alef 0.67.5

//! The single place that answers "how does serde put this enum on the JSON wire?".
//!
//! serde has exactly four enum representations and the choice between them is made by the
//! `tag`/`content`/`untagged` container attributes alone — never by the shape of the variants.
//! Backends that re-derive the answer drift apart from each other and from Rust: the Swift
//! trait-bridge result encoder assumed serde's *external* default for every enum, so an
//! adjacently tagged enum went out as the bare string `"Variant"` and Rust rejected every
//! callback with `invalid type: string "...", expected adjacently tagged enum ...`, while Go
//! (which did consult `serde_content`) stayed correct.
//!
//! Every backend that emits or parses the JSON form of an IR enum must classify it through
//! [`serde_enum_repr`] so a future edit cannot reintroduce that divergence.

use crate::core::ir::EnumDef;

/// serde's four enum representations, carrying the key names each one needs.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum SerdeEnumRepr {
    /// serde's default. Unit variants are the bare string `"Variant"`; data variants are
    /// `{"Variant": payload}`.
    External,
    /// `#[serde(tag = "t")]`. The variant name is a field of the payload object:
    /// `{"t":"variant", ...payload fields}`.
    Internal { tag: String },
    /// `#[serde(tag = "t", content = "c")]`. The payload sits under its own key:
    /// `{"t":"variant","c":payload}`, and unit variants are just `{"t":"variant"}`.
    Adjacent { tag: String, content: String },
    /// `#[serde(untagged)]`. The payload alone, with no variant name anywhere; unit variants
    /// are `null`.
    Untagged,
}

impl SerdeEnumRepr {
    /// A stable lower-case discriminator suitable for a template context value.
    pub fn kind(&self) -> &'static str {
        match self {
            Self::External => "external",
            Self::Internal { .. } => "internal",
            Self::Adjacent { .. } => "adjacent",
            Self::Untagged => "untagged",
        }
    }

    /// The wire key holding the variant name, for the representations that have one.
    pub fn tag(&self) -> Option<&str> {
        match self {
            Self::Internal { tag } | Self::Adjacent { tag, .. } => Some(tag),
            Self::External | Self::Untagged => None,
        }
    }

    /// The wire key holding the payload — adjacent tagging only.
    pub fn content(&self) -> Option<&str> {
        match self {
            Self::Adjacent { content, .. } => Some(content),
            Self::External | Self::Internal { .. } | Self::Untagged => None,
        }
    }
}

/// Classify an IR enum's serde JSON representation.
///
/// `serde_untagged` wins over a stray tag because `#[serde(untagged)]` and `#[serde(tag = ...)]`
/// are mutually exclusive in serde, and the absence of `serde_tag` means *external*, not
/// untagged — the two are different wire forms and conflating them is exactly the bug this
/// module exists to prevent.
pub fn serde_enum_repr(enum_def: &EnumDef) -> SerdeEnumRepr {
    if enum_def.serde_untagged {
        return SerdeEnumRepr::Untagged;
    }
    match (enum_def.serde_tag.as_deref(), enum_def.serde_content.as_deref()) {
        (Some(tag), Some(content)) => SerdeEnumRepr::Adjacent {
            tag: tag.to_string(),
            content: content.to_string(),
        },
        (Some(tag), None) => SerdeEnumRepr::Internal { tag: tag.to_string() },
        (None, _) => SerdeEnumRepr::External,
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    fn enum_def(tag: Option<&str>, content: Option<&str>, untagged: bool) -> EnumDef {
        EnumDef {
            name: "Sample".to_string(),
            serde_tag: tag.map(str::to_string),
            serde_content: content.map(str::to_string),
            serde_untagged: untagged,
            ..EnumDef::default()
        }
    }

    #[test]
    fn should_be_external_when_no_serde_container_attribute_is_present() {
        let repr = serde_enum_repr(&enum_def(None, None, false));
        assert_eq!(repr, SerdeEnumRepr::External);
        assert_eq!(repr.kind(), "external");
        assert_eq!(repr.tag(), None);
        assert_eq!(repr.content(), None);
    }

    #[test]
    fn should_be_internal_when_only_tag_is_present() {
        let repr = serde_enum_repr(&enum_def(Some("type"), None, false));
        assert_eq!(
            repr,
            SerdeEnumRepr::Internal {
                tag: "type".to_string()
            }
        );
        assert_eq!(repr.kind(), "internal");
        assert_eq!(repr.tag(), Some("type"));
        assert_eq!(repr.content(), None);
    }

    #[test]
    fn should_be_adjacent_when_tag_and_content_are_both_present() {
        let repr = serde_enum_repr(&enum_def(Some("kind"), Some("payload"), false));
        assert_eq!(
            repr,
            SerdeEnumRepr::Adjacent {
                tag: "kind".to_string(),
                content: "payload".to_string()
            }
        );
        assert_eq!(repr.kind(), "adjacent");
        assert_eq!(repr.tag(), Some("kind"));
        assert_eq!(repr.content(), Some("payload"));
    }

    #[test]
    fn should_be_untagged_when_untagged_is_set() {
        let repr = serde_enum_repr(&enum_def(None, None, true));
        assert_eq!(repr, SerdeEnumRepr::Untagged);
        assert_eq!(repr.kind(), "untagged");
        assert_eq!(repr.tag(), None);
    }

    #[test]
    fn should_prefer_untagged_over_a_stray_tag() {
        assert_eq!(
            serde_enum_repr(&enum_def(Some("type"), None, true)),
            SerdeEnumRepr::Untagged
        );
    }

    #[test]
    fn should_ignore_content_without_a_tag() {
        assert_eq!(
            serde_enum_repr(&enum_def(None, Some("payload"), false)),
            SerdeEnumRepr::External
        );
    }
}