minimal extraction of enum_metadata from PR #207.

strum/src/lib.rs

@@ -30,6 +30,8 @@
 // only for documentation purposes
 pub mod additional_attributes;
 
+use core::ops;
+
 /// The ParseError enum is a collection of all the possible reasons
 /// an enum can fail to parse from a string.
 #[derive(Debug, Clone, Copy, Eq, PartialEq, Hash)]
@@ -190,6 +192,44 @@ pub trait VariantNames {
     const VARIANTS: &'static [&'static str];
 }
 
+pub trait EnumMetadata {
+    /// The repr type as a trait associated type.
+    type Repr: Copy
+        + ops::BitOr
+        + ops::BitAnd
+        + ops::BitXor
+        + ops::Shr
+        + ops::Shl
+        + ops::Not
+        + ops::BitOrAssign
+        + ops::BitAndAssign
+        + ops::BitXorAssign
+        + ops::ShrAssign
+        + ops::ShlAssign
+        + core::cmp::Eq
+        + core::cmp::Ord
+        + core::cmp::PartialEq
+        + core::cmp::PartialOrd
+        + core::fmt::Display
+        + core::fmt::Debug;
+
+    /// The enum type, generally Self unless deriving EnumMetadata for a type which returns
+    /// metadata for another enum.
+    type EnumT: EnumMetadata;
+
+    /// Variant names
+    const VARIANTS: &'static [&'static str];
+    /// Number of variants
+    const COUNT: usize;
+    /// std::mem::size_of<Self::Repr>().
+    const REPR_SIZE: usize;
+
+    /// convert to the enums #[repr(..)] equivalent to `self as ..`
+    fn to_repr(self) -> Self::Repr;
+    /// Trait equivalent of EnumT::from_repr(...)
+    fn from_repr(repr: Self::Repr) -> Option<Self::EnumT>;
+}
+
 #[cfg(feature = "derive")]
 pub use strum_macros::*;
 
@@ -214,6 +254,7 @@ DocumentMacroRexports! {
     EnumCount,
     EnumDiscriminants,
     EnumIter,
+    EnumMetadata,
     EnumMessage,
     EnumProperty,
     EnumString,

strum_macros/src/helpers/metadata_impl.rs

@@ -0,0 +1,214 @@
+use proc_macro2::{Span, TokenStream};
+use quote::{format_ident, quote};
+use syn::{
+    Data, DeriveInput, ImplGenerics, PathArguments, Type, TypeGenerics, TypeParen, WhereClause,
+};
+
+use crate::helpers::{non_enum_error, HasStrumVariantProperties, HasTypeProperties};
+
+pub struct MetadataImpl<'a> {
+    ast: &'a syn::DeriveInput,
+    variants: &'a syn::punctuated::Punctuated<syn::Variant, syn::token::Comma>,
+    gen_names: Option<Vec<syn::LitStr>>,
+    gen_from_repr: Option<FromReprTokens>,
+    generics_split: (ImplGenerics<'a>, TypeGenerics<'a>, Option<&'a WhereClause>),
+    pub enum_count: usize,
+    pub has_additional_data: bool,
+}
+
+pub struct FromReprTokens {
+    pub constant_defs: Vec<TokenStream>,
+    pub match_arms: Vec<TokenStream>,
+}
+
+impl<'a> MetadataImpl<'a> {
+    pub fn new(ast: &'a DeriveInput) -> syn::Result<Self> {
+        let gen = &ast.generics;
+        let generics_split = gen.split_for_impl();
+
+        match &ast.data {
+            Data::Enum(_) => (),
+            _ => return Err(non_enum_error()),
+        };
+
+        let variants = match &ast.data {
+            Data::Enum(v) => &v.variants,
+            _ => return Err(non_enum_error()),
+        };
+        let enum_count = variants.len();
+
+        Ok(MetadataImpl {
+            ast,
+            enum_count,
+            gen_names: None,
+            gen_from_repr: None,
+            generics_split,
+            variants,
+            has_additional_data: false,
+        })
+    }
+
+    pub fn use_name_info(mut self) -> Self {
+        self.gen_names = Some(Vec::new());
+        self
+    }
+
+    pub fn use_from_repr(mut self) -> syn::Result<Self> {
+        let gen = &self.ast.generics;
+        if gen.lifetimes().count() > 0 {
+            return Err(syn::Error::new(
+                Span::call_site(),
+                "This macro doesn't support enums with lifetimes. \
+                 The resulting enums would be unbounded.",
+            ));
+        }
+
+        self.gen_from_repr = Some(FromReprTokens {
+            constant_defs: Vec::new(),
+            match_arms: Vec::new(),
+        });
+        Ok(self)
+    }
+
+    pub fn discriminant_type(&self) -> Type {
+        let mut discriminant_type: Type = syn::parse("usize".parse().unwrap()).unwrap();
+        for attr in &self.ast.attrs {
+            let path = &attr.path;
+            let tokens = &attr.tokens;
+            if path.leading_colon.is_some() {
+                continue;
+            }
+            if path.segments.len() != 1 {
+                continue;
+            }
+            let segment = path.segments.first().unwrap();
+            if segment.ident != "repr" {
+                continue;
+            }
+            if segment.arguments != PathArguments::None {
+                continue;
+            }
+            let typ_paren = match syn::parse2::<Type>(tokens.clone()) {
+                Ok(Type::Paren(TypeParen { elem, .. })) => *elem,
+                _ => continue,
+            };
+            let inner_path = match &typ_paren {
+                Type::Path(t) => t,
+                _ => continue,
+            };
+            if let Some(seg) = inner_path.path.segments.last() {
+                for t in &[
+                    "u8", "u16", "u32", "u64", "usize", "i8", "i16", "i32", "i64", "isize",
+                ] {
+                    if seg.ident == t {
+                        discriminant_type = typ_paren;
+                        break;
+                    }
+                }
+            }
+        }
+        discriminant_type
+    }
+
+    fn params_from_fields(fields: &syn::Fields) -> (TokenStream, bool) {
+        use syn::Fields::*;
+        match &fields {
+            Unit => (quote! {}, false),
+            Unnamed(fields) => {
+                let defaults = ::std::iter::repeat(quote!(::core::default::Default::default()))
+                    .take(fields.unnamed.len());
+                (quote! { (#(#defaults),*) }, true)
+            }
+            Named(fields) => {
+                let fields = fields
+                    .named
+                    .iter()
+                    .map(|field| field.ident.as_ref().unwrap());
+                (
+                    quote! { {#(#fields: ::core::default::Default::default()),*} },
+                    true,
+                )
+            }
+        }
+    }
+
+    fn case_style(&self) -> Option<crate::helpers::case_style::CaseStyle> {
+        if let Ok(props) = self.ast.get_type_properties() {
+            props.case_style
+        } else {
+            None
+        }
+    }
+
+    pub fn generate(&mut self) -> syn::Result<()> {
+        let name = &self.ast.ident;
+        let discriminant_type = self.discriminant_type();
+
+        let case_style = self.case_style();
+        let mut prev_const_var_ident = None;
+
+        for variant in self.variants {
+            let props = variant.get_variant_properties()?;
+
+            if let Some(variant_names) = &mut self.gen_names {
+                variant_names.push(props.get_preferred_name(case_style));
+            }
+
+            if let Some(FromReprTokens {
+                match_arms,
+                constant_defs,
+            }) = &mut self.gen_from_repr
+            {
+                if props.disabled.is_some() {
+                    continue;
+                }
+
+                let ident = &variant.ident;
+                let (params, has_additional_data) = Self::params_from_fields(&variant.fields);
+                if has_additional_data {
+                    self.has_additional_data = has_additional_data;
+                }
+
+                let const_var_ident = {
+                    use heck::ToShoutySnakeCase;
+                    let const_var_str = format!("{}_DISCRIMINANT", ident).to_shouty_snake_case();
+                    format_ident!("{}", const_var_str)
+                };
+
+                let const_val_expr = match &variant.discriminant {
+                    Some((_, expr)) => quote! { #expr },
+                    None => match &prev_const_var_ident {
+                        Some(prev) => quote! { #prev + 1 },
+                        None => quote! { 0 },
+                    },
+                };
+
+                constant_defs
+                    .push(quote! {const #const_var_ident: #discriminant_type = #const_val_expr;});
+                match_arms.push(quote! {v if v == #const_var_ident => ::core::option::Option::Some(#name::#ident #params)});
+                prev_const_var_ident = Some(const_var_ident);
+            }
+        }
+        if let Some(FromReprTokens { match_arms, .. }) = &mut self.gen_from_repr {
+            match_arms.push(quote! { _ => ::core::option::Option::None });
+        }
+
+        Ok(())
+    }
+
+    pub fn variant_names(&self) -> &Option<Vec<syn::LitStr>> {
+        &self.gen_names
+    }
+
+    pub fn from_repr(&self) -> &Option<FromReprTokens> {
+        &self.gen_from_repr
+    }
+
+    pub fn enum_count(&self) -> usize {
+        self.enum_count
+    }
+
+    pub fn generics_split(&self) -> &(ImplGenerics<'a>, TypeGenerics<'a>, Option<&'a WhereClause>) {
+        &self.generics_split
+    }
+}

strum_macros/src/helpers/mod.rs

@@ -4,6 +4,7 @@ pub use self::variant_props::HasStrumVariantProperties;
 
 pub mod case_style;
 mod metadata;
+pub mod metadata_impl;
 pub mod type_props;
 pub mod variant_props;
 

strum_macros/src/lib.rs

@@ -466,6 +466,17 @@ pub fn from_repr(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
     toks.into()
 }
 
+/// Enum Metadata documentation.
+#[proc_macro_derive(EnumMetadata, attributes(strum))]
+pub fn enum_metadata(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
+    let ast = syn::parse_macro_input!(input as DeriveInput);
+
+    let toks = macros::enum_metadata::enum_metadata_inner(&ast)
+        .unwrap_or_else(|err| err.to_compile_error());
+    debug_print_generated(&ast, &toks);
+    toks.into()
+}
+
 /// Add a verbose message to an enum variant.
 ///
 /// Encode strings into the enum itself. The `strum_macros::EmumMessage` macro implements the `strum::EnumMessage` trait.

strum_macros/src/macros/enum_count.rs

@@ -1,14 +1,12 @@
+use crate::helpers::metadata_impl::MetadataImpl;
 use proc_macro2::TokenStream;
 use quote::quote;
-use syn::{Data, DeriveInput};
+use syn::DeriveInput;
 
-use crate::helpers::{non_enum_error, HasTypeProperties};
+use crate::helpers::HasTypeProperties;
 
 pub(crate) fn enum_count_inner(ast: &DeriveInput) -> syn::Result<TokenStream> {
-    let n = match &ast.data {
-        Data::Enum(v) => v.variants.len(),
-        _ => return Err(non_enum_error()),
-    };
+    let n = MetadataImpl::new(ast)?.enum_count;
     let type_properties = ast.get_type_properties()?;
     let strum_module_path = type_properties.crate_module_path();
 

strum_macros/src/macros/enum_metadata.rs

@@ -0,0 +1,47 @@
+use crate::helpers::metadata_impl::{FromReprTokens, MetadataImpl};
+use proc_macro2::TokenStream;
+use quote::quote;
+use syn::DeriveInput;
+
+pub fn enum_metadata_inner(ast: &DeriveInput) -> syn::Result<TokenStream> {
+    let name = &ast.ident;
+    let mut metadata = MetadataImpl::new(ast)?.use_name_info().use_from_repr()?;
+    let discriminant_type = metadata.discriminant_type();
+    metadata.generate()?;
+    let enum_count = metadata.enum_count();
+    let variant_names = metadata.variant_names().as_ref().unwrap();
+    let (impl_generics, ty_generics, where_clause) = &metadata.generics_split();
+
+    let FromReprTokens {
+        constant_defs,
+        match_arms,
+    } = &metadata.from_repr().as_ref().unwrap();
+
+    Ok(quote! {
+        impl #impl_generics EnumMetadata for #name #ty_generics #where_clause {
+            #[doc = "The Repr type."]
+            type Repr = #discriminant_type;
+            #[doc = "The Enum type, typically Self unless implementing EnumMetadata for another enum type."]
+            type EnumT = Self;
+
+            const VARIANTS: &'static [&'static str] = &[ #(#variant_names),* ];
+            const COUNT: usize = #enum_count;
+            const REPR_SIZE: usize = ::core::mem::size_of::<Self::Repr>();
+
+            fn to_repr(self) -> #discriminant_type {
+               self as #discriminant_type
+            }
+
+            // Note: synchronize changes with `FromRepr::from_repr`,
+            // it duplicates this logic in an inherent impl.
+            // Making it possible to have both impls on the same type;
+            // so their behavior must be kept the same.
+            fn from_repr(discriminant: #discriminant_type) -> Option<Self> {
+                #(#constant_defs)*
+                match discriminant {
+                    #(#match_arms),*
+                }
+            }
+        }
+    })
+}