1use proc_macro2::{Span, TokenStream, TokenTree};
2use quote::{quote, ToTokens};
3use syn::parse_quote;
4use syn::{Data, DeriveInput, Fields};
5
6use crate::helpers::{non_enum_error, strum_discriminants_passthrough_error, HasTypeProperties};
7
8/// Attributes to copy from the main enum's variants to the discriminant enum's variants.
9///
10/// Attributes not in this list may be for other `proc_macro`s on the main enum, and may cause
11/// compilation problems when copied across.
12const ATTRIBUTES_TO_COPY: &[&str] = &["doc", "cfg", "allow", "deny", "strum_discriminants"];
13
14pub fn enum_discriminants_inner(ast: &DeriveInput) -> syn::Result<TokenStream> {
15 let name = &ast.ident;
16 let vis = &ast.vis;
17
18 let variants = match &ast.data {
19 Data::Enum(v) => &v.variants,
20 _ => return Err(non_enum_error()),
21 };
22
23 // Derives for the generated enum
24 let type_properties = ast.get_type_properties()?;
25
26 let derives = type_properties.discriminant_derives;
27
28 let derives = quote! {
29 #[derive(Clone, Copy, Debug, PartialEq, Eq, #(#derives),*)]
30 };
31
32 // Work out the name
33 let default_name = syn::Ident::new(&format!("{}Discriminants", name), Span::call_site());
34
35 let discriminants_name = type_properties.discriminant_name.unwrap_or(default_name);
36 let discriminants_vis = type_properties
37 .discriminant_vis
38 .unwrap_or_else(|| vis.clone());
39
40 // Pass through all other attributes
41 let pass_though_attributes = type_properties.discriminant_others;
42
43 let repr = type_properties.enum_repr.map(|repr| quote!(#[repr(#repr)]));
44
45 // Add the variants without fields, but exclude the `strum` meta item
46 let mut discriminants = Vec::new();
47 for variant in variants {
48 let ident = &variant.ident;
49 let discriminant = variant
50 .discriminant
51 .as_ref()
52 .map(|(_, expr)| quote!( = #expr));
53
54 // Don't copy across the "strum" meta attribute. Only passthrough the whitelisted
55 // attributes and proxy `#[strum_discriminants(...)]` attributes
56 let attrs = variant
57 .attrs
58 .iter()
59 .filter(|attr| {
60 ATTRIBUTES_TO_COPY
61 .iter()
62 .any(|attr_whitelisted| attr.path().is_ident(attr_whitelisted))
63 })
64 .map(|attr| {
65 if attr.path().is_ident("strum_discriminants") {
66 let mut ts = attr.meta.require_list()?.to_token_stream().into_iter();
67
68 // Discard strum_discriminants(...)
69 let _ = ts.next();
70
71 let passthrough_group = ts
72 .next()
73 .ok_or_else(|| strum_discriminants_passthrough_error(attr))?;
74 let passthrough_attribute = match passthrough_group {
75 TokenTree::Group(ref group) => group.stream(),
76 _ => {
77 return Err(strum_discriminants_passthrough_error(&passthrough_group));
78 }
79 };
80 if passthrough_attribute.is_empty() {
81 return Err(strum_discriminants_passthrough_error(&passthrough_group));
82 }
83 Ok(quote! { #[#passthrough_attribute] })
84 } else {
85 Ok(attr.to_token_stream())
86 }
87 })
88 .collect::<Result<Vec<_>, _>>()?;
89
90 discriminants.push(quote! { #(#attrs)* #ident #discriminant});
91 }
92
93 // Ideally:
94 //
95 // * For `Copy` types, we `impl From<TheEnum> for TheEnumDiscriminants`
96 // * For `!Copy` types, we `impl<'enum> From<&'enum TheEnum> for TheEnumDiscriminants`
97 //
98 // That way we ensure users are not able to pass a `Copy` type by reference. However, the
99 // `#[derive(..)]` attributes are not in the parsed tokens, so we are not able to check if a
100 // type is `Copy`, so we just implement both.
101 //
102 // See <https://github.com/dtolnay/syn/issues/433>
103 // ---
104 // let is_copy = unique_meta_list(type_meta.iter(), "derive")
105 // .map(extract_list_metas)
106 // .map(|metas| {
107 // metas
108 // .filter_map(get_meta_ident)
109 // .any(|derive| derive.to_string() == "Copy")
110 // }).unwrap_or(false);
111
112 let arms = variants
113 .iter()
114 .map(|variant| {
115 let ident = &variant.ident;
116 let params = match &variant.fields {
117 Fields::Unit => quote! {},
118 Fields::Unnamed(_fields) => {
119 quote! { (..) }
120 }
121 Fields::Named(_fields) => {
122 quote! { { .. } }
123 }
124 };
125
126 quote! { #name::#ident #params => #discriminants_name::#ident }
127 })
128 .collect::<Vec<_>>();
129
130 let from_fn_body = quote! { match val { #(#arms),* } };
131
132 let (impl_generics, ty_generics, where_clause) = ast.generics.split_for_impl();
133 let impl_from = quote! {
134 impl #impl_generics ::core::convert::From< #name #ty_generics > for #discriminants_name #where_clause {
135 fn from(val: #name #ty_generics) -> #discriminants_name {
136 #from_fn_body
137 }
138 }
139 };
140 let impl_from_ref = {
141 let mut generics = ast.generics.clone();
142
143 let lifetime = parse_quote!('_enum);
144 let enum_life = quote! { & #lifetime };
145 generics.params.push(lifetime);
146
147 // Shadows the earlier `impl_generics`
148 let (impl_generics, _, _) = generics.split_for_impl();
149
150 quote! {
151 impl #impl_generics ::core::convert::From< #enum_life #name #ty_generics > for #discriminants_name #where_clause {
152 fn from(val: #enum_life #name #ty_generics) -> #discriminants_name {
153 #from_fn_body
154 }
155 }
156 }
157 };
158
159 Ok(quote! {
160 /// Auto-generated discriminant enum variants
161 #derives
162 #repr
163 #(#[ #pass_though_attributes ])*
164 #discriminants_vis enum #discriminants_name {
165 #(#discriminants),*
166 }
167
168 #impl_from
169 #impl_from_ref
170 })
171}
172