Deserializing to `Value` silently ignores duplicate keys

[stevenengler]

The YAML spec disallows duplicate keys:

The content of a mapping node is an unordered set of key: value node pairs, with the restriction that each of the keys is unique.

When deserializing to a custom serde-derive struct we can use #[serde(with = "serde_with::rust::maps_duplicate_key_is_error")] to prevent duplicate keys. But if we want to deserialize to a serde_yaml::Value, serde-yaml's mappings will ignore duplicate keys. Is there any way to achieve the same behaviour when deserializing to a serde_yaml::Value?

The reason for wanting to deserialize to a serde_yaml::Value is that I would like to take advantage of the new apply_merge() feature. So what I'm doing is:

let mut config: serde_yaml::Value = serde_yaml::from_reader(file).unwrap();
config.apply_merge().unwrap();
let config: MyConfig = serde_yaml::from_value(config).unwrap();

[dtolnay]

Fixed in 0.9.4. I didn't realize this was specified as disallowed by the YAML spec.

[stevenengler]

Wow that was quick, thanks!

[NigelThorpe]

Except that the majority of YAML parsers out there allow duplicate keys, and take a "last key wins" philosophy. See e.g. https://github.com/chyh1990/yaml-rust/issues/19

I've got some things which were considered valid before but now aren't. I'd argue this was a breaking change, despite the YAML spec.

[conradludgate]

Being devils advocate, semver is based around documented behaviour. Since it's documented in the YAML spec as invalid behaviour, and since serde-yaml doesn't document otherwise, this is a valid bug fix.

Obligatory XKCD: obviously this is hyperbole, but no matter where you draw the line someone will find an issue with the change.

That being said, it did cause our codebase to break 😅