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

extern crate proc_macro;

use proc_macro::TokenStream;
use proc_macro_error::{abort, proc_macro_error};
use quote::quote;
use syn::{
    AngleBracketedGenericArguments, AttrStyle::Outer, Attribute, Expr::Lit, ExprLit, Field,
    Fields::Named, GenericArgument, Lit::Str, Meta::NameValue, MetaNameValue, PathArguments,
    PathSegment, Type, TypePath,
};

fn default_value(ty: String) -> String {
    match ty.as_str() {
        "usize" | "u8" | "u16" | "u32" | "u64" | "u128" | "isize" | "i8" | "i16" | "i32"
        | "i64" | "i128" => "0",
        "f32" | "f64" => "0.0",
        _ => "\"\"",
    }
    .to_string()
}

fn parse_type(ty: &Type, default: &mut Option<String>, optional: &mut bool) {
    if let Type::Path(TypePath { path, .. }) = ty {
        if let Some(PathSegment { ident, arguments }) = path.segments.last() {
            let id = ident.to_string();
            if arguments.is_none() {
                *default = Some(default_value(id));
            } else if id == "Option" {
                *optional = true;
                if let PathArguments::AngleBracketed(AngleBracketedGenericArguments {
                    args, ..
                }) = arguments
                {
                    if let Some(GenericArgument::Type(ty)) = args.first() {
                        parse_type(&ty, default, &mut false);
                    }
                }
            } else if id == "Vec" {
                if let PathArguments::AngleBracketed(AngleBracketedGenericArguments {
                    args, ..
                }) = arguments
                {
                    if let Some(GenericArgument::Type(ty)) = args.first() {
                        let mut item_default = None;
                        parse_type(&ty, &mut item_default, &mut false);
                        *default = if let Some(item_default) = item_default {
                            Some(format!("[ {item_default:}, ]"))
                        } else {
                            Some(format!("[  ]"))
                        }
                    }
                }
            }
            // TODO else Complex struct in else
        }
    }
}

fn parse_docs(attrs: &Vec<Attribute>) -> Vec<String> {
    let mut docs = Vec::new();
    for attr in attrs.iter() {
        match (attr.style, &attr.meta) {
            (Outer, NameValue(MetaNameValue { path, value, .. })) => {
                for seg in path.segments.iter() {
                    if seg.ident.to_string() == "doc" {
                        if let Lit(ExprLit {
                            lit: Str(lit_str), ..
                        }) = value
                        {
                            docs.push(lit_str.value());
                        }
                    }
                }
            }
            _ => (),
        }
    }
    docs
}

fn get_default_and_doc_from_field(field: &Field) -> (Option<String>, Vec<String>, bool) {
    let mut default = None;
    let mut optional = false;
    parse_type(&field.ty, &mut default, &mut optional);
    (
        default.map(|s| s.to_string()),
        parse_docs(&field.attrs),
        optional,
    )
}

fn push_doc_string(example: &mut String, docs: Vec<String>, paragraph: bool) {
    let has_docs = !docs.is_empty();
    for doc in docs.into_iter() {
        example.push('#');
        example.push_str(&doc);
        example.push('\n');
    }

    if has_docs && paragraph {
        example.push('\n');
    }
}

#[proc_macro_derive(TomlExample)]
#[proc_macro_error]
pub fn derive_patch(item: TokenStream) -> TokenStream {
    let input = syn::parse_macro_input!(item as syn::DeriveInput);
    let struct_name = &input.ident;
    let mut example = String::new();
    push_doc_string(&mut example, parse_docs(&input.attrs), true);

    let fields = if let syn::Data::Struct(syn::DataStruct { fields, .. }) = &input.data {
        fields
    } else {
        abort!(&input.ident, "TomlExample derive only use for struct")
    };
    if let Named(fields_named) = fields {
        for f in fields_named.named.iter() {
            if let Some(field_name) = f.ident.as_ref().map(|i| i.to_string()) {
                let (default, doc_str, optional) = get_default_and_doc_from_field(&f);
                push_doc_string(&mut example, doc_str, false);

                if optional {
                    example.push_str("# ");
                }

                if let Some(default) = default {
                    example.push_str(&field_name);
                    example.push_str(" = ");
                    example.push_str(&default);
                } else {
                    example.push_str(&field_name);
                    example.push_str(" = \"\"");
                }
                example.push('\n');
            }
        }
    }

    let output = quote! {
        impl toml_example::TomlExample for #struct_name {
            fn toml_example() -> &'static str {
                #example
            }
        }
    };
    TokenStream::from(output)
}