Regression in quality of TOML type mismatch error messages

[crumblingstatue]

$ cargo +stable build
...
  expected a value of type `string`, but found a value of type `integer` for the key `dependencies.regex`

$ cargo build
...
  data did not match any variant of untagged enum TomlDependency for key `dependencies.regex`

Fixing this might require changes in toml, or maybe even serde, but I don't think we should sacrifice user experience for the developer productivity that serde provides.

[alexcrichton]

Thanks for the report!

@dtolnay do you know if there's an easy-ish way to configure that error message locally?

[dtolnay]

Yes the untagged enum error messages need some work. We are tracking this in serde-rs/serde#773.

For now you can provide a custom message by writing the Deserialize impl yourself.

invalid type: integer `1`, expected a version string like "0.9.8" or a detailed dependency like {version = "0.9.8"} for key `dependencies.regex`

use serde::de::{self, Deserialize, Deserializer, Visitor};

impl Deserialize for TomlDependency {
    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
        where D: Deserializer
    {
        struct TomlDependencyVisitor;

        impl Visitor for TomlDependencyVisitor {
            type Value = TomlDependency;

            fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
                formatter.write_str("a version string like \"0.9.8\" or a \
                                     detailed dependency like {version = \"0.9.8\"}")
            }

            fn visit_str<E>(self, s: &str) -> Result<Self::Value, E>
                where E: de::Error
            {
                Ok(TomlDependency::Simple(s.to_owned()))
            }

            fn visit_map<V>(self, map: V) -> Result<Self::Value, V::Error>
                where V: de::MapVisitor
            {
                let mvd = de::value::MapVisitorDeserializer::new(map);
                DetailedTomlDependency::deserialize(mvd).map(TomlDependency::Detailed)
            }
        }

        deserializer.deserialize(TomlDependencyVisitor)
    }
}

[crumblingstatue]

Might be worth the extra effort to implement the above, since that error message is even better than the original.

[alexcrichton]

@dtolnay awesome, thanks! @crumblingstatue yeah I'll send a PR.