Initial vendor packages

Signed-off-by: Valentin Popov <valentin@popov.link>

1 files changed, 1881 insertions, 0 deletions

diff --git a/vendor/serde_derive/src/internals/attr.rs b/vendor/serde_derive/src/internals/attr.rs
new file mode 100644
index 0000000..bb9de32
--- /dev/null
+++ b/vendor/serde_derive/src/internals/attr.rs
@@ -0,0 +1,1881 @@
+use crate::internals::symbol::*;
+use crate::internals::{ungroup, Ctxt};
+use proc_macro2::{Spacing, Span, TokenStream, TokenTree};
+use quote::ToTokens;
+use std::borrow::Cow;
+use std::collections::BTreeSet;
+use std::iter::FromIterator;
+use syn::meta::ParseNestedMeta;
+use syn::parse::ParseStream;
+use syn::punctuated::Punctuated;
+use syn::{parse_quote, token, Ident, Lifetime, Token};
+
+// 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 crate::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() {
+            let msg = format!("duplicate serde attribute `{}`", self.name);
+            self.cx.error_spanned_by(tokens, msg);
+        } 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) -> Option<T> {
+        if self.values.len() > 1 {
+            let dup_token = self.first_dup_tokens;
+            let msg = format!("duplicate serde attribute `{}`", self.name);
+            self.cx.error_spanned_by(dup_token, msg);
+            None
+        } else {
+            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: BTreeSet<String>,
+}
+
+fn unraw(ident: &Ident) -> String {
+    ident.to_string().trim_start_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 mut alias_set = BTreeSet::new();
+        if let Some(de_aliases) = de_aliases {
+            for alias_name in de_aliases.get() {
+                alias_set.insert(alias_name);
+            }
+        }
+
+        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: alias_set,
+        }
+    }
+
+    /// Return the container name for the container when serializing.
+    pub fn serialize_name(&self) -> &str {
+        &self.serialize
+    }
+
+    /// Return the container name for the container when deserializing.
+    pub fn deserialize_name(&self) -> &str {
+        &self.deserialize
+    }
+
+    fn deserialize_aliases(&self) -> &BTreeSet<String> {
+        &self.deserialize_aliases
+    }
+}
+
+#[derive(Copy, Clone)]
+pub struct RenameAllRules {
+    serialize: RenameRule,
+    deserialize: RenameRule,
+}
+
+impl RenameAllRules {
+    /// Returns a new `RenameAllRules` with the individual rules of `self` and
+    /// `other_rules` joined by `RenameRules::or`.
+    pub fn or(self, other_rules: Self) -> Self {
+        Self {
+            serialize: self.serialize.or(other_rules.serialize),
+            deserialize: self.deserialize.or(other_rules.deserialize),
+        }
+    }
+}
+
+/// Represents struct or enum attribute information.
+pub struct Container {
+    name: Name,
+    transparent: bool,
+    deny_unknown_fields: bool,
+    default: Default,
+    rename_all_rules: RenameAllRules,
+    rename_all_fields_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>,
+    non_exhaustive: bool,
+}
+
+/// 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 rename_all_fields_ser_rule = Attr::none(cx, RENAME_ALL_FIELDS);
+        let mut rename_all_fields_de_rule = Attr::none(cx, RENAME_ALL_FIELDS);
+        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);
+        let mut non_exhaustive = false;
+
+        for attr in &item.attrs {
+            if attr.path() != SERDE {
+                non_exhaustive |=
+                    matches!(&attr.meta, syn::Meta::Path(path) if path == NON_EXHAUSTIVE);
+                continue;
+            }
+
+            if let syn::Meta::List(meta) = &attr.meta {
+                if meta.tokens.is_empty() {
+                    continue;
+                }
+            }
+
+            if let Err(err) = attr.parse_nested_meta(|meta| {
+                if meta.path == RENAME {
+                    // #[serde(rename = "foo")]
+                    // #[serde(rename(serialize = "foo", deserialize = "bar"))]
+                    let (ser, de) = get_renames(cx, RENAME, &meta)?;
+                    ser_name.set_opt(&meta.path, ser.as_ref().map(syn::LitStr::value));
+                    de_name.set_opt(&meta.path, de.as_ref().map(syn::LitStr::value));
+                } else if meta.path == RENAME_ALL {
+                    // #[serde(rename_all = "foo")]
+                    // #[serde(rename_all(serialize = "foo", deserialize = "bar"))]
+                    let one_name = meta.input.peek(Token![=]);
+                    let (ser, de) = get_renames(cx, RENAME_ALL, &meta)?;
+                    if let Some(ser) = ser {
+                        match RenameRule::from_str(&ser.value()) {
+                            Ok(rename_rule) => rename_all_ser_rule.set(&meta.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(&meta.path, rename_rule),
+                            Err(err) => {
+                                if !one_name {
+                                    cx.error_spanned_by(de, err);
+                                }
+                            }
+                        }
+                    }
+                } else if meta.path == RENAME_ALL_FIELDS {
+                    // #[serde(rename_all_fields = "foo")]
+                    // #[serde(rename_all_fields(serialize = "foo", deserialize = "bar"))]
+                    let one_name = meta.input.peek(Token![=]);
+                    let (ser, de) = get_renames(cx, RENAME_ALL_FIELDS, &meta)?;
+
+                    match item.data {
+                        syn::Data::Enum(_) => {
+                            if let Some(ser) = ser {
+                                match RenameRule::from_str(&ser.value()) {
+                                    Ok(rename_rule) => {
+                                        rename_all_fields_ser_rule.set(&meta.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_fields_de_rule.set(&meta.path, rename_rule);
+                                    }
+                                    Err(err) => {
+                                        if !one_name {
+                                            cx.error_spanned_by(de, err);
+                                        }
+                                    }
+                                }
+                            }
+                        }
+                        syn::Data::Struct(_) => {
+                            let msg = "#[serde(rename_all_fields)] can only be used on enums";
+                            cx.syn_error(meta.error(msg));
+                        }
+                        syn::Data::Union(_) => {
+                            let msg = "#[serde(rename_all_fields)] can only be used on enums";
+                            cx.syn_error(meta.error(msg));
+                        }
+                    }
+                } else if meta.path == TRANSPARENT {
+                    // #[serde(transparent)]
+                    transparent.set_true(meta.path);
+                } else if meta.path == DENY_UNKNOWN_FIELDS {
+                    // #[serde(deny_unknown_fields)]
+                    deny_unknown_fields.set_true(meta.path);
+                } else if meta.path == DEFAULT {
+                    if meta.input.peek(Token![=]) {
+                        // #[serde(default = "...")]
+                        if let Some(path) = parse_lit_into_expr_path(cx, DEFAULT, &meta)? {
+                            match &item.data {
+                                syn::Data::Struct(syn::DataStruct { fields, .. }) => match fields {
+                                    syn::Fields::Named(_) | syn::Fields::Unnamed(_) => {
+                                        default.set(&meta.path, Default::Path(path));
+                                    }
+                                    syn::Fields::Unit => {
+                                        let msg = "#[serde(default = \"...\")] can only be used on structs that have fields";
+                                        cx.syn_error(meta.error(msg));
+                                    }
+                                },
+                                syn::Data::Enum(_) => {
+                                    let msg = "#[serde(default = \"...\")] can only be used on structs";
+                                    cx.syn_error(meta.error(msg));
+                                }
+                                syn::Data::Union(_) => {
+                                    let msg = "#[serde(default = \"...\")] can only be used on structs";
+                                    cx.syn_error(meta.error(msg));
+                                }
+                            }
+                        }
+                    } else {
+                        // #[serde(default)]
+                        match &item.data {
+                            syn::Data::Struct(syn::DataStruct { fields, .. }) => match fields {
+                                syn::Fields::Named(_) | syn::Fields::Unnamed(_) => {
+                                    default.set(meta.path, Default::Default);
+                                }
+                                syn::Fields::Unit => {
+                                    let msg = "#[serde(default)] can only be used on structs that have fields";
+                                    cx.error_spanned_by(fields, msg);
+                                }
+                            },
+                            syn::Data::Enum(_) => {
+                                let msg = "#[serde(default)] can only be used on structs";
+                                cx.syn_error(meta.error(msg));
+                            }
+                            syn::Data::Union(_) => {
+                                let msg = "#[serde(default)] can only be used on structs";
+                                cx.syn_error(meta.error(msg));
+                            }
+                        }
+                    }
+                } else if meta.path == BOUND {
+                    // #[serde(bound = "T: SomeBound")]
+                    // #[serde(bound(serialize = "...", deserialize = "..."))]
+                    let (ser, de) = get_where_predicates(cx, &meta)?;
+                    ser_bound.set_opt(&meta.path, ser);
+                    de_bound.set_opt(&meta.path, de);
+                } else if meta.path == UNTAGGED {
+                    // #[serde(untagged)]
+                    match item.data {
+                        syn::Data::Enum(_) => {
+                            untagged.set_true(&meta.path);
+                        }
+                        syn::Data::Struct(_) => {
+                            let msg = "#[serde(untagged)] can only be used on enums";
+                            cx.syn_error(meta.error(msg));
+                        }
+                        syn::Data::Union(_) => {
+                            let msg = "#[serde(untagged)] can only be used on enums";
+                            cx.syn_error(meta.error(msg));
+                        }
+                    }
+                } else if meta.path == TAG {
+                    // #[serde(tag = "type")]
+                    if let Some(s) = get_lit_str(cx, TAG, &meta)? {
+                        match &item.data {
+                            syn::Data::Enum(_) => {
+                                internal_tag.set(&meta.path, s.value());
+                            }
+                            syn::Data::Struct(syn::DataStruct { fields, .. }) => match fields {
+                                syn::Fields::Named(_) => {
+                                    internal_tag.set(&meta.path, s.value());
+                                }
+                                syn::Fields::Unnamed(_) | syn::Fields::Unit => {
+                                    let msg = "#[serde(tag = \"...\")] can only be used on enums and structs with named fields";
+                                    cx.syn_error(meta.error(msg));
+                                }
+                            },
+                            syn::Data::Union(_) => {
+                                let msg = "#[serde(tag = \"...\")] can only be used on enums and structs with named fields";
+                                cx.syn_error(meta.error(msg));
+                            }
+                        }
+                    }
+                } else if meta.path == CONTENT {
+                    // #[serde(content = "c")]
+                    if let Some(s) = get_lit_str(cx, CONTENT, &meta)? {
+                        match &item.data {
+                            syn::Data::Enum(_) => {
+                                content.set(&meta.path, s.value());
+                            }
+                            syn::Data::Struct(_) => {
+                                let msg = "#[serde(content = \"...\")] can only be used on enums";
+                                cx.syn_error(meta.error(msg));
+                            }
+                            syn::Data::Union(_) => {
+                                let msg = "#[serde(content = \"...\")] can only be used on enums";
+                                cx.syn_error(meta.error(msg));
+                            }
+                        }
+                    }
+                } else if meta.path == FROM {
+                    // #[serde(from = "Type")]
+                    if let Some(from_ty) = parse_lit_into_ty(cx, FROM, &meta)? {
+                        type_from.set_opt(&meta.path, Some(from_ty));
+                    }
+                } else if meta.path == TRY_FROM {
+                    // #[serde(try_from = "Type")]
+                    if let Some(try_from_ty) = parse_lit_into_ty(cx, TRY_FROM, &meta)? {
+                        type_try_from.set_opt(&meta.path, Some(try_from_ty));
+                    }
+                } else if meta.path == INTO {
+                    // #[serde(into = "Type")]
+                    if let Some(into_ty) = parse_lit_into_ty(cx, INTO, &meta)? {
+                        type_into.set_opt(&meta.path, Some(into_ty));
+                    }
+                } else if meta.path == REMOTE {
+                    // #[serde(remote = "...")]
+                    if let Some(path) = parse_lit_into_path(cx, REMOTE, &meta)? {
+                        if is_primitive_path(&path, "Self") {
+                            remote.set(&meta.path, item.ident.clone().into());
+                        } else {
+                            remote.set(&meta.path, path);
+                        }
+                    }
+                } else if meta.path == FIELD_IDENTIFIER {
+                    // #[serde(field_identifier)]
+                    field_identifier.set_true(&meta.path);
+                } else if meta.path == VARIANT_IDENTIFIER {
+                    // #[serde(variant_identifier)]
+                    variant_identifier.set_true(&meta.path);
+                } else if meta.path == CRATE {
+                    // #[serde(crate = "foo")]
+                    if let Some(path) = parse_lit_into_path(cx, CRATE, &meta)? {
+                        serde_path.set(&meta.path, path);
+                    }
+                } else if meta.path == EXPECTING {
+                    // #[serde(expecting = "a message")]
+                    if let Some(s) = get_lit_str(cx, EXPECTING, &meta)? {
+                        expecting.set(&meta.path, s.value());
+                    }
+                } else {
+                    let path = meta.path.to_token_stream().to_string().replace(' ', "");
+                    return Err(
+                        meta.error(format_args!("unknown serde container attribute `{}`", path))
+                    );
+                }
+                Ok(())
+            }) {
+                cx.syn_error(err);
+            }
+        }
+
+        let mut is_packed = false;
+        for attr in &item.attrs {
+            if attr.path() == 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),
+            },
+            rename_all_fields_rules: RenameAllRules {
+                serialize: rename_all_fields_ser_rule.get().unwrap_or(RenameRule::None),
+                deserialize: rename_all_fields_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(),
+            non_exhaustive,
+        }
+    }
+
+    pub fn name(&self) -> &Name {
+        &self.name
+    }
+
+    pub fn rename_all_rules(&self) -> RenameAllRules {
+        self.rename_all_rules
+    }
+
+    pub fn rename_all_fields_rules(&self) -> RenameAllRules {
+        self.rename_all_fields_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)
+    }
+
+    pub fn non_exhaustive(&self) -> bool {
+        self.non_exhaustive
+    }
+}
+
+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 {
+                                let msg =
+                                    "#[serde(tag = \"...\")] cannot be used with tuple variants";
+                                cx.error_spanned_by(variant, msg);
+                                break;
+                            }
+                        }
+                    }
+                }
+            }
+            TagType::Internal { tag }
+        }
+        (Some((untagged_tokens, ())), Some((tag_tokens, _)), None) => {
+            let msg = "enum cannot be both untagged and internally tagged";
+            cx.error_spanned_by(untagged_tokens, msg);
+            cx.error_spanned_by(tag_tokens, msg);
+            TagType::External // doesn't matter, will error
+        }
+        (None, None, Some((content_tokens, _))) => {
+            let msg = "#[serde(tag = \"...\", content = \"...\")] must be used together";
+            cx.error_spanned_by(content_tokens, msg);
+            TagType::External
+        }
+        (Some((untagged_tokens, ())), None, Some((content_tokens, _))) => {
+            let msg = "untagged enum cannot have #[serde(content = \"...\")]";
+            cx.error_spanned_by(untagged_tokens, msg);
+            cx.error_spanned_by(content_tokens, msg);
+            TagType::External
+        }
+        (None, Some((_, tag)), Some((_, content))) => TagType::Adjacent { tag, content },
+        (Some((untagged_tokens, ())), Some((tag_tokens, _)), Some((content_tokens, _))) => {
+            let msg = "untagged enum cannot have #[serde(tag = \"...\", content = \"...\")]";
+            cx.error_spanned_by(untagged_tokens, msg);
+            cx.error_spanned_by(tag_tokens, msg);
+            cx.error_spanned_by(content_tokens, msg);
+            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, ()))) => {
+            let msg =
+                "#[serde(field_identifier)] and #[serde(variant_identifier)] cannot both be set";
+            cx.error_spanned_by(field_identifier_tokens, msg);
+            cx.error_spanned_by(variant_identifier_tokens, msg);
+            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) => {
+            let msg = "#[serde(field_identifier)] can only be used on an enum";
+            cx.error_spanned_by(struct_token, msg);
+            Identifier::No
+        }
+        (syn::Data::Union(syn::DataUnion { union_token, .. }), Some(_), None) => {
+            let msg = "#[serde(field_identifier)] can only be used on an enum";
+            cx.error_spanned_by(union_token, msg);
+            Identifier::No
+        }
+        (syn::Data::Struct(syn::DataStruct { struct_token, .. }), None, Some(_)) => {
+            let msg = "#[serde(variant_identifier)] can only be used on an enum";
+            cx.error_spanned_by(struct_token, msg);
+            Identifier::No
+        }
+        (syn::Data::Union(syn::DataUnion { union_token, .. }), None, Some(_)) => {
+            let msg = "#[serde(variant_identifier)] can only be used on an enum";
+            cx.error_spanned_by(union_token, msg);
+            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<BorrowAttribute>,
+    untagged: bool,
+}
+
+struct BorrowAttribute {
+    path: syn::Path,
+    lifetimes: Option<BTreeSet<syn::Lifetime>>,
+}
+
+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);
+        let mut untagged = BoolAttr::none(cx, UNTAGGED);
+
+        for attr in &variant.attrs {
+            if attr.path() != SERDE {
+                continue;
+            }
+
+            if let syn::Meta::List(meta) = &attr.meta {
+                if meta.tokens.is_empty() {
+                    continue;
+                }
+            }
+
+            if let Err(err) = attr.parse_nested_meta(|meta| {
+                if meta.path == RENAME {
+                    // #[serde(rename = "foo")]
+                    // #[serde(rename(serialize = "foo", deserialize = "bar"))]
+                    let (ser, de) = get_multiple_renames(cx, &meta)?;
+                    ser_name.set_opt(&meta.path, ser.as_ref().map(syn::LitStr::value));
+                    for de_value in de {
+                        de_name.set_if_none(de_value.value());
+                        de_aliases.insert(&meta.path, de_value.value());
+                    }
+                } else if meta.path == ALIAS {
+                    // #[serde(alias = "foo")]
+                    if let Some(s) = get_lit_str(cx, ALIAS, &meta)? {
+                        de_aliases.insert(&meta.path, s.value());
+                    }
+                } else if meta.path == RENAME_ALL {
+                    // #[serde(rename_all = "foo")]
+                    // #[serde(rename_all(serialize = "foo", deserialize = "bar"))]
+                    let one_name = meta.input.peek(Token![=]);
+                    let (ser, de) = get_renames(cx, RENAME_ALL, &meta)?;
+                    if let Some(ser) = ser {
+                        match RenameRule::from_str(&ser.value()) {
+                            Ok(rename_rule) => rename_all_ser_rule.set(&meta.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(&meta.path, rename_rule),
+                            Err(err) => {
+                                if !one_name {
+                                    cx.error_spanned_by(de, err);
+                                }
+                            }
+                        }
+                    }
+                } else if meta.path == SKIP {
+                    // #[serde(skip)]
+                    skip_serializing.set_true(&meta.path);
+                    skip_deserializing.set_true(&meta.path);
+                } else if meta.path == SKIP_DESERIALIZING {
+                    // #[serde(skip_deserializing)]
+                    skip_deserializing.set_true(&meta.path);
+                } else if meta.path == SKIP_SERIALIZING {
+                    // #[serde(skip_serializing)]
+                    skip_serializing.set_true(&meta.path);
+                } else if meta.path == OTHER {
+                    // #[serde(other)]
+                    other.set_true(&meta.path);
+                } else if meta.path == BOUND {
+                    // #[serde(bound = "T: SomeBound")]
+                    // #[serde(bound(serialize = "...", deserialize = "..."))]
+                    let (ser, de) = get_where_predicates(cx, &meta)?;
+                    ser_bound.set_opt(&meta.path, ser);
+                    de_bound.set_opt(&meta.path, de);
+                } else if meta.path == WITH {
+                    // #[serde(with = "...")]
+                    if let Some(path) = parse_lit_into_expr_path(cx, WITH, &meta)? {
+                        let mut ser_path = path.clone();
+                        ser_path
+                            .path
+                            .segments
+                            .push(Ident::new("serialize", Span::call_site()).into());
+                        serialize_with.set(&meta.path, ser_path);
+                        let mut de_path = path;
+                        de_path
+                            .path
+                            .segments
+                            .push(Ident::new("deserialize", Span::call_site()).into());
+                        deserialize_with.set(&meta.path, de_path);
+                    }
+                } else if meta.path == SERIALIZE_WITH {
+                    // #[serde(serialize_with = "...")]
+                    if let Some(path) = parse_lit_into_expr_path(cx, SERIALIZE_WITH, &meta)? {
+                        serialize_with.set(&meta.path, path);
+                    }
+                } else if meta.path == DESERIALIZE_WITH {
+                    // #[serde(deserialize_with = "...")]
+                    if let Some(path) = parse_lit_into_expr_path(cx, DESERIALIZE_WITH, &meta)? {
+                        deserialize_with.set(&meta.path, path);
+                    }
+                } else if meta.path == BORROW {
+                    let borrow_attribute = if meta.input.peek(Token![=]) {
+                        // #[serde(borrow = "'a + 'b")]
+                        let lifetimes = parse_lit_into_lifetimes(cx, &meta)?;
+                        BorrowAttribute {
+                            path: meta.path.clone(),
+                            lifetimes: Some(lifetimes),
+                        }
+                    } else {
+                        // #[serde(borrow)]
+                        BorrowAttribute {
+                            path: meta.path.clone(),
+                            lifetimes: None,
+                        }
+                    };
+                    match &variant.fields {
+                        syn::Fields::Unnamed(fields) if fields.unnamed.len() == 1 => {
+                            borrow.set(&meta.path, borrow_attribute);
+                        }
+                        _ => {
+                            let msg = "#[serde(borrow)] may only be used on newtype variants";
+                            cx.error_spanned_by(variant, msg);
+                        }
+                    }
+                } else if meta.path == UNTAGGED {
+                    untagged.set_true(&meta.path);
+                } else {
+                    let path = meta.path.to_token_stream().to_string().replace(' ', "");
+                    return Err(
+                        meta.error(format_args!("unknown serde variant attribute `{}`", path))
+                    );
+                }
+                Ok(())
+            }) {
+                cx.syn_error(err);
+            }
+        }
+
+        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(),
+            untagged: untagged.get(),
+        }
+    }
+
+    pub fn name(&self) -> &Name {
+        &self.name
+    }
+
+    pub fn aliases(&self) -> &BTreeSet<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);
+        }
+        self.name
+            .deserialize_aliases
+            .insert(self.name.deserialize.clone());
+    }
+
+    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()
+    }
+
+    pub fn untagged(&self) -> bool {
+        self.untagged
+    }
+}
+
+/// 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(),
+        };
+
+        if let Some(borrow_attribute) = attrs.and_then(|variant| variant.borrow.as_ref()) {
+            if let Ok(borrowable) = borrowable_lifetimes(cx, &ident, field) {
+                if let Some(lifetimes) = &borrow_attribute.lifetimes {
+                    for lifetime in lifetimes {
+                        if !borrowable.contains(lifetime) {
+                            let msg =
+                                format!("field `{}` does not have lifetime {}", ident, lifetime);
+                            cx.error_spanned_by(field, msg);
+                        }
+                    }
+                    borrowed_lifetimes.set(&borrow_attribute.path, lifetimes.clone());
+                } else {
+                    borrowed_lifetimes.set(&borrow_attribute.path, borrowable);
+                }
+            }
+        }
+
+        for attr in &field.attrs {
+            if attr.path() != SERDE {
+                continue;
+            }
+
+            if let syn::Meta::List(meta) = &attr.meta {
+                if meta.tokens.is_empty() {
+                    continue;
+                }
+            }
+
+            if let Err(err) = attr.parse_nested_meta(|meta| {
+                if meta.path == RENAME {
+                    // #[serde(rename = "foo")]
+                    // #[serde(rename(serialize = "foo", deserialize = "bar"))]
+                    let (ser, de) = get_multiple_renames(cx, &meta)?;
+                    ser_name.set_opt(&meta.path, ser.as_ref().map(syn::LitStr::value));
+                    for de_value in de {
+                        de_name.set_if_none(de_value.value());
+                        de_aliases.insert(&meta.path, de_value.value());
+                    }
+                } else if meta.path == ALIAS {
+                    // #[serde(alias = "foo")]
+                    if let Some(s) = get_lit_str(cx, ALIAS, &meta)? {
+                        de_aliases.insert(&meta.path, s.value());
+                    }
+                } else if meta.path == DEFAULT {
+                    if meta.input.peek(Token![=]) {
+                        // #[serde(default = "...")]
+                        if let Some(path) = parse_lit_into_expr_path(cx, DEFAULT, &meta)? {
+                            default.set(&meta.path, Default::Path(path));
+                        }
+                    } else {
+                        // #[serde(default)]
+                        default.set(&meta.path, Default::Default);
+                    }
+                } else if meta.path == SKIP_SERIALIZING {
+                    // #[serde(skip_serializing)]
+                    skip_serializing.set_true(&meta.path);
+                } else if meta.path == SKIP_DESERIALIZING {
+                    // #[serde(skip_deserializing)]
+                    skip_deserializing.set_true(&meta.path);
+                } else if meta.path == SKIP {
+                    // #[serde(skip)]
+                    skip_serializing.set_true(&meta.path);
+                    skip_deserializing.set_true(&meta.path);
+                } else if meta.path == SKIP_SERIALIZING_IF {
+                    // #[serde(skip_serializing_if = "...")]
+                    if let Some(path) = parse_lit_into_expr_path(cx, SKIP_SERIALIZING_IF, &meta)? {
+                        skip_serializing_if.set(&meta.path, path);
+                    }
+                } else if meta.path == SERIALIZE_WITH {
+                    // #[serde(serialize_with = "...")]
+                    if let Some(path) = parse_lit_into_expr_path(cx, SERIALIZE_WITH, &meta)? {
+                        serialize_with.set(&meta.path, path);
+                    }
+                } else if meta.path == DESERIALIZE_WITH {
+                    // #[serde(deserialize_with = "...")]
+                    if let Some(path) = parse_lit_into_expr_path(cx, DESERIALIZE_WITH, &meta)? {
+                        deserialize_with.set(&meta.path, path);
+                    }
+                } else if meta.path == WITH {
+                    // #[serde(with = "...")]
+                    if let Some(path) = parse_lit_into_expr_path(cx, WITH, &meta)? {
+                        let mut ser_path = path.clone();
+                        ser_path
+                            .path
+                            .segments
+                            .push(Ident::new("serialize", Span::call_site()).into());
+                        serialize_with.set(&meta.path, ser_path);
+                        let mut de_path = path;
+                        de_path
+                            .path
+                            .segments
+                            .push(Ident::new("deserialize", Span::call_site()).into());
+                        deserialize_with.set(&meta.path, de_path);
+                    }
+                } else if meta.path == BOUND {
+                    // #[serde(bound = "T: SomeBound")]
+                    // #[serde(bound(serialize = "...", deserialize = "..."))]
+                    let (ser, de) = get_where_predicates(cx, &meta)?;
+                    ser_bound.set_opt(&meta.path, ser);
+                    de_bound.set_opt(&meta.path, de);
+                } else if meta.path == BORROW {
+                    if meta.input.peek(Token![=]) {
+                        // #[serde(borrow = "'a + 'b")]
+                        let lifetimes = parse_lit_into_lifetimes(cx, &meta)?;
+                        if let Ok(borrowable) = borrowable_lifetimes(cx, &ident, field) {
+                            for lifetime in &lifetimes {
+                                if !borrowable.contains(lifetime) {
+                                    let msg = format!(
+                                        "field `{}` does not have lifetime {}",
+                                        ident, lifetime,
+                                    );
+                                    cx.error_spanned_by(field, msg);
+                                }
+                            }
+                            borrowed_lifetimes.set(&meta.path, lifetimes);
+                        }
+                    } else {
+                        // #[serde(borrow)]
+                        if let Ok(borrowable) = borrowable_lifetimes(cx, &ident, field) {
+                            borrowed_lifetimes.set(&meta.path, borrowable);
+                        }
+                    }
+                } else if meta.path == GETTER {
+                    // #[serde(getter = "...")]
+                    if let Some(path) = parse_lit_into_expr_path(cx, GETTER, &meta)? {
+                        getter.set(&meta.path, path);
+                    }
+                } else if meta.path == FLATTEN {
+                    // #[serde(flatten)]
+                    flatten.set_true(&meta.path);
+                } else {
+                    let path = meta.path.to_token_stream().to_string().replace(' ', "");
+                    return Err(
+                        meta.error(format_args!("unknown serde field attribute `{}`", path))
+                    );
+                }
+                Ok(())
+            }) {
+                cx.syn_error(err);
+            }
+        }
+
+        // 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) -> &BTreeSet<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);
+        }
+        self.name
+            .deserialize_aliases
+            .insert(self.name.deserialize.clone());
+    }
+
+    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<'c, T, F, R>(
+    cx: &'c Ctxt,
+    attr_name: Symbol,
+    meta: &ParseNestedMeta,
+    f: F,
+) -> syn::Result<(VecAttr<'c, T>, VecAttr<'c, T>)>
+where
+    T: Clone,
+    F: Fn(&Ctxt, Symbol, Symbol, &ParseNestedMeta) -> syn::Result<R>,
+    R: Into<Option<T>>,
+{
+    let mut ser_meta = VecAttr::none(cx, attr_name);
+    let mut de_meta = VecAttr::none(cx, attr_name);
+
+    let lookahead = meta.input.lookahead1();
+    if lookahead.peek(Token![=]) {
+        if let Some(both) = f(cx, attr_name, attr_name, meta)?.into() {
+            ser_meta.insert(&meta.path, both.clone());
+            de_meta.insert(&meta.path, both);
+        }
+    } else if lookahead.peek(token::Paren) {
+        meta.parse_nested_meta(|meta| {
+            if meta.path == SERIALIZE {
+                if let Some(v) = f(cx, attr_name, SERIALIZE, &meta)?.into() {
+                    ser_meta.insert(&meta.path, v);
+                }
+            } else if meta.path == DESERIALIZE {
+                if let Some(v) = f(cx, attr_name, DESERIALIZE, &meta)?.into() {
+                    de_meta.insert(&meta.path, v);
+                }
+            } else {
+                return Err(meta.error(format_args!(
+                    "malformed {0} attribute, expected `{0}(serialize = ..., deserialize = ...)`",
+                    attr_name,
+                )));
+            }
+            Ok(())
+        })?;
+    } else {
+        return Err(lookahead.error());
+    }
+
+    Ok((ser_meta, de_meta))
+}
+
+fn get_renames(
+    cx: &Ctxt,
+    attr_name: Symbol,
+    meta: &ParseNestedMeta,
+) -> syn::Result<SerAndDe<syn::LitStr>> {
+    let (ser, de) = get_ser_and_de(cx, attr_name, meta, get_lit_str2)?;
+    Ok((ser.at_most_one(), de.at_most_one()))
+}
+
+fn get_multiple_renames(
+    cx: &Ctxt,
+    meta: &ParseNestedMeta,
+) -> syn::Result<(Option<syn::LitStr>, Vec<syn::LitStr>)> {
+    let (ser, de) = get_ser_and_de(cx, RENAME, meta, get_lit_str2)?;
+    Ok((ser.at_most_one(), de.get()))
+}
+
+fn get_where_predicates(
+    cx: &Ctxt,
+    meta: &ParseNestedMeta,
+) -> syn::Result<SerAndDe<Vec<syn::WherePredicate>>> {
+    let (ser, de) = get_ser_and_de(cx, BOUND, meta, parse_lit_into_where)?;
+    Ok((ser.at_most_one(), de.at_most_one()))
+}
+
+fn get_lit_str(
+    cx: &Ctxt,
+    attr_name: Symbol,
+    meta: &ParseNestedMeta,
+) -> syn::Result<Option<syn::LitStr>> {
+    get_lit_str2(cx, attr_name, attr_name, meta)
+}
+
+fn get_lit_str2(
+    cx: &Ctxt,
+    attr_name: Symbol,
+    meta_item_name: Symbol,
+    meta: &ParseNestedMeta,
+) -> syn::Result<Option<syn::LitStr>> {
+    let expr: syn::Expr = meta.value()?.parse()?;
+    let mut value = &expr;
+    while let syn::Expr::Group(e) = value {
+        value = &e.expr;
+    }
+    if let syn::Expr::Lit(syn::ExprLit {
+        lit: syn::Lit::Str(lit),
+        ..
+    }) = value
+    {
+        let suffix = lit.suffix();
+        if !suffix.is_empty() {
+            cx.error_spanned_by(
+                lit,
+                format!("unexpected suffix `{}` on string literal", suffix),
+            );
+        }
+        Ok(Some(lit.clone()))
+    } else {
+        cx.error_spanned_by(
+            expr,
+            format!(
+                "expected serde {} attribute to be a string: `{} = \"...\"`",
+                attr_name, meta_item_name
+            ),
+        );
+        Ok(None)
+    }
+}
+
+fn parse_lit_into_path(
+    cx: &Ctxt,
+    attr_name: Symbol,
+    meta: &ParseNestedMeta,
+) -> syn::Result<Option<syn::Path>> {
+    let string = match get_lit_str(cx, attr_name, meta)? {
+        Some(string) => string,
+        None => return Ok(None),
+    };
+
+    Ok(match string.parse() {
+        Ok(path) => Some(path),
+        Err(_) => {
+            cx.error_spanned_by(
+                &string,
+                format!("failed to parse path: {:?}", string.value()),
+            );
+            None
+        }
+    })
+}
+
+fn parse_lit_into_expr_path(
+    cx: &Ctxt,
+    attr_name: Symbol,
+    meta: &ParseNestedMeta,
+) -> syn::Result<Option<syn::ExprPath>> {
+    let string = match get_lit_str(cx, attr_name, meta)? {
+        Some(string) => string,
+        None => return Ok(None),
+    };
+
+    Ok(match string.parse() {
+        Ok(expr) => Some(expr),
+        Err(_) => {
+            cx.error_spanned_by(
+                &string,
+                format!("failed to parse path: {:?}", string.value()),
+            );
+            None
+        }
+    })
+}
+
+fn parse_lit_into_where(
+    cx: &Ctxt,
+    attr_name: Symbol,
+    meta_item_name: Symbol,
+    meta: &ParseNestedMeta,
+) -> syn::Result<Vec<syn::WherePredicate>> {
+    let string = match get_lit_str2(cx, attr_name, meta_item_name, meta)? {
+        Some(string) => string,
+        None => return Ok(Vec::new()),
+    };
+
+    Ok(
+        match string.parse_with(Punctuated::<syn::WherePredicate, Token![,]>::parse_terminated) {
+            Ok(predicates) => Vec::from_iter(predicates),
+            Err(err) => {
+                cx.error_spanned_by(string, err);
+                Vec::new()
+            }
+        },
+    )
+}
+
+fn parse_lit_into_ty(
+    cx: &Ctxt,
+    attr_name: Symbol,
+    meta: &ParseNestedMeta,
+) -> syn::Result<Option<syn::Type>> {
+    let string = match get_lit_str(cx, attr_name, meta)? {
+        Some(string) => string,
+        None => return Ok(None),
+    };
+
+    Ok(match string.parse() {
+        Ok(ty) => Some(ty),
+        Err(_) => {
+            cx.error_spanned_by(
+                &string,
+                format!("failed to parse type: {} = {:?}", attr_name, string.value()),
+            );
+            None
+        }
+    })
+}
+
+// Parses a string literal like "'a + 'b + 'c" containing a nonempty list of
+// lifetimes separated by `+`.
+fn parse_lit_into_lifetimes(
+    cx: &Ctxt,
+    meta: &ParseNestedMeta,
+) -> syn::Result<BTreeSet<syn::Lifetime>> {
+    let string = match get_lit_str(cx, BORROW, meta)? {
+        Some(string) => string,
+        None => return Ok(BTreeSet::new()),
+    };
+
+    if let Ok(lifetimes) = string.parse_with(|input: ParseStream| {
+        let mut set = BTreeSet::new();
+        while !input.is_empty() {
+            let lifetime: Lifetime = input.parse()?;
+            if !set.insert(lifetime.clone()) {
+                cx.error_spanned_by(
+                    &string,
+                    format!("duplicate borrowed lifetime `{}`", lifetime),
+                );
+            }
+            if input.is_empty() {
+                break;
+            }
+            input.parse::<Token![+]>()?;
+        }
+        Ok(set)
+    }) {
+        if lifetimes.is_empty() {
+            cx.error_spanned_by(string, "at least one lifetime must be borrowed");
+        }
+        return Ok(lifetimes);
+    }
+
+    cx.error_spanned_by(
+        &string,
+        format!("failed to parse borrowed lifetimes: {:?}", string.value()),
+    );
+    Ok(BTreeSet::new())
+}
+
+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() {
+        let msg = format!("field `{}` has no lifetimes to borrow", name);
+        cx.error_spanned_by(field, msg);
+        Err(())
+    } else {
+        Ok(lifetimes)
+    }
+}
+
+fn collect_lifetimes(ty: &syn::Type, out: &mut BTreeSet<syn::Lifetime>) {
+    match ty {
+        #![cfg_attr(all(test, exhaustive), deny(non_exhaustive_omitted_patterns))]
+        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::AssocType(binding) => {
+                                collect_lifetimes(&binding.ty, out);
+                            }
+                            syn::GenericArgument::Const(_)
+                            | syn::GenericArgument::AssocConst(_)
+                            | syn::GenericArgument::Constraint(_)
+                            | _ => {}
+                        }
+                    }
+                }
+            }
+        }
+        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(_) => {}
+
+        _ => {}
+    }
+}
+
+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);
+            }
+            _ => {}
+        }
+    }
+}