Trait I18nPlugin 

pub trait I18nPlugin: Send + Sync {
    // Required methods
    fn name(&self) -> &str;
    fn translate(
        &self,
        key: &str,
        locale: &str,
        vars: &TransVars,
    ) -> Result<String, I18nError>;
    fn supported_locales(&self) -> Vec<String>;
    fn fallback_locale(&self) -> String;

    // Provided methods
    fn has_locale(&self, locale: &str) -> bool { ... }
    fn negotiate(&self, requested: &str) -> String { ... }
}

A plugin that translates keys into localized strings.

Host functions i18n.t(key, vars) and i18n.locales() call through to the currently registered implementation. The host layer is responsible for negotiating the request’s locale from headers / session / URL before calling translate.

Required Methods

fn name(&self) -> &str

Unique identifier, e.g. "static", "fluent", "icu".

fn translate( &self, key: &str, locale: &str, vars: &TransVars, ) -> Result<String, I18nError>

Translate key into locale, interpolating vars.

Returns Ok(rendered) in the overwhelming majority of cases, including when the key is unknown — backends render missing keys as a safe marker (the key itself, or "{key}") rather than returning an error, because a missing translation must never take down a UI.

Returns Err(I18nError) only for genuine backend failures: a locale that was promised but failed to load, a format error, a backend in a transiently unavailable state.

fn supported_locales(&self) -> Vec<String>

All locales this plugin can serve, as BCP 47 tags ("en", "en-US", "fr-CA", "zh-Hant-TW"). The order is display order, not preference order; callers negotiate via negotiate.

fn fallback_locale(&self) -> String

The locale to use when the caller asks for one that is not in supported_locales() and no partial match negotiated. A BCP 47 tag; MUST appear in supported_locales().

Provided Methods

fn has_locale(&self, locale: &str) -> bool

Whether the plugin recognises the given locale tag exactly (case-insensitive). The default implementation linearly scans supported_locales(); backends that maintain a fast lookup structure override this.

fn negotiate(&self, requested: &str) -> String

Negotiate a requested locale down to a locale the plugin can actually serve, per BCP 47 Lookup (RFC 4647 §3.4).

The default implementation implements the standard walk: given "fr-CA", try "fr-CA", then "fr", then fallback_locale(). Backends are free to override with a richer matcher (Fluent, for example, has its own negotiation that also considers script subtags).

Implementors