Initial experimental version

.gitignore

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+.DS_Store
+
+.idea
+.vscode
+
+/target
+Cargo.lock

Cargo.toml

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+[package]
+name = "enumscribe"
+version = "0.1.0"
+authors = ["Tom Panton <pantonshire@gmail.com>"]
+edition = "2018"
+license = "MIT"
+description = "Procedural macros for converting between enums and strings"
+
+[lib]
+proc-macro = true
+
+[dependencies]
+syn = "1.0"
+quote = "1.0"

src/lib.rs

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+use proc_macro::TokenStream;
+
+use quote::quote;
+use syn::{Attribute, Data, DeriveInput, LitStr};
+
+/// Derives `serde::Deserialize` for an enum with variants associated with strings.
+/// The `#[str_name("...")]` attribute is used to specify the string associated with each variant.
+///
+/// An "other" variant can be specified with `#[other]`. This variant should have a parameter
+/// which implements `From<String>` to store the string that could not be deserialized to any
+/// of the other variants.
+///
+/// If no "other" variant is specified, strings which are not associated with any of the variants
+/// will produce a deserialization error.
+///
+/// The enum may have the attribute `#[case_insensitive]`, in which case string comparisons will
+/// be done case-insensitively.
+#[proc_macro_derive(EnumStrDeserialize, attributes(case_insensitive, str_name, other))]
+pub fn derive_enum_str_de(ast: TokenStream) -> TokenStream {
+    const ATTR_CASE_INSENSITIVE: &'static str = "case_insensitive";
+    const ATTR_STR_NAME: &'static str = "str_name";
+    const ATTR_OTHER: &'static str = "other";
+
+    let ast: DeriveInput = syn::parse(ast).unwrap();
+
+    let enum_name = &ast.ident;
+    let enum_names = std::iter::repeat(enum_name);
+
+    let case_insensitive = find_attribute(ATTR_CASE_INSENSITIVE, &ast.attrs).is_some();
+
+    let enum_data = match ast.data {
+        Data::Enum(e) => e,
+        _ => panic!("cannot derive EnumStrDeserialize for anything other than an enum"),
+    };
+
+    let (variant_names, variant_strings): (Vec<_>, Vec<_>) = enum_data.variants.iter()
+        .map(|variant| (&variant.ident, find_attribute(ATTR_STR_NAME, &variant.attrs)))
+        .filter(|(_, attribute)| attribute.is_some())
+        .map(|(variant_ident, attribute)| (variant_ident, attribute
+            .unwrap()
+            .parse_args::<LitStr>()
+            .unwrap()
+            .value()))
+        .map(|(variant_ident, attribute)| (variant_ident, if case_insensitive {
+            attribute.to_lowercase()
+        } else {
+            attribute
+        }))
+        .unzip();
+
+    let other_variant = enum_data.variants.iter()
+        .find(|variant| find_attribute(ATTR_OTHER, &variant.attrs).is_some());
+
+    let matching_string = if case_insensitive {
+        quote! { deserialized_string.to_lowercase() }
+    } else {
+        quote! { deserialized_string }
+    };
+
+    let (base_case_pattern, base_case_value) = if let Some(other_variant) = other_variant {
+        let other_variant_name = &other_variant.ident;
+        (quote! { _ }, quote! { ::core::result::Result::Ok(#enum_name::#other_variant_name(deserialized_string.into())) })
+    } else {
+        (quote! { s }, quote! { ::core::result::Result::Err(::serde::de::Error::unknown_variant(s, &[#(#variant_strings),*])) })
+    };
+
+    (quote! {
+        impl<'de> ::serde::Deserialize<'de> for #enum_name {
+            fn deserialize<D>(deserializer: D) -> ::core::result::Result<Self, D::Error>
+                where
+                    D: ::serde::Deserializer<'de>,
+            {
+                let deserialized_string = ::std::string::String::deserialize(deserializer)?;
+                match #matching_string.as_str() {
+                    #(#variant_strings => ::core::result::Result::Ok(#enum_names::#variant_names),)*
+                    #base_case_pattern => #base_case_value,
+                }
+            }
+        }
+    }).into()
+}
+
+fn find_attribute<'a>(name: &str, attributes: &'a [Attribute]) -> Option<&'a Attribute> {
+    attributes
+        .iter()
+        .find(|attribute| attribute.path.is_ident(name))
+}
+