smart-default 0.5.0

use proc_macro2::TokenStream;
use syn::parse::Error;
use syn::spanned::Spanned;
use quote::ToTokens;

use util::{find_only, single_value};

#[derive(Debug, Clone, Copy)]
pub enum ConversionStrategy {
    NoConversion,
    Into,
}

pub struct DefaultAttr {
    pub code: Option<TokenStream>,
    conversion_strategy: Option<ConversionStrategy>,
}

impl DefaultAttr {
    pub fn find_in_attributes(attrs: &[syn::Attribute]) -> Result<Option<Self>, Error> {
        if let Some(default_attr) = find_only(attrs.iter(), |attr| is_default_attr(attr))? {
            match default_attr.parse_meta() {
                Ok(syn::Meta::Word(_)) => Ok(Some(Self {
                    code: None,
                    conversion_strategy: None,
                })),
                Ok(syn::Meta::List(meta)) => {
                    if let Some(field_value) = parse_code_hack(&meta)? { // #[default(_code = "...")]
                        Ok(Some(Self {
                            code: Some(field_value.into_token_stream()),
                            conversion_strategy: Some(ConversionStrategy::NoConversion),
                        }))
                    } else if let Some(field_value) = single_value(meta.nested.iter()) { // #[default(...)]
                        Ok(Some(Self {
                            code: Some(field_value.into_token_stream()),
                            conversion_strategy: None,
                        }))
                    } else {
                        return Err(Error::new(
                                if meta.nested.is_empty() {
                                    meta.span()
                                } else {
                                    meta.nested.span()
                                },
                                "Expected signle value in #[default(...)]"));
                    }
                }
                Ok(syn::Meta::NameValue(meta)) => {
                    Ok(Some(Self {
                        code: Some(meta.lit.into_token_stream()),
                        conversion_strategy: None,
                    }))
                }
                Err(error) => {
                    if let syn::Expr::Paren(as_parens) = syn::parse(default_attr.tts.clone().into())? {
                        Ok(Some(Self {
                            code: Some(as_parens.expr.into_token_stream()),
                            conversion_strategy: None,
                        }))
                    } else {
                        Err(error)
                    }
                }
            }
        } else {
            Ok(None)
        }
    }

    pub fn conversion_strategy(&self) -> ConversionStrategy {
        if let Some(conversion_strategy) = self.conversion_strategy {
            // Conversion strategy already set
            return conversion_strategy;
        }
        let code = if let Some(code) = &self.code {
            code
        } else {
            // #[default] - so no conversion (`Default::default()` already has the correct type)
            return ConversionStrategy::NoConversion;
        };
        if let Ok(syn::Lit::Str(_)) | Ok(syn::Lit::ByteStr(_)) = syn::parse::<syn::Lit>(code.clone().into()) {
            // A string literal - so we need a conversion in case we need to make it a `String`
            return ConversionStrategy::Into;
        }
        // Not handled by one of the rules, so we don't convert it to avoid causing trouble
        ConversionStrategy::NoConversion
    }
}

fn is_default_attr(attr: &syn::Attribute) -> Result<bool, Error> {
    let path = &attr.path;
    if path.leading_colon.is_some() {
        return Ok(false);
    }
    let segment = if let Some(segment) = single_value(path.segments.iter()) {
        segment
    } else {
        return Ok(false);
    };

    if let syn::PathArguments::None = segment.arguments {
    } else {
        return Ok(false);
    }

    Ok(segment.ident.to_string() == "default")
}

fn parse_code_hack(meta: &syn::MetaList) -> Result<Option<TokenStream>, Error> {
    for meta in meta.nested.iter() {
        if let syn::NestedMeta::Meta(syn::Meta::NameValue(meta)) = meta {
            if meta.ident != "_code" {
                continue;
            }
            if let syn::Lit::Str(lit) = &meta.lit {
                use std::str::FromStr;
                return Ok(Some(TokenStream::from_str(&lit.value())?));
            }
        };
    }
    Ok(None)
}