uniffi::custom_type! should have try_lower instead of just lower

[kriswuollett]

A possible common use case could be serializing a complex value instead of using Uniffi for everything like a structured record like a JSON type. For example I'd expect to be able to have a failure when lowering:

#[derive(Clone, Debug, serde::Deserialize, serde::Serialize)]
#[cfg_attr(feature = "uniffi", derive(uniffi::Record))]
pub struct Metadata(serde_json::Value);

uniffi::custom_type!(Metadata, String, {
    try_lower: |s| serde_json::to_string(&s).context("Metadata serialization failed"),
    try_lift: |s| serde_json::from_str(&s).context("Metadata deserialization failed"),
});

I assumed that was possible, but found out only lower that doesn't return an error is available. A generic structure like a serde_json::Value may be invalid if non-string keys are used:

/// Serialize the given data structure as a String of JSON.
///
/// # Errors
///
/// Serialization can fail if `T`'s implementation of `Serialize` decides to
/// fail, or if `T` contains a map with non-string keys.
#[inline]
pub fn to_string<T>(value: &T) -> Result<String>
where
    T: ?Sized + Serialize,
{
    let vec = tri!(to_vec(value));
    let string = unsafe {
        // We do not emit invalid UTF-8.
        String::from_utf8_unchecked(vec)
    };
    Ok(string)
}

[kriswuollett]

[dependencies]
async-trait = "0.1.85"
serde = { version = "1.0.217", features = ["derive"] }
serde_json = { version = "1.0.138" }
thiserror = "2.0.11"
uniffi = { git = "http://github.com/mozilla/uniffi-rs", rev = "9c83e1de164b4168bd33bd991d22184ee9f6c856", optional = true }

[bendk]

I agree with this. The current system is based on the general lift/lower system, where we assume that any Rust type can be lowered and the only error possibility is when lifting data from the other side of the FFI. This is usually true, but not always.

I feel like I remember some corner cases with callback interfaces that are related to this as well, but I'm coming up blank when trying to recall them.