1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158

use proc_macro2::{Delimiter, Group, TokenStream, TokenTree};
use quote::{format_ident, quote, ToTokens};
use std::iter::FromIterator;
use syn::parse::{Nothing, ParseStream};
use syn::{
    braced, bracketed, parenthesized, token, Attribute, Error, Ident, Index, LitInt, LitStr,
    Result, Token,
};

pub struct Attrs<'a> {
    pub display: Option<Display<'a>>,
    pub source: Option<&'a Attribute>,
    pub backtrace: Option<&'a Attribute>,
    pub from: Option<&'a Attribute>,
    pub transparent: Option<&'a Attribute>,
}

#[derive(Clone)]
pub struct Display<'a> {
    pub original: &'a Attribute,
    pub fmt: LitStr,
    pub args: TokenStream,
    pub has_bonus_display: bool,
}

pub fn get(input: &[Attribute]) -> Result<Attrs> {
    let mut attrs = Attrs {
        display: None,
        source: None,
        backtrace: None,
        from: None,
        transparent: None,
    };

    for attr in input {
        if attr.path.is_ident("error") {
            parse_error_attribute(&mut attrs, attr)?;
        } else if attr.path.is_ident("source") {
            require_empty_attribute(attr)?;
            if attrs.source.is_some() {
                return Err(Error::new_spanned(attr, "duplicate #[source] attribute"));
            }
            attrs.source = Some(attr);
        } else if attr.path.is_ident("backtrace") {
            require_empty_attribute(attr)?;
            if attrs.backtrace.is_some() {
                return Err(Error::new_spanned(attr, "duplicate #[backtrace] attribute"));
            }
            attrs.backtrace = Some(attr);
        } else if attr.path.is_ident("from") {
            if !attr.tokens.is_empty() {
                // Assume this is meant for derive_more crate or something.
                continue;
            }
            if attrs.from.is_some() {
                return Err(Error::new_spanned(attr, "duplicate #[from] attribute"));
            }
            attrs.from = Some(attr);
        }
    }

    Ok(attrs)
}

fn parse_error_attribute<'a>(attrs: &mut Attrs<'a>, attr: &'a Attribute) -> Result<()> {
    syn::custom_keyword!(transparent);

    attr.parse_args_with(|input: ParseStream| {
        if input.parse::<Option<transparent>>()?.is_some() {
            if attrs.transparent.is_some() {
                return Err(Error::new_spanned(
                    attr,
                    "duplicate #[error(transparent)] attribute",
                ));
            }
            attrs.transparent = Some(attr);
            return Ok(());
        }

        let display = Display {
            original: attr,
            fmt: input.parse()?,
            args: parse_token_expr(input, false)?,
            has_bonus_display: false,
        };
        if attrs.display.is_some() {
            return Err(Error::new_spanned(
                attr,
                "only one #[error(...)] attribute is allowed",
            ));
        }
        attrs.display = Some(display);
        Ok(())
    })
}

fn parse_token_expr(input: ParseStream, mut last_is_comma: bool) -> Result<TokenStream> {
    let mut tokens = Vec::new();
    while !input.is_empty() {
        if last_is_comma && input.peek(Token![.]) {
            if input.peek2(Ident) {
                input.parse::<Token![.]>()?;
                last_is_comma = false;
                continue;
            }
            if input.peek2(LitInt) {
                input.parse::<Token![.]>()?;
                let int: Index = input.parse()?;
                let ident = format_ident!("_{}", int.index, span = int.span);
                tokens.push(TokenTree::Ident(ident));
                last_is_comma = false;
                continue;
            }
        }
        last_is_comma = input.peek(Token![,]);
        let token: TokenTree = if input.peek(token::Paren) {
            let content;
            let delimiter = parenthesized!(content in input);
            let nested = parse_token_expr(&content, true)?;
            let mut group = Group::new(Delimiter::Parenthesis, nested);
            group.set_span(delimiter.span);
            TokenTree::Group(group)
        } else if input.peek(token::Brace) {
            let content;
            let delimiter = braced!(content in input);
            let nested = parse_token_expr(&content, true)?;
            let mut group = Group::new(Delimiter::Brace, nested);
            group.set_span(delimiter.span);
            TokenTree::Group(group)
        } else if input.peek(token::Bracket) {
            let content;
            let delimiter = bracketed!(content in input);
            let nested = parse_token_expr(&content, true)?;
            let mut group = Group::new(Delimiter::Bracket, nested);
            group.set_span(delimiter.span);
            TokenTree::Group(group)
        } else {
            input.parse()?
        };
        tokens.push(token);
    }
    Ok(TokenStream::from_iter(tokens))
}

fn require_empty_attribute(attr: &Attribute) -> Result<()> {
    syn::parse2::<Nothing>(attr.tokens.clone())?;
    Ok(())
}

impl ToTokens for Display<'_> {
    fn to_tokens(&self, tokens: &mut TokenStream) {
        let fmt = &self.fmt;
        let args = &self.args;
        tokens.extend(quote! {
            write!(__formatter, #fmt #args)
        });
    }
}