makeover_webview/

form.rs

//! Phase B, the forms half: [`makeover_layout::Field`] rendered to HTML.
//!
//! # Why this emits strings
//!
//! Both webview apps build their markup as strings and hand it to `innerHTML`:
//! goingson's `renderFormField` returns a template literal that fifteen call
//! sites interpolate into larger literals, and Balanced Breakfast's builds
//! nodes but appends them into the same string-built forms. Returning nodes
//! would rewrite the surrounding templates as well, which makes it a migration
//! rather than an adoption. So: strings, and the escaping comes with them.
//!
//! # Why one escaper is enough here
//!
//! goingson carries four escapers and 543 call sites that must pick between
//! them, because `escapeHtml` is built on `textContent` serialization and
//! **`textContent` refuses to encode `"`**. That is what makes it unsound in an
//! attribute, and it is the whole reason the choice exists. Its `escape.js`
//! records the finding as the CHRONIC-XSS seal, and its test suite has a gate
//! keeping the unsafe one off the namespace.
//!
//! [`escape`] here is not built on that, so it encodes the quote along with
//! everything else, which makes one function sound in both sinks. The four-way
//! choice does not move into Rust: it disappears. Nothing in this module hands
//! an unescaped value to the output except through [`Markup`], which a caller
//! has to name.
//!
//! # What the description does not carry
//!
//! One thing: the **current value**, which arrives in [`Filling`].
//!
//! It used to be three. Writing this emitter is what found them, and the other
//! two turned out not to be renderer state at all — the placeholder is
//! user-facing text that sits with `label` and `hint`, and a select's options
//! are needed by every renderer, which is how each of them ends up inventing a
//! near-miss of the same struct. Both moved down into `makeover-layout` 0.8.0,
//! `Choice` included, and this crate reads them off [`Field`] now.
//!
//! The value stays, and it is not a leftover. A webview reads it back out of
//! the DOM, an immediate-mode renderer writes through a `&mut`, and a terminal
//! keeps an edit buffer; a description carrying it would have to carry a way to
//! write it back, at which point it is a form model.

use crate::{Emit, class};
use makeover_layout::{Choice, Field, FieldKind};
use std::fmt::Write as _;

/// A string that is already markup, and is emitted without escaping.
///
/// The one hole in the escaping, and it has to be named to be used. goingson
/// has two live callers that need it, both passing a recurrence-config block
/// built elsewhere, and both would otherwise have their markup rendered as
/// visible angle brackets. A caller constructing this is stating that the
/// contents are trusted; nothing here can check that for them.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct Markup<'a>(pub &'a str);

/// What the field currently holds.
///
/// An enum rather than a bag of optional fields, on the same reasoning
/// [`makeover_layout::Depth`] is one: a checkbox holding a string is unsayable
/// here, where a struct would let it be said and then have to cope.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
pub enum Value<'a> {
    /// Nothing yet.
    #[default]
    Absent,
    /// The value of anything that takes typed text, a select included: what a
    /// select holds is the `value` of one of [`Field::options`]'s
    /// [`Choice`]s.
    ///
    /// It carried the options too until makeover-layout 0.8.0 moved them onto
    /// the field, which collapsed a `Chosen { options, value }` variant into
    /// this one. `makeover-immediate` arrived at the same single-variant shape
    /// on its own, from the other direction.
    Text(&'a str),
    /// A checkbox, on or off.
    On(bool),
}

impl<'a> Value<'a> {
    /// The value as text, for the kinds that submit one.
    const fn as_text(&self) -> &'a str {
        match self {
            Self::Text(text) => text,
            Self::Absent | Self::On(_) => "",
        }
    }
}

/// Everything about the field that the description does not carry.
#[derive(Debug, Clone, Copy, Default)]
pub struct Filling<'a> {
    /// What the field holds now.
    pub value: Value<'a>,
    /// Markup appended inside the group, after the hint. Not escaped.
    pub trailing: Option<Markup<'a>>,
    /// Scopes the `id` attributes to one instance of the form.
    ///
    /// The field's `name` is what the value submits under and is the same
    /// wherever the form appears; its `id` has to be unique in the document,
    /// and those two facts stop agreeing the moment a form appears twice.
    /// goingson hits this directly: its new-task and edit-task modals are the
    /// same field set, so it prefixes `form-modal-task-new` or `-edit` to keep
    /// `label for` and `aria-describedby` pointing at the right control.
    ///
    /// Applies to `id`, `for` and the `-hint` / `-error` associations. Never to
    /// `name`, which would change what the form submits.
    pub id_prefix: Option<&'a str>,
}

impl<'a> Filling<'a> {
    /// A filling that carries a value and nothing else.
    #[must_use]
    pub const fn of(value: Value<'a>) -> Self {
        Self {
            value,
            trailing: None,
            id_prefix: None,
        }
    }

    /// The document-unique id for a field of this name.
    fn id_for(&self, name: &str) -> String {
        match self.id_prefix {
            Some(prefix) => format!("{}-{}", escape(prefix), escape(name)),
            None => escape(name),
        }
    }
}

/// Encode the five characters that let a value stop being a value.
///
/// Sound in element text and in a double-quoted attribute alike, which is the
/// property `textContent`-based escaping cannot have. Both sinks are covered by
/// one function so that no call site has to choose, here or downstream.
#[must_use]
pub fn escape(text: &str) -> String {
    let mut out = String::with_capacity(text.len());
    for ch in text.chars() {
        match ch {
            '&' => out.push_str("&amp;"),
            '<' => out.push_str("&lt;"),
            '>' => out.push_str("&gt;"),
            '"' => out.push_str("&quot;"),
            '\'' => out.push_str("&#39;"),
            other => out.push(other),
        }
    }
    out
}

/// The `type` an input takes for a kind.
///
/// [`FieldKind::Secret`] is `password`, which both apps already map by hand.
const fn input_type(kind: FieldKind) -> &'static str {
    match kind {
        FieldKind::Secret => "password",
        FieldKind::Number => "number",
        FieldKind::Checkbox => "checkbox",
        FieldKind::File => "file",
        FieldKind::Hidden => "hidden",
        // Not decoration. Each of these changes the keyboard a touch device
        // offers and turns on the platform's own validation, which is why the
        // description names them apart from text rather than letting the app
        // pass an HTML type through.
        FieldKind::Email => "email",
        FieldKind::Url => "url",
        FieldKind::Tel => "tel",
        FieldKind::Radio => "radio",
        // Select and Textarea are not inputs at all; they never reach here.
        // Radio is one, but it is emitted once per option by `radio_html` and
        // so does not reach here either.
        FieldKind::Text | FieldKind::Select | FieldKind::Textarea => "text",
        // A kind added to the description since this renderer was built. Text
        // accepts any value the others would, so it degrades rather than
        // dropping the field.
        _ => "text",
    }
}

/// The attributes every visible control carries, error state included.
///
/// `aria-invalid` is the whole reason the error state is readable at all: the
/// generated stylesheet keys the danger ring on `[aria-invalid="true"]` rather
/// than on a class, so a control rendered already-invalid without it is styled
/// as if nothing were wrong. goingson's runtime validation path sets the
/// attribute and its initial render does not, which is exactly the drift one
/// emitter removes.
/// `id` and `name` arrive separately because they are not the same fact. The
/// name is what submits and is fixed by the description; the id has to be
/// unique in the document and so carries [`Filling::id_prefix`] when a form
/// appears more than once.
fn control_attributes(field: &Field<'_>, id: &str, name: &str) -> String {
    let mut attrs = format!(" id=\"{id}\" name=\"{}\"", escape(name));
    if field.required {
        attrs.push_str(" required");
    }
    // makeover-layout 0.11.0's constraints. The description carries the rule and
    // this emits the browser's idiom for it, which is the model `required` has
    // been using since before the crate wrote down that it carried none.
    // Enforcement is still whoever validated's, and arrives back as `error`.
    if let Some(limit) = field.max_length {
        let _ = write!(attrs, " maxlength=\"{limit}\"");
    }
    if let Some(min) = field.min {
        let _ = write!(attrs, " min=\"{}\"", escape(min));
    }
    if let Some(max) = field.max {
        let _ = write!(attrs, " max=\"{}\"", escape(max));
    }
    if field.invalid() {
        attrs.push_str(" aria-invalid=\"true\"");
    }

    attrs.push_str(&described_by(field, id));
    attrs
}

/// The `aria-describedby` naming whatever of the hint and the error exist.
///
/// Both associations, in the order they are useful: the standing help, then
/// what is currently wrong. goingson's runtime path points describedby at the
/// error alone and drops the hint association it never made in the first place;
/// naming both here means the hint survives an error appearing.
///
/// Its own function because a radio group carries it on the group rather than
/// on a control, and one reading of "what describes this field" is the point.
fn described_by(field: &Field<'_>, id: &str) -> String {
    let mut described = Vec::new();
    if field.hint.is_some() {
        described.push(format!("{id}-hint"));
    }
    if field.error.is_some() {
        described.push(format!("{id}-error"));
    }
    if described.is_empty() {
        return String::new();
    }
    format!(" aria-describedby=\"{}\"", described.join(" "))
}

/// Whether the field's control is a set of elements rather than one.
///
/// A DOM concern rather than a description one, which is why it is decided here
/// and not in `makeover-layout`: `for` and `id` are an HTML association and
/// egui has no counterpart to get wrong. A `<label for>` aimed at a radio group
/// points at nothing, because no single element carries the group's id, so the
/// association has to invert — the label takes an id and the group names itself
/// with `aria-labelledby`.
const fn is_group_control(kind: FieldKind) -> bool {
    matches!(kind, FieldKind::Radio)
}

/// A radio group: the options as sibling inputs sharing one `name`.
///
/// The group carries the error state and the descriptions, and the inputs carry
/// what submits. That split is [`Field::invalid`]'s reasoning applied one level
/// down: marking a single input invalid would say the wrong thing, since what
/// is wrong is the answer to the question and not one of the alternatives.
///
/// Ids are numbered rather than built from the option values, which can hold
/// anything a `&str` can — spaces and quotes included — and would otherwise
/// have to be slugged into something unique by a rule this crate would then own.
///
/// `required` lands on every input, which is how HTML says a group is
/// compulsory: the constraint is satisfied when any one of them is checked.
fn radio_html(field: &Field<'_>, filling: &Filling<'_>, opts: &Emit) -> String {
    let id = filling.id_for(field.name);
    let value = filling.value.as_text();
    let name = escape(field.name);

    let mut html = format!(
        "<div class=\"{}\" role=\"radiogroup\"",
        class("form-radio-group", opts)
    );
    let _ = write!(html, " aria-labelledby=\"{id}-label\"");
    if field.invalid() {
        html.push_str(" aria-invalid=\"true\"");
    }
    html.push_str(&described_by(field, &id));
    html.push('>');

    // A group described with no options emits an empty group, for the reason
    // `Field::options` gives: an app whose option list has not loaded has
    // exactly that, and an empty group says so on screen rather than in a log.
    for (index, opt) in field.options.iter().enumerate() {
        let checked = if opt.value == value { " checked" } else { "" };
        let required = if field.required { " required" } else { "" };
        let _ = write!(
            html,
            "<label class=\"{}\"><input type=\"radio\" id=\"{id}-{index}\" name=\"{name}\" \
             value=\"{}\"{checked}{required}><span>{}</span></label>",
            class("form-radio-label", opts),
            escape(opt.value),
            escape(opt.label)
        );
    }

    html.push_str("</div>");
    html
}

/// The options of a select, with an unmatched current value carried as its own.
///
/// A select handed a value no option carries renders with nothing selected, the
/// browser falls back to the first option, and the next save writes a value
/// nobody chose. goingson hit exactly that with a backup-retention default of
/// 10 against a 1/3/7/14/0 list, and grew this stray-option fix locally; it is
/// here so the second app gets it without hitting the bug first.
fn options_html(options: &[Choice<'_>], value: &str) -> String {
    let mut html = String::new();
    if !value.is_empty() && !options.iter().any(|opt| opt.value == value) {
        let escaped = escape(value);
        let _ = write!(
            html,
            "<option value=\"{escaped}\" selected data-unmatched=\"true\">{escaped}</option>"
        );
    }
    for opt in options {
        let selected = if opt.value == value { " selected" } else { "" };
        let _ = write!(
            html,
            "<option value=\"{}\"{selected}>{}</option>",
            escape(opt.value),
            escape(opt.label)
        );
    }
    html
}

/// The control itself, without its label, hint or error.
fn control_html(field: &Field<'_>, filling: &Filling<'_>, opts: &Emit) -> String {
    let id = filling.id_for(field.name);
    let attrs = control_attributes(field, &id, field.name);
    let field_class = class("field", opts);
    let placeholder = field.placeholder.map_or_else(String::new, |text| {
        format!(" placeholder=\"{}\"", escape(text))
    });

    match field.kind {
        FieldKind::Radio => radio_html(field, filling, opts),
        FieldKind::Textarea => format!(
            "<textarea class=\"{field_class}\"{attrs}{placeholder}>{}</textarea>",
            escape(filling.value.as_text())
        ),
        FieldKind::Select => {
            // A select described with no options emits an empty select, which
            // says so on screen rather than in a log. That is the description's
            // own position on `Field::options`, not a fallback invented here.
            let options = options_html(field.options, filling.value.as_text());
            format!("<select class=\"{field_class}\"{attrs}>{options}</select>")
        }
        FieldKind::Checkbox => {
            let checked = if matches!(filling.value, Value::On(true)) {
                " checked"
            } else {
                ""
            };
            format!(
                "<label class=\"{}\"><input type=\"checkbox\"{attrs}{checked}><span>{}</span></label>",
                class("form-checkbox-label", opts),
                escape(field.label)
            )
        }
        // A secret never carries its value into the markup. `FieldKind::secret`
        // is documented as a value that must not be round-tripped through
        // anything that might persist it, and the DOM is such a thing: it is
        // read by every extension on the page and is the first thing a crash
        // reporter serialises. Neither app pre-fills one today, so this costs
        // nothing and closes the door before something does.
        FieldKind::Secret => {
            format!("<input type=\"password\" class=\"{field_class}\"{attrs}{placeholder}>")
        }
        // A file input carries no value, and this is the browser's rule rather
        // than a preference: setting one from markup is refused, because a page
        // that could preselect a path could read a file the user never offered.
        // Nothing upstream needs to know, which is why the exception is here.
        FieldKind::File => {
            format!("<input type=\"file\" class=\"{field_class}\"{attrs}>")
        }
        kind => format!(
            "<input type=\"{}\" class=\"{field_class}\"{attrs}{placeholder} value=\"{}\">",
            input_type(kind),
            escape(filling.value.as_text())
        ),
    }
}

/// One field, as the group the app drops into its form.
///
/// The shape is goingson's, down to the class names, so adoption there deletes
/// `renderFormField` rather than restyling anything. That is also why the class
/// names are not emitted by [`crate::stylesheet`]: `.form-group`, `.form-label`,
/// `.form-hint` and `.form-error` are the apps' own, and phase A deliberately
/// emits only what it can generate from the description. Whether they should
/// move into the description is the next question this raises, not one it
/// answers.
///
/// A [`FieldKind::Hidden`] field is the input alone: no group, no label, and
/// nothing drawn, which is what [`FieldKind::visible`] means.
///
/// The error marks the group as well as the control. That is
/// [`Field::invalid`]'s own reasoning: a renderer with no descendant selectors
/// cannot find the group from the message, so the group has to be told.
///
/// ```
/// use makeover_layout::{Field, FieldKind};
/// use makeover_webview::{Emit, form::{Filling, Value, field_html}};
///
/// let field = Field::new(FieldKind::Text, "title", "Title");
/// let html = field_html(&field, &Filling::of(Value::Text("Ship it")), &Emit::default());
///
/// assert!(html.contains(r#"<label class="form-label" for="title">Title</label>"#));
/// assert!(html.contains(r#"value="Ship it""#));
/// ```
#[must_use]
pub fn field_html(field: &Field<'_>, filling: &Filling<'_>, opts: &Emit) -> String {
    let id = filling.id_for(field.name);

    if !field.kind.visible() {
        // Name only, no id: a hidden field is never pointed at by a label or a
        // description, so the one attribute it needs is the one that submits.
        return format!(
            "<input type=\"hidden\" name=\"{}\" value=\"{}\">",
            escape(field.name),
            escape(filling.value.as_text())
        );
    }

    let mut html = format!("<div class=\"{}", class("form-group", opts));
    if field.invalid() {
        html.push_str(" has-error");
    }
    if field.extended {
        // The disclosure that hides these is a property of the form, not of the
        // field, so the field is marked and the app opens or closes the group.
        html.push_str("\" data-extended=\"true");
    }
    html.push_str("\">");

    // A checkbox labels itself, on the right of the box. Both apps special-case
    // this inline today, which is the tell that it belongs in the description;
    // `FieldKind::labels_itself` is where it went.
    if !field.kind.labels_itself() {
        // A group control is named *by* its label rather than pointing at it,
        // so the two carry opposite halves of the association. See
        // `is_group_control`.
        let association = if is_group_control(field.kind) {
            format!(" id=\"{id}-label\"")
        } else {
            format!(" for=\"{id}\"")
        };
        let _ = write!(
            html,
            "<label class=\"{}\"{association}>{}</label>",
            class("form-label", opts),
            escape(field.label)
        );
    }

    html.push_str(&control_html(field, filling, opts));

    if let Some(hint) = field.hint {
        let _ = write!(
            html,
            "<div class=\"{}\" id=\"{id}-hint\">{}</div>",
            class("form-hint", opts),
            escape(hint)
        );
    }
    if let Some(Markup(markup)) = filling.trailing {
        html.push_str(markup);
    }
    if let Some(error) = field.error {
        let _ = write!(
            html,
            "<div class=\"{} visible\" id=\"{id}-error\" role=\"alert\">{}</div>",
            class("form-error", opts),
            escape(error)
        );
    }

    html.push_str("</div>");
    html
}

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

    fn field(kind: FieldKind) -> Field<'static> {
        Field::new(kind, "title", "Title")
    }

    #[test]
    fn a_value_cannot_break_out_of_the_attribute_it_sits_in() {
        // The payload from goingson's own CHRONIC-XSS regression test.
        let filling = Filling::of(Value::Text("x\" onfocus=alert(1) autofocus=\""));
        let html = field_html(&field(FieldKind::Text), &filling, &Emit::default());
        // The payload survives as text, which is the point: it is inert
        // because the quote that would have closed the attribute is encoded,
        // not because the words were filtered.
        assert!(!html.contains("\" onfocus"), "{html}");
        assert!(
            html.contains("value=\"x&quot; onfocus=alert(1) autofocus=&quot;\""),
            "{html}"
        );
    }

    #[test]
    fn a_label_cannot_open_a_tag() {
        let mut f = field(FieldKind::Text);
        f.label = "<script>alert(1)</script>";
        let html = field_html(&f, &Filling::default(), &Emit::default());
        assert!(!html.contains("<script>"), "{html}");
        assert!(html.contains("&lt;script&gt;"), "{html}");
    }

    #[test]
    fn every_escaped_sink_is_covered_by_the_one_escaper() {
        assert_eq!(escape("&<>\"'"), "&amp;&lt;&gt;&quot;&#39;");
        // The character `textContent` serialization leaves alone, which is why
        // the app needs two escapers and this needs one.
        assert!(escape("\"").contains("&quot;"));
    }

    #[test]
    fn markup_is_the_only_way_past_the_escaping() {
        let filling = Filling {
            trailing: Some(Markup("<div class=\"recurrence-config\"></div>")),
            ..Filling::default()
        };
        let html = field_html(&field(FieldKind::Text), &filling, &Emit::default());
        assert!(
            html.contains("<div class=\"recurrence-config\"></div>"),
            "{html}"
        );
    }

    #[test]
    fn an_invalid_field_carries_the_attribute_its_own_stylesheet_keys_on() {
        let mut f = field(FieldKind::Text);
        f.error = Some("Required");
        let opts = Emit::default();
        let html = field_html(&f, &Filling::default(), &opts);
        assert!(html.contains("aria-invalid=\"true\""), "{html}");
        // The selector the CSS side emits for exactly this state.
        assert!(crate::stylesheet(&opts).contains("[aria-invalid=\"true\"]"));
        // And the group is marked too, which a renderer without descendant
        // selectors depends on.
        assert!(html.contains("has-error"), "{html}");
    }

    #[test]
    fn a_valid_field_claims_nothing_about_being_invalid() {
        let html = field_html(
            &field(FieldKind::Text),
            &Filling::default(),
            &Emit::default(),
        );
        assert!(!html.contains("aria-invalid"), "{html}");
        assert!(!html.contains("has-error"), "{html}");
    }

    #[test]
    fn the_hint_survives_an_error_arriving() {
        let mut f = field(FieldKind::Text);
        f.hint = Some("Keep it short");
        f.error = Some("Required");
        let html = field_html(&f, &Filling::default(), &Emit::default());
        assert!(
            html.contains("aria-describedby=\"title-hint title-error\""),
            "{html}"
        );
    }

    #[test]
    fn a_secret_never_carries_its_value_into_the_markup() {
        let filling = Filling::of(Value::Text("hunter2"));
        let html = field_html(&field(FieldKind::Secret), &filling, &Emit::default());
        assert!(!html.contains("hunter2"), "{html}");
        assert!(html.contains("type=\"password\""), "{html}");
    }

    #[test]
    fn a_hidden_field_is_the_input_and_nothing_else() {
        let filling = Filling::of(Value::Text("42"));
        let html = field_html(&field(FieldKind::Hidden), &filling, &Emit::default());
        assert_eq!(html, "<input type=\"hidden\" name=\"title\" value=\"42\">");
    }

    #[test]
    fn a_checkbox_labels_itself_and_takes_no_separate_label() {
        let html = field_html(
            &field(FieldKind::Checkbox),
            &Filling::of(Value::On(true)),
            &Emit::default(),
        );
        assert!(!html.contains("form-label"), "{html}");
        assert!(html.contains("checked"), "{html}");
        assert!(html.contains("<span>Title</span>"), "{html}");
    }

    #[test]
    fn a_select_keeps_a_value_no_option_carries() {
        let options = [Choice::plain("1"), Choice::plain("3"), Choice::plain("7")];
        let f = Field::select("title", "Title", &options);
        let html = field_html(&f, &Filling::of(Value::Text("10")), &Emit::default());
        assert!(html.contains("data-unmatched=\"true\""), "{html}");
        // Selected, so the next save round-trips it rather than writing the
        // first option over the top of it.
        assert!(html.contains("<option value=\"10\" selected"), "{html}");
    }

    #[test]
    fn a_select_with_no_options_emits_an_empty_select() {
        // The description says a select with no options is sayable, because an
        // app whose option list has not loaded has exactly that. Emitting the
        // empty select reports it on screen rather than in a log.
        let f = Field::select("title", "Title", &[]);
        let html = field_html(&f, &Filling::default(), &Emit::default());
        assert!(html.contains("<select"), "{html}");
        assert!(!html.contains("<option"), "{html}");
    }

    #[test]
    fn a_radio_group_is_named_by_its_label_instead_of_pointing_at_it() {
        // The association inverts, and getting it wrong is silent: a
        // `<label for>` aimed at a group points at no element, so the group
        // simply has no accessible name and nothing reports that.
        let options = [Choice::plain("copy"), Choice::plain("reference")];
        let f = Field::radio("storage", "Storage style", &options);
        let html = field_html(&f, &Filling::of(Value::Text("copy")), &Emit::default());

        assert!(html.contains("id=\"storage-label\""), "{html}");
        assert!(!html.contains("for=\"storage\""), "{html}");
        assert!(html.contains("role=\"radiogroup\""), "{html}");
        assert!(html.contains("aria-labelledby=\"storage-label\""), "{html}");
    }

    #[test]
    fn every_option_shares_the_name_and_only_the_current_one_is_checked() {
        // One `name` is what makes them one answer rather than three; distinct
        // ids are what keep each `<label>` wrapping its own input.
        let options = [
            Choice::plain("copy"),
            Choice::plain("reference"),
            Choice::plain("link"),
        ];
        let f = Field::radio("storage", "Storage style", &options);
        let html = field_html(&f, &Filling::of(Value::Text("reference")), &Emit::default());

        assert_eq!(html.matches("name=\"storage\"").count(), 3, "{html}");
        assert_eq!(html.matches(" checked").count(), 1, "{html}");
        assert!(
            html.contains("value=\"reference\" checked"),
            "the checked one is the one held: {html}"
        );
        for index in 0..3 {
            assert!(html.contains(&format!("id=\"storage-{index}\"")), "{html}");
        }
    }

    #[test]
    fn a_radio_group_carries_the_error_rather_than_any_one_option() {
        // What is wrong is the answer, not one of the alternatives, so marking
        // a single input invalid would say something false. Same reading
        // `Field::invalid` gives one level up.
        let options = [Choice::plain("copy"), Choice::plain("reference")];
        let f = Field {
            error: Some("Pick one."),
            hint: Some("Cannot be changed later."),
            ..Field::radio("storage", "Storage style", &options)
        };
        let html = field_html(&f, &Filling::default(), &Emit::default());

        assert_eq!(html.matches("aria-invalid=\"true\"").count(), 1, "{html}");
        assert!(
            html.contains("aria-describedby=\"storage-hint storage-error\""),
            "{html}"
        );
        // The group is the element that carries them, so they land before the
        // first option rather than on it.
        let group = html.find("role=\"radiogroup\"").expect("group");
        let first = html.find("type=\"radio\"").expect("an option");
        assert!(group < first, "{html}");
    }

    #[test]
    fn a_compulsory_radio_group_marks_every_option() {
        // How HTML says a group is compulsory: the constraint reads as
        // satisfied when any one of them is checked.
        let options = [Choice::plain("copy"), Choice::plain("reference")];
        let f = Field {
            required: true,
            ..Field::radio("storage", "Storage style", &options)
        };
        let html = field_html(&f, &Filling::default(), &Emit::default());
        assert_eq!(html.matches(" required").count(), 2, "{html}");
    }

    #[test]
    fn a_radio_option_cannot_break_out_of_its_attribute() {
        // Values are `&str` and carry whatever the app put in them. The ids are
        // numbered rather than derived from the value for the same reason.
        let hostile = [Choice {
            value: "x\" onclick=alert(1) data-x=\"",
            label: "<script>alert(1)</script>",
        }];
        let f = Field::radio("storage", "Storage style", &hostile);
        let html = field_html(&f, &Filling::default(), &Emit::default());

        // The payload survives as text; what must not survive is the quote
        // that would end the attribute and let the rest of it become markup.
        assert!(html.contains("value=\"x&quot; onclick=alert(1)"), "{html}");
        assert!(!html.contains("<script>"), "{html}");
        assert!(html.contains("id=\"storage-0\""), "{html}");
    }

    #[test]
    fn a_radio_group_with_no_options_emits_an_empty_group() {
        // Same position the select takes, and the description's own.
        let f = Field::radio("storage", "Storage style", &[]);
        let html = field_html(&f, &Filling::default(), &Emit::default());
        assert!(html.contains("role=\"radiogroup\""), "{html}");
        assert!(!html.contains("type=\"radio\""), "{html}");
    }

    #[test]
    fn a_placeholder_comes_off_the_description_and_is_escaped() {
        // It arrived in `Filling` until makeover-layout 0.8.0 and was never
        // covered here; it is a value in an attribute like any other.
        let f = Field {
            placeholder: Some("x\" onfocus=alert(1) autofocus=\""),
            ..field(FieldKind::Text)
        };
        let html = field_html(&f, &Filling::default(), &Emit::default());
        assert!(html.contains("placeholder=\""), "{html}");
        assert!(!html.contains("\" onfocus"), "{html}");
    }

    #[test]
    fn a_select_marks_the_option_that_matches() {
        let options = [Choice::plain("1"), Choice::plain("3")];
        let f = Field::select("title", "Title", &options);
        let html = field_html(&f, &Filling::of(Value::Text("3")), &Emit::default());
        assert!(
            html.contains("<option value=\"3\" selected>3</option>"),
            "{html}"
        );
        assert!(html.contains("<option value=\"1\">1</option>"), "{html}");
        assert!(!html.contains("data-unmatched"), "{html}");
    }

    #[test]
    fn a_textarea_carries_its_value_as_text_and_not_as_an_attribute() {
        let filling = Filling::of(Value::Text("two\nlines"));
        let html = field_html(&field(FieldKind::Textarea), &filling, &Emit::default());
        assert!(html.contains(">two\nlines</textarea>"), "{html}");
    }

    #[test]
    fn the_class_prefix_reaches_the_markup_as_well_as_the_stylesheet() {
        let opts = Emit {
            class_prefix: "mk-",
            ..Emit::default()
        };
        let html = field_html(&field(FieldKind::Text), &Filling::default(), &opts);
        assert!(html.contains("class=\"mk-form-group\""), "{html}");
        assert!(html.contains("class=\"mk-field\""), "{html}");
    }

    #[test]
    fn an_extended_field_says_so_and_leaves_the_disclosure_to_the_form() {
        let mut f = field(FieldKind::Text);
        f.extended = true;
        let html = field_html(&f, &Filling::default(), &Emit::default());
        assert!(html.contains("data-extended=\"true\""), "{html}");
    }

    /// The prefix scopes the id and leaves the name alone. Prefixing the name
    /// too would change what the form submits, which is the failure this pair
    /// of assertions exists to catch rather than describe.
    #[test]
    fn the_id_prefix_scopes_the_id_and_never_the_name() {
        let mut f = field(FieldKind::Text);
        f.hint = Some("Keep it short");
        f.error = Some("Required");
        let filling = Filling {
            id_prefix: Some("form-modal-task-edit"),
            ..Filling::default()
        };
        let html = field_html(&f, &filling, &Emit::default());

        assert!(
            html.contains(r#"id="form-modal-task-edit-title""#),
            "{html}"
        );
        assert!(html.contains(r#"name="title""#), "{html}");
        assert!(
            !html.contains(r#"name="form-modal-task-edit-title""#),
            "{html}"
        );

        // The label and both associations follow the id, or they point at
        // nothing once the same form is on screen twice.
        assert!(
            html.contains(r#"for="form-modal-task-edit-title""#),
            "{html}"
        );
        assert!(
            html.contains(
                r#"aria-describedby="form-modal-task-edit-title-hint form-modal-task-edit-title-error""#
            ),
            "{html}"
        );
        assert!(
            html.contains(r#"id="form-modal-task-edit-title-hint""#),
            "{html}"
        );
    }

    #[test]
    fn a_hidden_field_submits_its_bare_name_under_a_prefix() {
        let filling = Filling {
            value: Value::Text("42"),
            id_prefix: Some("scoped"),
            ..Filling::default()
        };
        let html = field_html(&field(FieldKind::Hidden), &filling, &Emit::default());
        assert_eq!(html, r#"<input type="hidden" name="title" value="42">"#);
    }

    /// These three exist so a touch keyboard and the platform's validation
    /// arrive with the field. Emitting text for any of them is the regression
    /// the variants were added to prevent, so the type is asserted directly.
    #[test]
    fn a_constraint_becomes_the_browsers_own_attribute() {
        // makeover-layout 0.11.0's model: the description carries the rule and
        // each renderer emits its host's idiom for it. Enforcement is still
        // whoever validated's, and arrives back as `error`.
        let html = field_html(
            &Field {
                max_length: Some(100),
                min: Some("1"),
                max: Some("240"),
                required: true,
                ..Field::new(FieldKind::Number, "minutes", "Minutes")
            },
            &Filling::default(),
            &Emit::default(),
        );
        assert!(html.contains(r#"maxlength="100""#));
        assert!(html.contains(r#"min="1""#));
        assert!(html.contains(r#"max="240""#));
        assert!(html.contains(" required"));
    }

    #[test]
    fn a_bound_is_emitted_as_written_and_escaped_like_anything_else() {
        // The bound is text because it is only a number for some of the kinds
        // that take one; goingson's own sites are a duration and a datetime.
        let html = field_html(
            &Field {
                min: Some("2026-08-09T14:30"),
                ..Field::new(FieldKind::Text, "starts", "Starts")
            },
            &Filling::default(),
            &Emit::default(),
        );
        assert!(html.contains(r#"min="2026-08-09T14:30""#));
    }

    #[test]
    fn a_file_field_is_a_file_input() {
        // `844b5ae0`. It carries no `accept`, which is measured rather than
        // deferred: zero sites in either app.
        let html = field_html(
            &Field::new(FieldKind::File, "attachment", "Attachment"),
            &Filling::default(),
            &Emit::default(),
        );
        assert!(html.contains(r#"type="file""#));
        assert!(!html.contains("accept="));
        // And it never carries a value: a file input's value is not settable
        // from markup, and the browser refuses one that tries.
        assert!(!html.contains("value="));
    }

    #[test]
    fn the_typed_text_kinds_keep_their_input_type() {
        for (kind, expected) in [
            (FieldKind::Email, "email"),
            (FieldKind::Url, "url"),
            (FieldKind::Tel, "tel"),
        ] {
            let html = field_html(&field(kind), &Filling::default(), &Emit::default());
            assert!(
                html.contains(&format!(r#"type="{expected}""#)),
                "{kind:?} emitted {html}"
            );
        }
    }

    #[test]
    fn no_prefix_leaves_the_id_as_the_name() {
        let html = field_html(
            &field(FieldKind::Text),
            &Filling::default(),
            &Emit::default(),
        );
        assert!(html.contains(r#"id="title" name="title""#), "{html}");
    }
}