Add support for #derive[FromStr] for enums

This allows deriving FromStr for enums with no fields in the obvious way

fixes JelteF#59

Cargo.toml

@@ -29,7 +29,7 @@ proc-macro = true
 [dependencies]
 proc-macro2 = "1.0"
 quote = "1.0"
-syn = "1.0.3"
+syn = "1.0.81"
 convert_case = { version = "0.4", optional = true}
 
 [build-dependencies]
@@ -51,7 +51,7 @@ deref_mut = []
 display = ["syn/extra-traits"]
 error = ["syn/extra-traits"]
 from = ["syn/extra-traits"]
-from_str = []
+from_str = ["convert_case"]
 index = []
 index_mut = []
 into = ["syn/extra-traits"]

doc/from_str.md

@@ -1,6 +1,7 @@
 % What #[derive(FromStr)] generates
 
-Deriving `FromStr` only works for newtypes, i.e structs with only a single
+Deriving `FromStr` only works for enums with no fields
+or newtypes, i.e structs with only a single
 field. The result is that you will be able to call the `parse()` method on a
 string to convert it to your newtype. This only works when the type that is
 contained in the type implements `FromStr`.
@@ -77,4 +78,58 @@ impl ::core::str::FromStr for Point1D {
 
 # Enums
 
-Deriving `FromStr` is not supported for enums.
+When deriving `FromStr` for an enums with variants with no fields it will
+generate a `from_str` method that converts strings that match the variant name
+to the variant. If using a case insensitive match would give a unique variant
+(i.e you dont have both a `MyEnum::Foo` and a `MyEnum::foo` variant) then case
+insensitve matching will be used, otherwise it will fall back to exact string
+matchng.
+
+Since the string may not match any vairants an error type is needed so one
+will be generated of the format `Parse{}Error`
+
+e.g. Given the following enum:
+
+```rust
+# #[macro_use] extern crate derive_more;
+# fn main(){}
+#[derive(FromStr)]
+enum EnumNoFields {
+    Foo,
+    Bar,
+    Baz,
+}
+```
+
+Code like this will be generated:
+
+```rust
+# enum EnumNoFields {
+#     Foo,
+#     Bar,
+#     Baz,
+# }
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+struct ParseEnumNoFieldsError;
+
+impl std::fmt::Display for ParseEnumNoFieldsError {
+    fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        fmt.write_str("invalid enum no fields")
+    }
+}
+
+impl std::error::Error for ParseEnumNoFieldsError {}
+
+impl ::core::str::FromStr for EnumNoFields {
+    type Err = ParseEnumNoFieldsError;
+    fn from_str(src: &str) -> Result<Self, Self::Err> {
+        Ok(match src.to_lowercase().as_str() {
+            "foo" => EnumNoFields::Foo,
+            "bar" => EnumNoFields::Bar,
+            "baz" => EnumNoFields::Baz,
+            _ => return Err(ParseEnumNoFieldsError{}),
+        })
+    }
+}
+```

src/from_str.rs

@@ -1,6 +1,8 @@
+use crate::utils::{DeriveType, HashMap};
 use crate::utils::{SingleFieldData, State};
+use convert_case::{Case, Casing};
 use proc_macro2::TokenStream;
-use quote::quote;
+use quote::{format_ident, quote};
 use syn::{parse::Result, DeriveInput};
 
 /// Provides the hook to expand `#[derive(FromStr)]` into an implementation of `FromStr`
@@ -12,6 +14,14 @@ pub fn expand(input: &DeriveInput, trait_name: &'static str) -> Result<TokenStre
         trait_name.to_lowercase(),
     )?;
 
+    if state.derive_type == DeriveType::Enum {
+        Ok(enum_from(input, state, trait_name))
+    } else {
+        Ok(struct_from(&state, trait_name))
+    }
+}
+
+pub fn struct_from(state: &State, trait_name: &'static str) -> TokenStream {
     // We cannot set defaults for fields, once we do we can remove this check
     if state.fields.len() != 1 || state.enabled_fields().len() != 1 {
         panic_one_field(trait_name);
@@ -32,7 +42,7 @@ pub fn expand(input: &DeriveInput, trait_name: &'static str) -> Result<TokenStre
     let initializers = [quote!(#casted_trait::from_str(src)?)];
     let body = single_field_data.initializer(&initializers);
 
-    Ok(quote! {
+    quote! {
         impl#impl_generics #trait_path for #input_type#ty_generics #where_clause
         {
             type Err = <#field_type as #trait_path>::Err;
@@ -41,7 +51,81 @@ pub fn expand(input: &DeriveInput, trait_name: &'static str) -> Result<TokenStre
                 Ok(#body)
             }
         }
-    })
+    }
+}
+
+fn enum_from(
+    input: &DeriveInput,
+    state: State,
+    trait_name: &'static str,
+) -> TokenStream {
+    let mut variants_caseinsensitive = HashMap::default();
+    for variant_state in state.enabled_variant_data().variant_states {
+        let variant = variant_state.variant.unwrap();
+        if !variant.fields.is_empty() {
+            panic!("Only enums with no fields can derive({})", trait_name)
+        }
+
+        variants_caseinsensitive
+            .entry(variant.ident.to_string().to_lowercase())
+            .or_insert_with(Vec::new)
+            .push(variant.ident.clone());
+    }
+
+    let input_type = &input.ident;
+    let visibility = &input.vis;
+
+    let err_name = format_ident!("Parse{}Error", input_type);
+    let err_message =
+        format!("invalid {}", input_type.to_string().to_case(Case::Lower));
+
+    let mut cases = vec![];
+
+    // if a case insensitve match is unique match do that
+    // otherwise do a case sensitive match
+    for (ref canonical, ref variants) in variants_caseinsensitive {
+        if variants.len() == 1 {
+            let variant = &variants[0];
+            cases.push(quote! {
+                #canonical => #input_type::#variant,
+            })
+        } else {
+            for variant in variants {
+                let variant_str = variant.to_string();
+                cases.push(quote! {
+                    #canonical if(src == #variant_str) => #input_type::#variant,
+                })
+            }
+        }
+    }
+
+    let trait_path = state.trait_path;
+
+    quote! {
+
+        #[derive(Debug, Clone, PartialEq, Eq)]
+        #visibility struct #err_name;
+
+        impl std::fmt::Display for #err_name {
+            fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+                fmt.write_str(#err_message)
+            }
+        }
+
+        impl std::error::Error for #err_name {}
+
+        impl #trait_path for #input_type
+        {
+            type Err = #err_name;
+            #[inline]
+            fn from_str(src: &str) -> ::core::result::Result<Self, Self::Err> {
+                Ok(match src.to_lowercase().as_str() {
+                    #(#cases)*
+                    _ => return Err(#err_name{}),
+                })
+            }
+        }
+    }
 }
 
 fn panic_one_field(trait_name: &str) -> ! {

tests/from_str.rs

@@ -9,3 +9,25 @@ struct MyInt(i32);
 struct Point1D {
     x: i32,
 }
+
+#[derive(Debug, FromStr, PartialEq, Eq)]
+enum EnumNoFields {
+    Foo,
+    Bar,
+    Baz,
+}
+
+#[test]
+fn enum_test() {
+    assert_eq!("Foo".parse::<EnumNoFields>().unwrap(), EnumNoFields::Foo);
+    assert_eq!("FOO".parse::<EnumNoFields>().unwrap(), EnumNoFields::Foo);
+    assert_eq!("foo".parse::<EnumNoFields>().unwrap(), EnumNoFields::Foo);
+    assert_eq!(
+        "other".parse::<EnumNoFields>().unwrap_err(),
+        ParseEnumNoFieldsError {}
+    );
+    assert_eq!(
+        ParseEnumNoFieldsError {}.to_string(),
+        "invalid enum no fields"
+    );
+}