Features of mf2

An application names mf2 once, with the features it needs; everything else is left out of the build. None is on by default. The applications mf2 init makes choose them for each kind of application; this page says what each one does.

A feature decides which functions a message may use, so the build script and mf2 check read the features cargo resolves for the crate, and a message that calls a function whose feature is off is the gated-function error.

The Leptos line

leptos

The Leptos layer renders with Leptos 0.9, the default line.

leptos-0-8

The same layer with Leptos 0.8, for an application that stays on it.

A Leptos mode needs one line, and both at once is a compile error that says what to write. The line goes on the mf2 dependency (features = ["leptos"]); the mode goes in the application’s own feature of the same name, beside Leptos’s.

A Leptos mode

Exactly one, in the application’s feature of the same name (ssr = ["leptos/ssr", "mf2/ssr"]). Each brings mf2::leptos: tr! in text, attributes and props, the catalog of the request or the page, the live switch and the page’s components; and each implies its host.

ssr

Rendered on the server. Implies host-std.

hydrate

The server-rendered page, hydrated in the browser. Implies host-web.

csr

Built and rendered in the browser alone. Implies host-web.

static-locale

No live switch: a language switch sets a cookie and loads the page again, and rendered text registers nothing to update. It suits islands.

mark-fallback-lang

Text borrowed from a fallback language (a message not translated yet) is wrapped in a <span lang> of its own language, and dir when its direction differs from the page’s, identically on the server and in the browser (WCAG 2.2’s Language of Parts). Only a borrowed message in the page’s text is wrapped, so a page with no missing translation is unchanged; an attribute or a string cannot carry a lang and stays unmarked.

A server

axum

mf2::axum, with or without Leptos: the reader’s language chosen from the request, the catalogs served from the server binary under /i18n/ (precompressed), and the generated Locale as an extractor, with Locale::format. Implies host-std.

A native application

native

mf2::native, for a command-line tool or a terminal UI: one corpus’s catalogs embedded in the executable or shipped beside it (checked against the content hash in their names), installed once for the process, in the system’s language and time zone. A description’s Display, to_string() and to_cow() then show its text. Implies host-std.

ratatui

mf2::ratatui: a message as Ratatui Text or Line, its markup ({#name}…{/name}) as styles the application maps by name. It adds ratatui-core alone, whose types ratatui re-exports. Implies native.

clap

The generated Locale gets a clap value parser, so that --lang is matched by the same rules as the system’s language (fr_CA.UTF-8 is French), and --help lists the languages.

native, ratatui and axum are never in a browser build: beside hydrate or csr, each is a compile error when compiling for wasm32. On the host they compile together, as a workspace’s cargo check unifies features.

Functions

fn-number

Numbers in the reader’s language: its decimal and grouping separators, digits and numbering system for :number, :integer and unannotated numbers; and :percent, :currency and :unit. Without it, a number is written with neutral symbols (1234.5), and the build says so (neutral-numbers).

fn-datetime

:datetime, :date and :time, and date and time values without a function. On its own it formats with a neutral stand-in; a date backend (below) gives it the reader’s language. With a Leptos mode, dates are shown in the reader’s time zone: the browser reports its zone, a page the server rendered in another zone is corrected after hydrating, and the mf2_tz cookie lets the server render the next page in it. Off, none of this is in the client.

datetime-icu

Dates formatted by ICU4X on the server and in the browser, with the data each language needs in its catalog (icu.blob). Implies fn-datetime.

datetime-intl

Dates formatted by the browser’s Intl.DateTimeFormat in a browser build, and by ICU4X with its compiled data everywhere else. Implies fn-datetime.

intl

In a browser build, numbers are formatted and plurals chosen by the browser’s Intl.NumberFormat and Intl.PluralRules (which needs a browser with Intl.NumberFormat v3), instead of Rust code in the wasm. Every other build keeps the Rust code.

Hosts and tools

A mode implies its host; an application rarely names one.

host-std

Formatting on a native target: servers, tests, wasm32-wasip1.

host-web

Formatting in the browser.

compile

mf2::compile_str: an ad-hoc message compiled into a one-message catalog, for a server or a test. Never in a client.

What a browser build pays for text

A description (what tr! returns) turned into a String in a browser build costs code in the wasm, and the ways differ: .to_string() is the leanest; format!("{}", …) adds a few dozen bytes; {:?} adds about 1 KB, and so does an unwrap() or an assert_eq! that involves a description, since each reaches its Debug. In a view, tr! renders without any of them.