Add meaningful error messages

[russellbanks]

Without using nutype, I use the thiserror crate to create Error types and give them relevant error messages:

#[derive(Error, Debug, PartialEq)]
pub enum PackageVersionError {
    #[error(
        "Package version length must be between {} and {} characters",
        PackageVersion::MIN_LENGTH,
        PackageVersion::MAX_LENGTH
    )]
    InvalidLength,
    #[error("Package version may not contain whitespace")]
    ContainsWhitespace,
    #[error("Package version may not contain any control characters")]
    ContainsControlChars,
    #[error(
        "Package version may not contain any of the following characters: {:?}",
        PackageVersion::DISALLOWED_CHARACTERS
    )]
    DisallowedCharacters,
}

Currently, trying to achieve a similar thing with nutype will just return error messages like "too short". This isn't helpful as I show the error to the user.

#[nutype(
    sanitize(trim),
    validate(
        not_empty,
        char_len_min = 4,
        char_len_max = 128,
    ),
    default = "",
    derive(AsRef, Clone, Default, Deref, Display, FromStr, Deserialize, Serialize),
)]

Ideally, I'd like for nutype's error to instead say something like Value is too short. The value length must be between 4 and 128 characters, or the same message but have a value name, such as Package version in my case.

[russellbanks]

It's a similar issue when providing a custom predicate; the function can only return a bool so we lose any meaningful reasoning for the error.

[greyblake]

@russellbanks Thanks for reporting this!

[greyblake]

This was recently addressed in #125
And will be released in 0.4.1.
If you need this earlier please ping me I can release a beta.