use std::borrow::Borrow;
use std::collections::HashMap;
use std::convert::TryFrom;

use proc_macro2::{Ident, Span, TokenStream, TokenTree};
use quote::ToTokens;
use syn::parse::{Parse, ParseStream};
use syn::punctuated::Punctuated;
use syn::spanned::Spanned;
use syn::Token;

use anyhow::Error;

use crate::api::{self, Schema, SchemaItem};

/// A more relaxed version of Ident which allows hyphens.
///
/// Note that this acts both as an Ident and as a String so that we can easily access an &str
/// (which Ident does not provide, instead, Ident always requires you to produce a newly owned
/// `String`).
/// Because of this the user also needs to be aware of the differences between idents and strings,
/// and therefore we do not implement `Into<Ident>` anymore, but the user needs to explicitly ask
/// for it via the `.into_ident()` method.
#[derive(Clone, Debug)]
pub struct FieldName {
    ident: Ident,
    ident_str: String, // cached string version to avoid all the .to_string() calls
    string: String,    // hyphenated version
}

impl FieldName {
    pub fn new(name: String, span: Span) -> Self {
        let mut ident_str = name.replace(['-', '.', '+'].as_ref(), "_");

        if ident_str.chars().next().unwrap().is_numeric() {
            ident_str.insert(0, '_');
        }

        Self {
            ident: Ident::new(&ident_str, span),
            ident_str,
            string: name,
        }
    }

    #[inline]
    pub fn as_str(&self) -> &str {
        &self.string
    }

    #[inline]
    pub fn as_ident_str(&self) -> &str {
        &self.ident_str
    }

    #[inline]
    pub fn as_ident(&self) -> &Ident {
        &self.ident
    }

    #[inline]
    pub fn into_ident(self) -> Ident {
        self.ident
    }

    #[inline]
    pub fn span(&self) -> Span {
        self.ident.span()
    }

    pub fn cmp(&self, other: &Self) -> std::cmp::Ordering {
        self.string.cmp(&other.string)
    }

    pub fn into_lit_str(self) -> syn::LitStr {
        syn::LitStr::new(&self.string, self.ident.span())
    }

    pub fn into_str(self) -> String {
        self.string
    }
}

impl Eq for FieldName {}

impl PartialEq for FieldName {
    fn eq(&self, other: &Self) -> bool {
        self.string == other.string
    }
}

impl std::hash::Hash for FieldName {
    fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
        std::hash::Hash::hash(&self.string, state)
    }
}

impl From<Ident> for FieldName {
    fn from(ident: Ident) -> Self {
        let string = ident.to_string();
        Self {
            ident,
            ident_str: string.clone(),
            string,
        }
    }
}

impl From<&syn::LitStr> for FieldName {
    fn from(s: &syn::LitStr) -> Self {
        Self::new(s.value(), s.span())
    }
}

impl Borrow<str> for FieldName {
    #[inline]
    fn borrow(&self) -> &str {
        self.as_str()
    }
}

/// Note that the 'type' keyword is handled separately in `syn`. It's not an `Ident`:
impl Parse for FieldName {
    fn parse(input: ParseStream) -> syn::Result<Self> {
        let lookahead = input.lookahead1();
        Ok(if lookahead.peek(Token![type]) {
            let ty: Token![type] = input.parse()?;
            Self::new("type".to_string(), ty.span)
        } else if lookahead.peek(syn::LitStr) {
            let s: syn::LitStr = input.parse()?;
            Self::new(s.value(), s.span())
        } else {
            let ident: Ident = input.parse()?;
            Self::from(ident)
        })
    }
}

/// Most of our schema definition consists of a json-like notation.
/// For parsing we mostly just need to distinguish between objects and non-objects.
/// For specific expression types we match on the contained expression later on.
// FIXME: Expr(Box<syn::Expr>)
#[allow(clippy::large_enum_variant)]
pub enum JSONValue {
    Object(JSONObject),
    Expr(syn::Expr),
}

impl JSONValue {
    /// When we expect an object, it's nicer to know why/what kind, so instead of
    /// `TryInto<JSONObject>` we provide this method:
    pub fn into_object(self, expected: &str) -> Result<JSONObject, syn::Error> {
        match self {
            JSONValue::Object(s) => Ok(s),
            JSONValue::Expr(e) => bail!(e => "expected {}", expected),
        }
    }

    pub fn new_string(value: &str, span: Span) -> JSONValue {
        JSONValue::Expr(syn::Expr::Lit(syn::ExprLit {
            attrs: Vec::new(),
            lit: syn::Lit::Str(syn::LitStr::new(value, span)),
        }))
    }

    pub fn new_ident(ident: Ident) -> JSONValue {
        JSONValue::Expr(syn::Expr::Path(syn::ExprPath {
            attrs: Vec::new(),
            qself: None,
            path: syn::Path {
                leading_colon: None,
                segments: {
                    let mut p = Punctuated::new();
                    p.push(syn::PathSegment {
                        ident,
                        arguments: Default::default(),
                    });
                    p
                },
            },
        }))
    }

    pub fn span(&self) -> Span {
        match self {
            JSONValue::Object(obj) => obj.span(),
            JSONValue::Expr(expr) => expr.span(),
        }
    }
}

/// Expect a json value to be an expression, not an object:
impl TryFrom<JSONValue> for syn::Expr {
    type Error = syn::Error;
    fn try_from(value: JSONValue) -> Result<Self, syn::Error> {
        match value {
            JSONValue::Object(s) => bail!(s.span(), "unexpected object"),
            JSONValue::Expr(e) => Ok(e),
        }
    }
}

/// Expect a json value to be a literal string:
impl TryFrom<JSONValue> for syn::LitStr {
    type Error = syn::Error;
    fn try_from(value: JSONValue) -> Result<Self, syn::Error> {
        let expr = syn::Expr::try_from(value)?;
        if let syn::Expr::Lit(lit) = expr {
            if let syn::Lit::Str(lit) = lit.lit {
                return Ok(lit);
            }
            bail!(lit => "expected string literal");
        }
        bail!(expr => "expected string literal");
    }
}

/// Expect a json value to be a literal boolean:
impl TryFrom<JSONValue> for syn::LitBool {
    type Error = syn::Error;
    fn try_from(value: JSONValue) -> Result<Self, syn::Error> {
        let expr = syn::Expr::try_from(value)?;
        if let syn::Expr::Lit(lit) = expr {
            if let syn::Lit::Bool(lit) = lit.lit {
                return Ok(lit);
            }
            bail!(lit => "expected literal boolean");
        }
        bail!(expr => "expected literal boolean");
    }
}

/// Expect a json value to be a literal boolean:
impl TryFrom<JSONValue> for bool {
    type Error = syn::Error;
    fn try_from(value: JSONValue) -> Result<Self, syn::Error> {
        Ok(syn::LitBool::try_from(value)?.value)
    }
}

/// Expect a json value to be an identifier:
impl TryFrom<JSONValue> for Ident {
    type Error = syn::Error;
    fn try_from(value: JSONValue) -> Result<Self, syn::Error> {
        let expr = syn::Expr::try_from(value)?;
        let span = expr.span();
        if let syn::Expr::Path(path) = expr {
            let mut iter = path.path.segments.into_pairs();
            let segment = iter
                .next()
                .ok_or_else(|| format_err!(span, "expected an identify, got an empty path"))?
                .into_value();
            if iter.next().is_some() {
                bail!(span, "expected an identifier, not a path");
            }
            if !segment.arguments.is_empty() {
                bail!(segment.arguments => "unexpected path arguments, expected an identifier");
            }
            return Ok(segment.ident);
        }
        bail!(expr => "expected an identifier");
    }
}

/// Expect a json value to be our "simple" identifier, which can be either an Ident or a String, or
/// the 'type' keyword:
impl TryFrom<JSONValue> for FieldName {
    type Error = syn::Error;
    fn try_from(value: JSONValue) -> Result<Self, syn::Error> {
        Ok(FieldName::from(Ident::try_from(value)?))
    }
}

/// Expect a json value to be a path. This means it's supposed to be an expression which evaluates
/// to a path.
impl TryFrom<JSONValue> for syn::ExprPath {
    type Error = syn::Error;
    fn try_from(value: JSONValue) -> Result<Self, syn::Error> {
        match syn::Expr::try_from(value)? {
            syn::Expr::Path(path) => Ok(path),
            other => bail!(other => "expected a type path"),
        }
    }
}

/// Parsing a json value should be simple enough: braces means we have an object, otherwise it must
/// be an "expression".
impl Parse for JSONValue {
    fn parse(input: ParseStream) -> syn::Result<Self> {
        let lookahead = input.lookahead1();
        Ok(if lookahead.peek(syn::token::Brace) {
            JSONValue::Object(input.parse()?)
        } else {
            JSONValue::Expr(input.parse()?)
        })
    }
}

/// The "core" of our schema is a json object.
pub struct JSONObject {
    pub brace_token: Option<syn::token::Brace>,
    pub elements: HashMap<FieldName, JSONValue>,
}

impl JSONObject {
    pub fn is_empty(&self) -> bool {
        self.elements.is_empty()
    }

    fn parse_elements(input: ParseStream) -> syn::Result<HashMap<FieldName, JSONValue>> {
        let map_elems = input.parse_terminated(JSONMapEntry::parse, Token![,])?;
        let mut elems = HashMap::with_capacity(map_elems.len());
        for c in map_elems {
            if elems.insert(c.key.clone(), c.value).is_some() {
                bail!(c.key.span(), "duplicate '{}' in schema", c.key.as_str());
            }
        }
        Ok(elems)
    }

    pub fn parse_inner(input: ParseStream) -> syn::Result<Self> {
        Ok(Self {
            brace_token: None,
            elements: Self::parse_elements(input)?,
        })
    }
}

impl Parse for JSONObject {
    fn parse(input: ParseStream) -> syn::Result<Self> {
        let content;
        Ok(Self {
            brace_token: Some(syn::braced!(content in input)),
            elements: Self::parse_elements(&content)?,
        })
    }
}

impl std::ops::Deref for JSONObject {
    type Target = HashMap<FieldName, JSONValue>;

    fn deref(&self) -> &Self::Target {
        &self.elements
    }
}

impl std::ops::DerefMut for JSONObject {
    fn deref_mut(&mut self) -> &mut Self::Target {
        &mut self.elements
    }
}

impl JSONObject {
    pub fn span(&self) -> Span {
        match &self.brace_token {
            Some(brace) => brace.span.join(),
            None => Span::call_site(),
        }
    }

    pub fn remove_required_element(&mut self, name: &str) -> Result<JSONValue, syn::Error> {
        self.remove(name)
            .ok_or_else(|| format_err!(self.span(), "missing required element: {}", name))
    }
}

impl IntoIterator for JSONObject {
    type Item = <HashMap<FieldName, JSONValue> as IntoIterator>::Item;
    type IntoIter = <HashMap<FieldName, JSONValue> as IntoIterator>::IntoIter;

    fn into_iter(self) -> Self::IntoIter {
        self.elements.into_iter()
    }
}

/// An element in a json style map.
struct JSONMapEntry {
    pub key: FieldName,
    _colon_token: Token![:],
    pub value: JSONValue,
}

impl Parse for JSONMapEntry {
    fn parse(input: ParseStream) -> syn::Result<Self> {
        Ok(Self {
            key: input.parse()?,
            _colon_token: input.parse()?,
            value: input.parse()?,
        })
    }
}

/// We get macro attributes like `#[doc = "TEXT"]` with the `=` included.
pub struct BareAssignment<T: Parse> {
    pub _token: Token![=],
    pub _content: T,
}

impl<T: Parse> Parse for BareAssignment<T> {
    fn parse(input: ParseStream) -> syn::Result<Self> {
        Ok(Self {
            _token: input.parse()?,
            _content: input.parse()?,
        })
    }
}

pub fn get_doc_comments(attributes: &[syn::Attribute]) -> Result<(String, Span), syn::Error> {
    let mut doc_comment = String::new();
    let doc_span = Span::call_site(); // FIXME: set to first doc comment

    for attr in attributes {
        // skip #![...]
        if let syn::AttrStyle::Inner(_) = &attr.style {
            continue;
        }

        let nv = match &attr.meta {
            syn::Meta::NameValue(nv) if nv.path.is_ident("doc") => &nv.value,
            _ => continue,
        };

        if let syn::Expr::Lit(syn::ExprLit {
            lit: syn::Lit::Str(doc),
            ..
        }) = nv
        {
            if !doc_comment.is_empty() {
                doc_comment.push('\n');
            }
            doc_comment.push_str(doc.value().trim());
        }
    }

    Ok((doc_comment, doc_span))
}

pub fn derive_descriptions(
    input_schema: &mut Schema,
    returns_schema: Option<&mut Schema>,
    doc_comment: &str,
    doc_span: Span,
) -> Result<(), Error> {
    // If we have a doc comment, allow automatically inferring the description for the input and
    // output objects:
    if doc_comment.is_empty() {
        return Ok(());
    }

    let mut parts = doc_comment.split("\nReturns:");

    if let Some(first) = parts.next() {
        if input_schema.description.is_none() {
            input_schema.description = Maybe::Derived(syn::LitStr::new(first.trim(), doc_span));
        }
    }

    if let Some(second) = parts.next() {
        if let Some(returns_schema) = returns_schema {
            if returns_schema.description.is_none() {
                returns_schema.description =
                    Maybe::Derived(syn::LitStr::new(second.trim(), doc_span));
            }
        }

        if parts.next().is_some() {
            bail!(
                doc_span,
                "multiple 'Returns:' sections found in doc comment!"
            );
        }
    }

    Ok(())
}

pub fn infer_type(schema: &mut Schema, ty: &syn::Type) -> Result<bool, syn::Error> {
    if let SchemaItem::Inferred(_) = schema.item {
        //
    } else {
        return Ok(is_option_type(ty).is_some());
    }

    let (ty, is_option) = match is_option_type(ty) {
        Some(ty) => (ty, true),
        None => (ty, false),
    };

    // infer the type from a rust type:
    match ty {
        syn::Type::Path(path) if path.qself.is_none() => {
            if path.path.is_ident("String") {
                schema.item = SchemaItem::String(ty.span());
            } else if path.path.is_ident("bool") {
                schema.item = SchemaItem::Boolean(ty.span());
            } else if let Some(info) = api::INTTYPES.iter().find(|i| path.path.is_ident(i.name)) {
                schema.item = SchemaItem::Integer(ty.span());
                if let Some(min) = info.minimum {
                    schema.add_default_property("minimum", syn::Expr::Verbatim(min.parse()?));
                }
                if let Some(max) = info.maximum {
                    schema.add_default_property("maximum", syn::Expr::Verbatim(max.parse()?));
                }
            } else if api::NUMBERNAMES.iter().any(|n| path.path.is_ident(n)) {
                schema.item = SchemaItem::Number(ty.span());
            } else {
                // bail!(ty => "cannot infer parameter type from this rust type");
                schema.item = SchemaItem::ExternType(syn::ExprPath {
                    attrs: Vec::new(),
                    qself: path.qself.clone(),
                    path: path.path.clone(),
                });
            }
        }
        _ => (),
    }

    Ok(is_option)
}

/// Note that we cannot handle renamed imports at all here...
pub fn is_option_type(ty: &syn::Type) -> Option<&syn::Type> {
    if let syn::Type::Path(p) = ty {
        if p.qself.is_some() {
            return None;
        }
        let segs = &p.path.segments;
        let is_option = match segs.len() {
            1 => segs.last().unwrap().ident == "Option",
            2 => segs.first().unwrap().ident == "std" && segs.last().unwrap().ident == "Option",
            _ => false,
        };
        if !is_option {
            return None;
        }

        if let syn::PathArguments::AngleBracketed(generic) = &segs.last().unwrap().arguments {
            if generic.args.len() == 1 {
                if let syn::GenericArgument::Type(ty) = generic.args.first().unwrap() {
                    return Some(ty);
                }
            }
        }
    }
    None
}

pub fn make_ident_path(ident: Ident) -> syn::Path {
    syn::Path {
        leading_colon: None,
        segments: {
            let mut s = Punctuated::new();
            s.push(syn::PathSegment {
                ident,
                arguments: syn::PathArguments::None,
            });
            s
        },
    }
}

pub fn make_path(span: Span, leading_colon: bool, path: &[&str]) -> syn::Path {
    syn::Path {
        leading_colon: if leading_colon {
            Some(syn::token::PathSep {
                spans: [span, span],
            })
        } else {
            None
        },
        segments: path
            .iter()
            .map(|entry| syn::PathSegment {
                ident: Ident::new(entry, span),
                arguments: syn::PathArguments::None,
            })
            .collect(),
    }
}

/// Join an iterator over `Display` values.
pub fn join<T>(separator: &str, iter: impl Iterator<Item = T>) -> String
where
    T: std::fmt::Display,
{
    let mut text = String::new();
    let mut sep = "";
    for i in iter {
        text = format!("{}{}{}", text, sep, i);
        sep = separator;
    }
    text
}

/// Join an iterator over `Debug` values.
pub fn join_debug<T>(separator: &str, iter: impl Iterator<Item = T>) -> String
where
    T: std::fmt::Debug,
{
    let mut text = String::new();
    let mut sep = "";
    for i in iter {
        text = format!("{}{}{:?}", text, sep, i);
        sep = separator;
    }
    text
}

/// Helper to distinguish between explicitly set or derived data.
#[derive(Clone, Copy, Default, Eq, PartialEq)]
pub enum Maybe<T> {
    Explicit(T),
    Derived(T),
    #[default]
    None,
}

impl<T> Maybe<T> {
    pub fn as_ref(&self) -> Maybe<&T> {
        match self {
            Maybe::Explicit(t) => Maybe::Explicit(t),
            Maybe::Derived(t) => Maybe::Derived(t),
            Maybe::None => Maybe::None,
        }
    }

    pub fn explicit(t: Option<T>) -> Self {
        match t {
            Some(t) => Maybe::Explicit(t),
            None => Maybe::None,
        }
    }

    pub fn ok(self) -> Option<T> {
        match self {
            Maybe::Explicit(v) | Maybe::Derived(v) => Some(v),
            Maybe::None => None,
        }
    }

    pub fn ok_or_else<E, F>(self, other: F) -> Result<T, E>
    where
        F: FnOnce() -> E,
    {
        match self {
            Maybe::Explicit(t) | Maybe::Derived(t) => Ok(t),
            Maybe::None => Err(other()),
        }
    }

    pub fn is_none(&self) -> bool {
        matches!(self, Maybe::None)
    }

    pub fn is_explicit(&self) -> bool {
        matches!(self, Maybe::Explicit(_))
    }

    pub fn take(&mut self) -> Self {
        std::mem::take(self)
    }
}

impl<T> From<Maybe<T>> for Option<T> {
    fn from(maybe: Maybe<T>) -> Option<T> {
        match maybe {
            Maybe::Explicit(t) | Maybe::Derived(t) => Some(t),
            Maybe::None => None,
        }
    }
}

/// Helper to iterate over all the `#[derive(...)]` types found in an attribute list.
pub fn derived_items(attributes: &[syn::Attribute]) -> DerivedItems {
    DerivedItems {
        attributes: attributes.iter(),
        current: None,
    }
}

/// Helper to check if a certain trait is being derived.
pub fn derives_trait(attributes: &[syn::Attribute], ident: &str) -> bool {
    derived_items(attributes).any(|p| p.is_ident(ident))
}

/// Iterator over the types found in `#[derive(...)]` attributes.
pub struct DerivedItems<'a> {
    current: Option<<Punctuated<syn::Meta, Token![,]> as IntoIterator>::IntoIter>,
    attributes: std::slice::Iter<'a, syn::Attribute>,
}

impl<'a> Iterator for DerivedItems<'a> {
    type Item = syn::Path;

    fn next(&mut self) -> Option<Self::Item> {
        loop {
            if let Some(current) = &mut self.current {
                loop {
                    match current.next() {
                        Some(syn::Meta::Path(path)) => return Some(path),
                        Some(_) => continue,
                        None => {
                            self.current = None;
                            break;
                        }
                    }
                }
            }

            let attr = self.attributes.next()?;
            if attr.style != syn::AttrStyle::Outer {
                continue;
            }

            match &attr.meta {
                syn::Meta::List(list) if list.path.is_ident("derive") => {
                    if let Ok(items) =
                        list.parse_args_with(Punctuated::<syn::Meta, Token![,]>::parse_terminated)
                    {
                        self.current = Some(items.into_iter());
                    }
                    continue;
                }
                _ => continue,
            }
        }
    }
}

/// Helper to iterate over all the `#[derive(...)]` types found in an attribute list.
pub fn retain_derived_items<F>(attributes: &mut Vec<syn::Attribute>, mut func: F)
where
    F: FnMut(&syn::Path) -> bool,
{
    use syn::punctuated::Pair;

    let capacity = attributes.len();
    for mut attr in std::mem::replace(attributes, Vec::with_capacity(capacity)) {
        if attr.style != syn::AttrStyle::Outer {
            attributes.push(attr);
            continue;
        }

        let list = match &mut attr.meta {
            syn::Meta::List(list) if list.path.is_ident("derive") => list,
            _ => {
                attributes.push(attr);
                continue;
            }
        };

        let mut args =
            match list.parse_args_with(Punctuated::<syn::Meta, Token![,]>::parse_terminated) {
                Ok(args) => args,
                Err(_) => {
                    // if we can't parse it, we don't care
                    attributes.push(attr);
                    continue;
                }
            };

        for arg in std::mem::take(&mut args).into_pairs() {
            match arg {
                Pair::Punctuated(item, punct) => {
                    if let syn::Meta::Path(path) = &item {
                        if !func(path) {
                            continue;
                        }
                    }
                    args.push_value(item);
                    args.push_punct(punct);
                }
                Pair::End(item) => {
                    if let syn::Meta::Path(path) = &item {
                        if !func(path) {
                            continue;
                        }
                    }
                    args.push_value(item);
                }
            }
        }

        if !args.is_empty() {
            list.tokens = args.into_token_stream();
            attributes.push(attr);
        }
    }
}

pub fn make_derive_attribute(span: Span, content: TokenStream) -> syn::Attribute {
    // FIXME: syn2 wtf come on...
    let bracket_span =
        proc_macro2::Group::new(proc_macro2::Delimiter::Bracket, TokenStream::new()).delim_span();
    let paren_span =
        proc_macro2::Group::new(proc_macro2::Delimiter::Parenthesis, TokenStream::new())
            .delim_span();

    syn::Attribute {
        pound_token: syn::token::Pound { spans: [span] },
        style: syn::AttrStyle::Outer,
        bracket_token: syn::token::Bracket { span: bracket_span },
        //meta: syn::Meta::List(syn::parse_quote_spanned!(span => derive ( #content )).unwrap()),
        meta: syn::Meta::List(syn::MetaList {
            path: make_ident_path(Ident::new("derive", span)),
            delimiter: syn::MacroDelimiter::Paren(syn::token::Paren { span: paren_span }),
            tokens: content,
        }),
    }
}

/// Helper to create an error about some duplicate attribute.
pub fn duplicate<T>(prev: &Option<T>, attr: &syn::Path) {
    if prev.is_some() {
        error!(attr => "duplicate attribute: '{:?}'", attr)
    }
}

/// Set a boolean attribute to a value, producing a "duplication" error if it has already been set.
pub fn set_bool(b: &mut Option<syn::LitBool>, attr: &syn::Path, value: bool) {
    duplicate(&*b, attr);
    *b = Some(syn::LitBool::new(value, attr.span()));
}

pub fn default_false(o: Option<&syn::LitBool>) -> bool {
    o.as_ref().map(|b| b.value).unwrap_or(false)
}

/// Parse the contents of a `LitStr`, preserving its span.
pub fn parse_lit_str<T: Parse>(s: &syn::LitStr) -> syn::parse::Result<T> {
    parse_str(&s.value(), s.span())
}

/// Parse a literal string, giving the entire output the specified span.
pub fn parse_str<T: Parse>(s: &str, span: Span) -> syn::parse::Result<T> {
    syn::parse2(respan_tokens(syn::parse_str(s)?, span))
}

/// Apply a `Span` to an entire `TokenStream`.
pub fn respan_tokens(stream: TokenStream, span: Span) -> TokenStream {
    stream
        .into_iter()
        .map(|token| respan(token, span))
        .collect()
}

/// Apply a `Span` to a `TokenTree`, recursively if it is a `Group`.
pub fn respan(mut token: TokenTree, span: Span) -> TokenTree {
    use proc_macro2::Group;

    match &mut token {
        TokenTree::Group(g) => {
            *g = Group::new(g.delimiter(), respan_tokens(g.stream(), span));
        }
        other => other.set_span(span),
    }

    token
}

/// Parse a string attribute into a value, producing a duplication error if it has already been
/// set.
pub fn parse_str_value_to_option<T: Parse>(
    target: &mut Option<T>,
    path: &syn::Path,
    nv: syn::parse::ParseStream<'_>,
) {
    duplicate(&*target, path);
    match nv.parse().and_then(|lit| parse_lit_str(&lit)) {
        Ok(value) => *target = Some(value),
        Err(err) => crate::add_error(err),
    }
}

/*
pub fn parse_str_value<T: Parse>(nv: &syn::MetaNameValue) -> Result<T, syn::Error> {
    match &nv.lit {
        syn::Lit::Str(s) => super::parse_lit_str(s),
        other => bail!(other => "bad value for '{:?}' attribute", nv.path),
    }
}

pub fn default_true(o: Option<&syn::LitBool>) -> bool {
    o.as_ref().map(|b| b.value).unwrap_or(true)
}
*/