strum_macros/macros/
enum_discriminants.rs

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}
strum_macros/macros/enum_discriminants.rs

strum_macros/macros/
enum_discriminants.rs
