feat: change ConfigLayerName into a disjoint union rather than a simple enum

[bolinfest]

This attempts to tighten up the types related to "config layers." Currently, ConfigLayerEntry is defined as follows:

codex/codex-rs/core/src/config_loader/state.rs

Lines 19 to 25 in bef36f4

	#[derive(Debug, Clone)]
	pub struct ConfigLayerEntry {
	pub name: ConfigLayerName,
	pub source: PathBuf,
	pub config: TomlValue,
	pub version: String,
	}

but the source field is a bit of a lie, as:

• for ConfigLayerName::Mdm, it is "com.openai.codex/config_toml_base64"
• for ConfigLayerName::SessionFlags, it is "--config"
• for ConfigLayerName::User, it is "config.toml" (just the file name, not the path to the config.toml on disk that was read)
• for ConfigLayerName::System, it seems like it is usually /etc/codex/managed_config.toml in practice, though on Windows, it is %CODEX_HOME%/managed_config.toml:

https://github.com/openai/codex/blob/bef36f4ae765f471d7cd69372fcf1b92c8f0367a/codex-rs/core/src/config_loader/layer_io.rs#L84-L101

All that is to say, in three out of the four ConfigLayerName, source is a PathBuf that is not an absolute path (or even a true path).

This PR tries to uplevel things by eliminating source from ConfigLayerEntry and turning ConfigLayerName into a disjoint union named ConfigLayerSource that has the appropriate metadata for each variant, favoring the use of AbsolutePathBuf where appropriate:

pub enum ConfigLayerSource {
    /// Managed preferences layer delivered by MDM (macOS only).
    #[serde(rename_all = "camelCase")]
    #[ts(rename_all = "camelCase")]
    Mdm { domain: String, key: String },
    /// Managed config layer from a file (usually `managed_config.toml`).
    #[serde(rename_all = "camelCase")]
    #[ts(rename_all = "camelCase")]
    System { file: AbsolutePathBuf },
    /// Session-layer overrides supplied via `-c`/`--config`.
    SessionFlags,
    /// User config layer from a file (usually `config.toml`).
    #[serde(rename_all = "camelCase")]
    #[ts(rename_all = "camelCase")]
    User { file: AbsolutePathBuf },
}

[gt-oai]

override_keys on the SessionFlags variant doesn't feel right to me

Do you need it at all? Are the keys (and values) already in the ConfigLayerEntry.config?

ConfigLayerName now feels like it should be ConfigLayerLocation or something.

ConfigSource or ConfigLayerSource sound reasonable to me, as the source field is the one you've removed and whose information is now in the union.

[owenlin0]

+1 to ConfigLayerSource

[jif-oai]

I like it