added btree_map support to prost-serde

[siennathesane]

This fixes a hidden bug where users can use btree_map=. in protoc-gen-prost but it would break when uses with protoc-gen-prost-serde. Now users can use it in both places. I tested it with a complex workspace with buf and it worked as intended.

Example from generated code, see metadata__:

struct GeneratedVisitor;
impl<'de> serde::de::Visitor<'de> for GeneratedVisitor {
    type Value = ErrorInfo;

    fn expecting(&self, formatter: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        formatter.write_str("struct google.rpc.ErrorInfo")
    }

    fn visit_map<V>(self, mut map: V) -> std::result::Result<ErrorInfo, V::Error>
    where
        V: serde::de::MapAccess<'de>,
    {
        let mut reason__ = None;
        let mut domain__ = None;
        let mut metadata__ = None;
        while let Some(k) = map.next_key()? {
            match k {
                GeneratedField::Reason => {
                    if reason__.is_some() {
                        return Err(serde::de::Error::duplicate_field("reason"));
                    }
                    reason__ = Some(map.next_value()?);
                }
                GeneratedField::Domain => {
                    if domain__.is_some() {
                        return Err(serde::de::Error::duplicate_field("domain"));
                    }
                    domain__ = Some(map.next_value()?);
                }
                GeneratedField::Metadata => {
                    if metadata__.is_some() {
                        return Err(serde::de::Error::duplicate_field("metadata"));
                    }
                    metadata__ =
                        Some(map.next_value::<std::collections::BTreeMap<_, _>>()?);
                }
            }
        }
        Ok(ErrorInfo {
            reason: reason__.unwrap_or_default(),
            domain: domain__.unwrap_or_default(),
            metadata: metadata__.unwrap_or_default(),
        })
    }
}

[siennathesane]

Shouldn't CI have run after you approved it?

[neoeinstein]

Not sure why this never built, but I've now merged this in, and it should go out with the updates to prost, pbjson, and tonic.