What #[derive(FromStr)] generates

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.

Example usage

# use derive_more::FromStr;
#
#[derive(FromStr, Debug, Eq, PartialEq)]
struct MyInt(i32);

#[derive(FromStr, Debug, Eq, PartialEq)]
struct Point1D{
    x: i32,
}

assert_eq!(MyInt(5), "5".parse().unwrap());
assert_eq!(Point1D{x: 100}, "100".parse().unwrap());

Tuple structs

When deriving FromStr for a tuple struct with one field:

# use derive_more::FromStr;
#
#[derive(FromStr)]
struct MyInt(i32);

Code like this will be generated:

# struct MyInt(i32);
impl derive_more::core::str::FromStr for MyInt {
    type Err = <i32 as derive_more::core::str::FromStr>::Err;
    fn from_str(src: &str) -> Result<Self, <i32 as derive_more::core::str::FromStr>::Err> {
        return Ok(MyInt(i32::from_str(src)?));
    }
}

Regular structs

When deriving FromStr for a regular struct with one field:

# use derive_more::FromStr;
#
#[derive(FromStr)]
struct Point1D {
    x: i32,
}

Code like this will be generated:

# struct Point1D {
#     x: i32,
# }
impl derive_more::core::str::FromStr for Point1D {
    type Err = <i32 as derive_more::core::str::FromStr>::Err;
    fn from_str(src: &str) -> Result<Self, <i32 as derive_more::core::str::FromStr>::Err> {
        return Ok(Point1D {
            x: i32::from_str(src)?,
        });
    }
}

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 insensitive matching will be used, otherwise it will fall back to exact string matching.

Since the string may not match any variants an error type is needed, so the derive_more::FromStrError will be used for that purpose.

e.g. Given the following enum:

# use derive_more::FromStr;
#
#[derive(FromStr)]
enum EnumNoFields {
    Foo,
    Bar,
    Baz,
}

Code like this will be generated:

# enum EnumNoFields {
#     Foo,
#     Bar,
#     Baz,
# }
#
impl derive_more::core::str::FromStr for EnumNoFields {
    type Err = derive_more::FromStrError;
    fn from_str(src: &str) -> Result<Self, derive_more::FromStrError> {
        Ok(match src.to_lowercase().as_str() {
            "foo" => EnumNoFields::Foo,
            "bar" => EnumNoFields::Bar,
            "baz" => EnumNoFields::Baz,
            _ => return Err(derive_more::FromStrError::new("EnumNoFields")),
        })
    }
}