+            _ => false,
+        }
+}
+
+// Whether the type looks like it might be `&T` where elem="T". This can have
+// false negatives and false positives.
+//
+// False negative:
+//
+//     type Yarn = str;
+//
+//     #[derive(Deserialize)]
+//     struct S<'a> {
+//         r: &'a Yarn,
+//     }
+//
+// False positive:
+//
+//     type str = [i16];
+//
+//     #[derive(Deserialize)]
+//     struct S<'a> {
+//         r: &'a str,
+//     }
+fn is_reference(ty: &syn::Type, elem: fn(&syn::Type) -> bool) -> bool {
+    match ungroup(ty) {
+        syn::Type::Reference(ty) => ty.mutability.is_none() && elem(&ty.elem),
+        _ => false,
+    }
+}
+
+fn is_str(ty: &syn::Type) -> bool {
+    is_primitive_type(ty, "str")
+}
+
+fn is_slice_u8(ty: &syn::Type) -> bool {
+    match ungroup(ty) {
+        syn::Type::Slice(ty) => is_primitive_type(&ty.elem, "u8"),
+        _ => false,
+    }
+}
+
+fn is_primitive_type(ty: &syn::Type, primitive: &str) -> bool {
+    match ungroup(ty) {
+        syn::Type::Path(ty) => ty.qself.is_none() && is_primitive_path(&ty.path, primitive),
+        _ => false,
+    }
+}
+
+fn is_primitive_path(path: &syn::Path, primitive: &str) -> bool {
+    path.leading_colon.is_none()
+        && path.segments.len() == 1
+        && path.segments[0].ident == primitive
+        && path.segments[0].arguments.is_empty()
+}
+
+// All lifetimes that this type could borrow from a Deserializer.
+//
+// For example a type `S<'a, 'b>` could borrow `'a` and `'b`. On the other hand
+// a type `for<'a> fn(&'a str)` could not borrow `'a` from the Deserializer.
+//
+// This is used when there is an explicit or implicit `#[serde(borrow)]`
+// attribute on the field so there must be at least one borrowable lifetime.
+fn borrowable_lifetimes(
+    cx: &Ctxt,
+    name: &str,
+    field: &syn::Field,
+) -> Result<BTreeSet<syn::Lifetime>, ()> {
+    let mut lifetimes = BTreeSet::new();
+    collect_lifetimes(&field.ty, &mut lifetimes);
+    if lifetimes.is_empty() {
+        cx.error_spanned_by(
+            field,
+            format!("field `{}` has no lifetimes to borrow", name),
+        );
+        Err(())
+    } else {
+        Ok(lifetimes)
+    }
+}
+
+fn collect_lifetimes(ty: &syn::Type, out: &mut BTreeSet<syn::Lifetime>) {
+    match ty {
+        syn::Type::Slice(ty) => {
+            collect_lifetimes(&ty.elem, out);
+        }
+        syn::Type::Array(ty) => {
+            collect_lifetimes(&ty.elem, out);
+        }
+        syn::Type::Ptr(ty) => {
+            collect_lifetimes(&ty.elem, out);
+        }
+        syn::Type::Reference(ty) => {
+            out.extend(ty.lifetime.iter().cloned());
+            collect_lifetimes(&ty.elem, out);
+        }
+        syn::Type::Tuple(ty) => {
+            for elem in &ty.elems {
+                collect_lifetimes(elem, out);
+            }
+        }
+        syn::Type::Path(ty) => {
+            if let Some(qself) = &ty.qself {
+                collect_lifetimes(&qself.ty, out);
+            }
+            for seg in &ty.path.segments {
+                if let syn::PathArguments::AngleBracketed(bracketed) = &seg.arguments {
+                    for arg in &bracketed.args {
+                        match arg {
+                            syn::GenericArgument::Lifetime(lifetime) => {
+                                out.insert(lifetime.clone());
+                            }
+                            syn::GenericArgument::Type(ty) => {
+                                collect_lifetimes(ty, out);
+                            }
+                            syn::GenericArgument::Binding(binding) => {
+                                collect_lifetimes(&binding.ty, out);
+                            }
+                            syn::GenericArgument::Constraint(_)
+                            | syn::GenericArgument::Const(_) => {}
+                        }
+                    }
+                }
+            }
+        }
+        syn::Type::Paren(ty) => {
+            collect_lifetimes(&ty.elem, out);
+        }
+        syn::Type::Group(ty) => {
+            collect_lifetimes(&ty.elem, out);
+        }
+        syn::Type::Macro(ty) => {
+            collect_lifetimes_from_tokens(ty.mac.tokens.clone(), out);
+        }
+        syn::Type::BareFn(_)
+        | syn::Type::Never(_)
+        | syn::Type::TraitObject(_)
+        | syn::Type::ImplTrait(_)
+        | syn::Type::Infer(_)
+        | syn::Type::Verbatim(_) => {}
+
+        #[cfg_attr(all(test, exhaustive), deny(non_exhaustive_omitted_patterns))]
+        _ => {}
+    }
+}
+
+fn collect_lifetimes_from_tokens(tokens: TokenStream, out: &mut BTreeSet<syn::Lifetime>) {
+    let mut iter = tokens.into_iter();
+    while let Some(tt) = iter.next() {
+        match &tt {
+            TokenTree::Punct(op) if op.as_char() == '\'' && op.spacing() == Spacing::Joint => {
+                if let Some(TokenTree::Ident(ident)) = iter.next() {
+                    out.insert(syn::Lifetime {
+                        apostrophe: op.span(),
+                        ident,
+                    });
+                }
+            }
+            TokenTree::Group(group) => {
+                let tokens = group.stream();
+                collect_lifetimes_from_tokens(tokens, out);
+            }
+            _ => {}
+        }
+    }
+}
+
+fn parse_lit_str<T>(s: &syn::LitStr) -> parse::Result<T>
+where
+    T: Parse,
+{
+    let tokens = spanned_tokens(s)?;
+    syn::parse2(tokens)
+}
+
+fn spanned_tokens(s: &syn::LitStr) -> parse::Result<TokenStream> {
+    let stream = syn::parse_str(&s.value())?;
+    Ok(respan(stream, s.span()))
+}