renox_core/validation/

extract.rs

use axum::body::Bytes;
use std::collections::HashMap;

use axum::extract::multipart::MultipartError;
use axum::extract::{FromRequest, Multipart, Request};
use axum::http::header::CONTENT_TYPE;
use axum::http::{Method, StatusCode};
use axum::response::{IntoResponse, Response};
use serde::de::DeserializeOwned;
use serde_json::{Map, Value};

use super::messages::render;
use super::nested;
use super::{Errors, Validate, ValidationError, Validator};
use crate::upload::{self, Upload};
use crate::{AppState, Error};

/// Deserializes and validates a form (urlencoded or multipart with `Upload`
/// fields), a JSON body, or the query string for GET, with the type's
/// `Validate` rules.
///
/// On failure, HTMX and JSON requests get `422` with the errors as JSON (the
/// bundled script shows them next to the form's inputs); other requests are
/// redirected back with the errors and old input flashed.
///
/// Empty form fields count as missing, like Laravel: use `Option<T>` for
/// optional fields. A missing required field or a value of the wrong type
/// becomes a validation error rather than a 400.
pub struct Valid<T>(pub T);

/// Built-in messages in the request's language, with the app's overrides.
pub(crate) struct Messages {
    pub(crate) texts: crate::i18n::Texts,
}

impl Messages {
    fn template(&self, key: &str) -> std::borrow::Cow<'static, str> {
        super::messages::template_for(Some(&self.texts), key)
    }

    fn label(&self, field: &str) -> String {
        nested::label(field, |key| {
            self.texts
                .get(&format!("renox.validation.attributes.{key}"))
                .cloned()
        })
    }
}

pub(crate) enum Parsed<T> {
    /// Parsed, possibly with placeholders standing in for fields that
    /// didn't parse; `Errors` holds those fields' errors.
    Ok(T, Errors),
    Invalid(Errors),
}

impl<T> FromRequest<AppState> for Valid<T>
where
    T: DeserializeOwned + Validate + Send,
{
    type Rejection = Response;

    async fn from_request(req: Request, state: &AppState) -> Result<Self, Response> {
        validate_request(req, state, |_: &T, _, _| {})
            .await
            .map(|(data, _)| Valid(data))
    }
}

/// Sent by renox.js to validate one field as the user types (`data-live-validate`).
pub(crate) const LIVE_HEADER: &str = "x-renox-validate";

/// The input of the request's validated form, in [`crate::context`].
#[derive(Clone)]
pub(crate) struct SubmittedInput(pub Map<String, Value>);

/// What `Valid` does, with `extra` rules added to `T`'s own; returns the
/// data and the submitted fields (without files).
#[allow(clippy::result_large_err)] // the rejection is a response, like axum's
pub(crate) async fn validate_request<T>(
    req: Request,
    state: &AppState,
    extra: impl FnOnce(&T, &Map<String, Value>, &mut Validator) + Send,
) -> Result<(T, Map<String, Value>), Response>
where
    T: DeserializeOwned + Validate + Send,
{
    let live_field = req
        .headers()
        .get(LIVE_HEADER)
        .and_then(|v| v.to_str().ok())
        .filter(|f| !f.is_empty() && f.len() <= 200)
        .map(str::to_owned);
    // For `authorize` and `after`, taken before the body is read.
    let user = req
        .extensions()
        .get::<crate::auth::CurrentUser>()
        .and_then(|current| current.user.clone());
    let method = req.method().clone();
    let path = req.uri().path().to_owned();
    let locale_name = crate::i18n::request_locale(req.extensions(), state);
    let locale = &Messages {
        texts: state.translator.texts(&locale_name),
    };
    let content_type = req
        .headers()
        .get(CONTENT_TYPE)
        .and_then(|v| v.to_str().ok())
        .unwrap_or_default()
        .to_ascii_lowercase();
    let is_json = req
        .headers()
        .get(CONTENT_TYPE)
        .and_then(|v| v.to_str().ok())
        .is_some_and(|v| v.starts_with("application/json"));

    let is_multipart = req
        .headers()
        .get(CONTENT_TYPE)
        .and_then(|v| v.to_str().ok())
        .is_some_and(|v| v.starts_with("multipart/form-data"));

    let (parsed, input) = if matches!(*req.method(), Method::GET | Method::HEAD) {
        let query = req.uri().query().unwrap_or_default().as_bytes();
        let pairs = form_urlencoded::parse(query).into_owned().collect();
        parse_pairs(pairs, &HashMap::new(), locale)
    } else if is_multipart {
        let multipart = Multipart::from_request(req, state)
            .await
            .map_err(IntoResponse::into_response)?;
        // MultipartError keeps axum's status, e.g. 413 over UPLOAD_MAX_SIZE.
        let (pairs, uploads) = read_multipart(multipart)
            .await
            .map_err(IntoResponse::into_response)?;
        parse_pairs(pairs, &uploads, locale)
    } else {
        let bytes = Bytes::from_request(req, state)
            .await
            .map_err(IntoResponse::into_response)?;
        if is_json {
            parse_json(&bytes, locale).map_err(IntoResponse::into_response)?
        } else if !content_type.is_empty()
            && !content_type.starts_with("application/x-www-form-urlencoded")
        {
            return Err(
                (StatusCode::UNSUPPORTED_MEDIA_TYPE, "Send a form or JSON.").into_response()
            );
        } else {
            let pairs = form_urlencoded::parse(&bytes).into_owned().collect();
            parse_pairs(pairs, &HashMap::new(), locale)
        }
    };

    let (mut data, mut errors): (T, Errors) = match parsed {
        Parsed::Ok(data, errors) => (data, errors),
        Parsed::Invalid(errors) => {
            return Err(bagged::<T>(ValidationError::new(errors))
                .with_input_map(input)
                .into_response());
        }
    };
    data.prepare();
    let form = super::FormContext {
        state,
        user: user.as_deref(),
        method: &method,
        path: &path,
    };
    if !data
        .authorize(&form)
        .await
        .map_err(IntoResponse::into_response)?
    {
        return Err(Error::Forbidden.into_response());
    }
    let mut validator = Validator::rules_with_texts(&data, locale.texts.clone());
    extra(&data, &input, &mut validator);
    let rule_errors = validator
        .finish_for(state, user.as_deref())
        .await
        .map_err(IntoResponse::into_response)?;
    // A field that didn't parse was checked with a placeholder; its own
    // error is the one to show.
    for (field, messages) in rule_errors.iter() {
        if !errors.has(field) {
            for message in messages {
                errors.add(field, message.clone());
            }
        }
    }
    if errors.is_empty() {
        data.after(&form, &mut errors)
            .await
            .map_err(IntoResponse::into_response)?;
    }
    // Live validation (`data-live-validate` forms, renox.js): answer with
    // one field's errors and stop, whatever the rest of the form says; the
    // handler doesn't run, so nothing is saved.
    if let Some(field) = live_field {
        // `items[0][name]` from the page; its errors are keyed `items.0.name`.
        let key = nested::normalize(&field);
        // A list's own errors, or its items' (`tags.0`, `tags.1`) when
        // the field is the list: `each(…)` rules report on the items.
        let item = |name: &str| {
            name.strip_prefix(key.as_str())
                .and_then(|rest| rest.strip_prefix('.'))
                .is_some_and(|rest| !rest.is_empty() && rest.bytes().all(|b| b.is_ascii_digit()))
        };
        let messages: Vec<String> = errors
            .iter()
            .filter(|(name, _)| *name == key || item(name))
            .flat_map(|(_, messages)| messages.iter().cloned())
            .collect();
        let body = serde_json::json!({ "field": field, "errors": messages });
        return Err((axum::http::StatusCode::OK, axum::Json(body)).into_response());
    }
    if errors.is_empty() {
        // A later `ValidationError` (from a model's `saving` hook, say) is
        // sent back with this input, so the form is refilled.
        crate::context::set(SubmittedInput(input.clone()));
        Ok((data, input))
    } else {
        Err(bagged::<T>(ValidationError::new(errors))
            .with_input_map(input)
            .into_response())
    }
}

/// The error in `T`'s named bag, if it has one.
fn bagged<T: Validate>(error: ValidationError) -> ValidationError {
    match T::ERROR_BAG {
        Some(bag) => error.in_bag(bag),
        None => error,
    }
}

/// Text fields as pairs, and files replaced by tokens that `Upload`'s
/// `Deserialize` resolves. An empty file input counts as missing.
async fn read_multipart(
    mut multipart: Multipart,
) -> Result<(Vec<(String, String)>, HashMap<String, Upload>), MultipartError> {
    let mut pairs = Vec::new();
    let mut uploads = HashMap::new();
    while let Some(field) = multipart.next_field().await? {
        let Some(name) = field.name().map(str::to_owned) else {
            continue;
        };
        match field.file_name().map(str::to_owned) {
            Some(file_name) => {
                let content_type = field.content_type().unwrap_or_default().to_owned();
                let bytes = field.bytes().await?;
                if file_name.is_empty() && bytes.is_empty() {
                    continue;
                }
                let token = upload::token(uploads.len());
                uploads.insert(token.clone(), Upload::new(file_name, content_type, bytes));
                pairs.push((name, token));
            }
            None => pairs.push((name, field.text().await?)),
        }
    }
    Ok((pairs, uploads))
}

pub(crate) fn parse_pairs<T: DeserializeOwned>(
    pairs: Vec<(String, String)>,
    uploads: &HashMap<String, Upload>,
    locale: &Messages,
) -> (Parsed<T>, Map<String, Value>) {
    if nested::is_nested(pairs.iter().map(|(k, _)| k.as_str())) {
        return parse_nested(pairs, uploads, locale);
    }
    let mut input = Map::new();
    for (key, value) in pairs.iter().filter(|(_, v)| !uploads.contains_key(v)) {
        match input.get_mut(key) {
            Some(Value::Array(values)) => values.push(Value::String(value.clone())),
            Some(existing) => {
                let first = existing.take();
                *existing = Value::Array(vec![first, Value::String(value.clone())]);
            }
            None => {
                input.insert(key.clone(), Value::String(value.clone()));
            }
        }
    }

    // Empty inputs are dropped so `Option<T>` fields become `None`. A field
    // serde then reports missing is put back as "", so text fields still reach
    // the rules (and their labels); a number left blank becomes "required".
    //
    // A field that doesn't parse (`price=abc` for an i64) gets its error, then
    // a placeholder so the rest of the form still parses and every other
    // field's rules run too: all errors show at once.
    let mut filled: Vec<(String, String)> = pairs
        .iter()
        .filter(|(_, v)| !v.trim().is_empty())
        .cloned()
        .collect();
    let mut errors = Errors::new();
    let mut tries: HashMap<String, usize> = HashMap::new();
    let mut coerced: std::collections::HashSet<String> = std::collections::HashSet::new();
    let parsed = loop {
        let encoded = form_urlencoded::Serializer::new(String::new())
            .extend_pairs(&filled)
            .finish();
        // serde_html_form, unlike serde_urlencoded, reads repeated names (a
        // multi-select, a group of checkboxes) into a `Vec`.
        let deserializer = serde_html_form::Deserializer::from_bytes(encoded.as_bytes());
        match upload::with_uploads(uploads, || serde_path_to_error::deserialize(deserializer)) {
            Ok(data) => break Parsed::Ok(data, errors),
            Err(err) => {
                let message = err.inner().to_string();
                if let Some(field) = missing_field(&message)
                    && !filled.iter().any(|(k, _)| k == field)
                {
                    filled.push((field.to_owned(), String::new()));
                    continue;
                }
                let path = err.path().to_string();
                if coerce_browser_value(&mut filled, &path, &message, &mut coerced) {
                    continue;
                }
                let blank = filled.iter().any(|(k, v)| *k == path && v.is_empty());
                let tried = tries.entry(path.clone()).or_default();
                if *tried == 0 {
                    for (field, messages) in field_error(&path, &message, blank, locale).iter() {
                        for message in messages {
                            errors.add(field, message.clone());
                        }
                    }
                }
                // Stand-ins that parse as most field types: an enum's first
                // variant (named in the error), numbers and text, booleans.
                let placeholder = match *tried {
                    0 => expected_variant(&message)
                        .or_else(|| PLACEHOLDERS.first().map(|p| (*p).to_owned())),
                    n => PLACEHOLDERS.get(n).map(|p| (*p).to_owned()),
                };
                *tried += 1;
                match placeholder {
                    Some(value) if filled.iter().any(|(k, _)| *k == path) => {
                        filled.retain(|(k, _)| *k != path);
                        filled.push((path, value));
                    }
                    _ => break Parsed::Invalid(errors),
                }
            }
        }
    };
    (parsed, input)
}

const PLACEHOLDERS: &[&str] = &["0", "false"];

/// A form with nested names (`items[0][name]`): read as a tree, so lists of
/// structs and maps work. Empty values are kept (an `Option` reads "" as
/// `None`), so the rows of a list keep their numbers, and with them their
/// errors (`items.2.name`). Missing fields and fields that don't parse are
/// handled as for a plain form.
fn parse_nested<T: DeserializeOwned>(
    pairs: Vec<(String, String)>,
    uploads: &HashMap<String, Upload>,
    locale: &Messages,
) -> (Parsed<T>, Map<String, Value>) {
    let shown: Vec<(String, String)> = pairs
        .iter()
        .filter(|(_, v)| !uploads.contains_key(v))
        .cloned()
        .collect();
    let input = match nested::Node::build(&shown).into_json() {
        Value::Object(map) => map,
        _ => Map::new(),
    };
    // Dotted names, keeping a `[]` ending: `name[]` is a list even when
    // sent once, so it never fills a text field.
    let mut filled: Vec<(String, String)> = pairs
        .into_iter()
        .map(|(k, v)| {
            let key = nested::normalize(&k);
            if k.ends_with("[]") {
                (key + "[]", v)
            } else {
                (key, v)
            }
        })
        .collect();
    let mut errors = Errors::new();
    let mut tries: HashMap<String, usize> = HashMap::new();
    let mut coerced: std::collections::HashSet<String> = std::collections::HashSet::new();
    let parsed = loop {
        let tree = nested::Node::build(&filled);
        match upload::with_uploads(uploads, || nested::deserialize::<T>(tree)) {
            Ok(data) => break Parsed::Ok(data, errors),
            Err(err) => {
                let message = err.inner().to_string();
                let path = nested::normalize(&err.path().to_string());
                let path = path.trim_start_matches('.').to_owned();
                let path = if path.is_empty() {
                    ".".to_owned()
                } else {
                    path
                };
                if let Some(field) = missing_field(&message) {
                    let full = if path == "." {
                        field.to_owned()
                    } else {
                        format!("{path}.{field}")
                    };
                    if !filled.iter().any(|(k, _)| *k == full) {
                        filled.push((full, String::new()));
                        continue;
                    }
                }
                if coerce_browser_value(&mut filled, &path, &message, &mut coerced) {
                    continue;
                }
                let blank = filled
                    .iter()
                    .any(|(k, v)| *k == path && v.trim().is_empty());
                let tried = tries.entry(path.clone()).or_default();
                if *tried == 0 {
                    for (field, messages) in field_error(&path, &message, blank, locale).iter() {
                        for message in messages {
                            errors.add(field, message.clone());
                        }
                    }
                }
                let placeholder = match *tried {
                    0 => expected_variant(&message)
                        .or_else(|| PLACEHOLDERS.first().map(|p| (*p).to_owned())),
                    n => PLACEHOLDERS.get(n).map(|p| (*p).to_owned()),
                };
                *tried += 1;
                match placeholder {
                    Some(value) if filled.iter().any(|(k, _)| *k == path) => {
                        filled.retain(|(k, _)| *k != path);
                        filled.push((path, value));
                    }
                    _ => break Parsed::Invalid(errors),
                }
            }
        }
    };
    (parsed, input)
}

/// The first valid value an enum's error names: serde's "unknown variant
/// `x`, expected one of `a`, `b`" or DbEnum's "expected one of: a, b".
fn expected_variant(message: &str) -> Option<String> {
    let rest = message.split("expected one of").nth(1)?;
    let rest = rest.trim_start_matches([':', ' ']);
    let first = rest.split(',').next()?.trim().trim_matches('`');
    (!first.is_empty()).then(|| first.to_owned())
}

/// Whether `message` is serde_html_form's "this isn't a bool": 0.2 says
/// "provided string was not `true` or `false`" (as does `nested.rs`), 0.4
/// says `invalid value: string "", expected "true", "on" or "false"` (#160).
fn is_bool_error(message: &str) -> bool {
    message.contains("`true` or `false`") || message.contains("expected \"true\"")
}

/// Rewrites what browsers send into what Rust types parse, once per field:
/// a checkbox's `on` (or `1`, `yes`) is `true`, an unchecked one (missing)
/// is `false`; `<input type="datetime-local">` leaves out the seconds.
/// Returns whether `path` was rewritten, so deserializing can try again.
fn coerce_browser_value(
    filled: &mut [(String, String)],
    path: &str,
    message: &str,
    coerced: &mut std::collections::HashSet<String>,
) -> bool {
    if coerced.contains(path) {
        return false;
    }
    let mut changed = false;
    for (_, value) in filled.iter_mut().filter(|(k, _)| k == path) {
        let new = if is_bool_error(message) {
            match value.trim().to_ascii_lowercase().as_str() {
                "on" | "1" | "yes" | "checked" => Some("true".to_owned()),
                "" | "off" | "0" | "no" => Some("false".to_owned()),
                _ => None,
            }
        } else if is_minute_datetime(value) {
            Some(format!("{value}:00"))
        } else {
            None
        };
        if let Some(new) = new {
            *value = new;
            changed = true;
        }
    }
    if changed {
        coerced.insert(path.to_owned());
    }
    changed
}

/// `2026-10-01T10:30`, as `<input type="datetime-local">` sends it.
fn is_minute_datetime(value: &str) -> bool {
    let b = value.as_bytes();
    b.len() == 16
        && b[4] == b'-'
        && b[7] == b'-'
        && b[10] == b'T'
        && b[13] == b':'
        && b.iter()
            .enumerate()
            .all(|(i, c)| matches!(i, 4 | 7 | 10 | 13) || c.is_ascii_digit())
}

fn parse_json<T: DeserializeOwned>(
    bytes: &[u8],
    locale: &Messages,
) -> Result<(Parsed<T>, Map<String, Value>), Error> {
    let input = match serde_json::from_slice(bytes) {
        Ok(Value::Object(map)) => map,
        Ok(_) => return Err(Error::BadRequest("The JSON body must be an object.".into())),
        Err(err) => return Err(Error::BadRequest(format!("Invalid JSON: {err}"))),
    };
    // As with forms: a missing field is put back as "" so its rules (and
    // `required`) run, and a field of the wrong type gets its error and a
    // placeholder, so every field's errors show at once.
    let mut body = input.clone();
    let mut errors = Errors::new();
    let mut tries: HashMap<String, usize> = HashMap::new();
    let parsed = loop {
        match serde_path_to_error::deserialize(Value::Object(body.clone())) {
            Ok(data) => break Parsed::Ok(data, errors),
            Err(err) => {
                let message = err.inner().to_string();
                if let Some(field) = missing_field(&message)
                    && !body.contains_key(field)
                {
                    body.insert(field.to_owned(), Value::String(String::new()));
                    continue;
                }
                let path = err.path().to_string();
                let blank = body.get(&path).is_some_and(|v| {
                    v.is_null() || v.as_str().is_some_and(|s| s.trim().is_empty())
                });
                let tried = tries.entry(path.clone()).or_default();
                if *tried == 0 {
                    for (field, messages) in field_error(&path, &message, blank, locale).iter() {
                        for message in messages {
                            errors.add(field, message.clone());
                        }
                    }
                }
                let placeholder = match (*tried, expected_variant(&message)) {
                    (0, Some(variant)) => Some(Value::String(variant)),
                    (n, _) => JSON_PLACEHOLDERS.get(n).map(|p| p()),
                };
                *tried += 1;
                match placeholder {
                    Some(value) if body.contains_key(&path) => {
                        body.insert(path, value);
                    }
                    _ => break Parsed::Invalid(errors),
                }
            }
        }
    };
    Ok((parsed, input))
}

/// JSON stand-ins for a field of the wrong type, tried in turn.
const JSON_PLACEHOLDERS: &[fn() -> Value] = &[
    || Value::from(0),
    || Value::Bool(false),
    || Value::String(String::new()),
    || Value::Null,
];

fn missing_field(message: &str) -> Option<&str> {
    message
        .strip_prefix("missing field `")
        .and_then(|rest| rest.split('`').next())
}

/// Turns a deserialization error into a message for the field it concerns.
/// `blank` means the field was submitted empty.
fn field_error(path: &str, message: &str, blank: bool, locale: &Messages) -> Errors {
    let mut errors = Errors::new();
    if let Some(field) = missing_field(message) {
        let field = if path == "." {
            field.to_owned()
        } else {
            format!("{path}.{field}")
        };
        errors.add(
            &field,
            render(&locale.template("required"), &locale.label(&field), &[]),
        );
        return errors;
    }

    let field = if path == "." { "_form" } else { path };
    let numeric = [
        "invalid digit",
        "invalid float",
        "cannot parse integer",
        "number too large",
        "expected i",
        "expected u",
        "expected f",
    ]
    .iter()
    .any(|needle| message.contains(needle));
    let key = match (blank, numeric) {
        (true, _) => "required",
        _ if message.contains(upload::NOT_A_FILE) => "file",
        (false, true) => "numeric",
        (false, false) => "invalid",
    };
    errors.add(
        field,
        render(&locale.template(key), &locale.label(field), &[]),
    );
    errors
}

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

    fn plain() -> Messages {
        Messages {
            texts: Default::default(),
        }
    }

    #[derive(Deserialize, Debug)]
    #[allow(dead_code)]
    struct Form {
        name: String,
        price: i64,
        note: Option<String>,
    }

    fn parse(body: &str) -> Result<Form, Errors> {
        let pairs = form_urlencoded::parse(body.as_bytes())
            .into_owned()
            .collect();
        match parse_pairs::<Form>(pairs, &HashMap::new(), &plain()).0 {
            Parsed::Ok(form, errors) if errors.is_empty() => Ok(form),
            Parsed::Ok(_, errors) | Parsed::Invalid(errors) => Err(errors),
        }
    }

    #[derive(Deserialize, Debug)]
    struct Browser {
        agree: bool,
        news: bool,
        starts_at: chrono::NaiveDateTime,
        #[serde(default)]
        tags: Vec<String>,
        #[serde(default)]
        sizes: Vec<i64>,
    }

    fn parse_browser(body: &str) -> Result<Browser, Errors> {
        let pairs = form_urlencoded::parse(body.as_bytes())
            .into_owned()
            .collect();
        match parse_pairs::<Browser>(pairs, &HashMap::new(), &plain()).0 {
            Parsed::Ok(form, errors) if errors.is_empty() => Ok(form),
            Parsed::Ok(_, errors) | Parsed::Invalid(errors) => Err(errors),
        }
    }

    #[test]
    fn bool_errors_are_recognised_in_either_wording() {
        assert!(is_bool_error("provided string was not `true` or `false`"));
        assert!(is_bool_error(
            r#"invalid value: string "", expected "true", "on" or "false""#
        ));
        assert!(!is_bool_error("invalid digit found in string"));
        // What serde_html_form says today, whichever version Cargo picked.
        let err = serde_html_form::from_str::<Browser>("agree=&news=&starts_at=x").unwrap_err();
        assert!(is_bool_error(&err.to_string()), "{err}");
    }

    #[test]
    fn reads_what_browsers_send() {
        // A checked checkbox sends "on", an unchecked one nothing;
        // datetime-local has no seconds; multi-selects repeat the name.
        let form =
            parse_browser("agree=on&starts_at=2026-10-01T10%3A30&tags=a&tags=b&sizes=1&sizes=2")
                .unwrap();
        assert!(form.agree);
        assert!(!form.news);
        assert_eq!(form.starts_at.to_string(), "2026-10-01 10:30:00");
        assert_eq!(form.tags, ["a", "b"]);
        assert_eq!(form.sizes, [1, 2]);
        let form = parse_browser("agree=1&news=true&starts_at=2026-10-01T10%3A30%3A15").unwrap();
        assert!(form.agree && form.news);
        assert!(form.tags.is_empty());
        let errors = parse_browser("agree=maybe&starts_at=soon").unwrap_err();
        assert!(errors.has("agree") && errors.has("starts_at"));
        // A box sent empty or "off" (a hidden input before the checkbox, or a
        // script) is unchecked. serde_html_form 0.4 words the bool error
        // differently from 0.2, and the rewrite must catch both (#160).
        for unchecked in ["agree=on&news=", "agree=on&news=off", "agree=on&news=0"] {
            let form = parse_browser(&format!("{unchecked}&starts_at=2026-10-01T10%3A30"))
                .unwrap_or_else(|errors| panic!("{unchecked}: {errors:?}"));
            assert!(form.agree && !form.news, "{unchecked}");
        }
    }

    #[test]
    fn empty_fields_are_missing() {
        let form = parse("name=Coffee&price=5&note=").unwrap();
        assert!(form.note.is_none());
        // Text left empty reaches the rules as "".
        assert_eq!(parse("name=&price=5").unwrap().name, "");
        assert_eq!(parse("price=5").unwrap().name, "");
        let errors = parse("name=Coffee&price=").unwrap_err();
        assert_eq!(errors.first("price"), Some("The price field is required."));
    }

    #[test]
    fn wrong_types_name_the_field() {
        let errors = parse("name=Coffee&price=cheap").unwrap_err();
        assert_eq!(errors.first("price"), Some("The price must be a number."));
    }

    #[test]
    fn keeps_every_input_for_old_values() {
        let pairs = form_urlencoded::parse(b"name=Coffee&tag=a&tag=b&price=")
            .into_owned()
            .collect();
        let (_, input) = parse_pairs::<Form>(pairs, &HashMap::new(), &plain());
        assert_eq!(input["name"], "Coffee");
        assert_eq!(input["tag"], serde_json::json!(["a", "b"]));
        assert_eq!(input["price"], "");
    }

    // #250: JSON bodies and nested forms beyond the happy path.

    fn errors<T: DeserializeOwned>(parsed: Parsed<T>) -> Errors {
        match parsed {
            Parsed::Ok(_, errors) | Parsed::Invalid(errors) => errors,
        }
    }

    #[test]
    fn a_json_body_must_be_an_object() {
        for body in [&b"[1, 2]"[..], b"\"name\"", b"42"] {
            match parse_json::<Form>(body, &plain()) {
                Err(Error::BadRequest(message)) => {
                    assert_eq!(message, "The JSON body must be an object.")
                }
                Err(other) => panic!("{other:?}"),
                Ok(_) => panic!("accepted a body that isn't an object"),
            }
        }
        assert!(matches!(
            parse_json::<Form>(b"{", &plain()),
            Err(Error::BadRequest(m)) if m.starts_with("Invalid JSON")
        ));
    }

    #[derive(Deserialize, Debug)]
    #[serde(deny_unknown_fields)]
    #[allow(dead_code)]
    struct Strict {
        name: String,
    }

    #[test]
    fn an_unknown_json_field_is_reported_under_its_name() {
        let (parsed, _) = parse_json::<Strict>(br#"{"name": "a", "extra": 1}"#, &plain()).unwrap();
        let errors = errors(parsed);
        assert_eq!(errors.first("extra"), Some("The extra is invalid."));
    }

    #[test]
    fn errors_at_the_root_name_the_field_or_the_form() {
        // A missing field reported at the root is that field's error.
        let errors = field_error(".", "missing field `name`", false, &plain());
        assert_eq!(errors.first("name"), Some("The name field is required."));
        // Anything else at the root belongs to the whole form.
        let errors = field_error(".", "invalid type: sequence", false, &plain());
        assert!(errors.has("_form"), "{errors:?}");
        // A missing field inside an object keeps its path.
        let errors = field_error("address", "missing field `city`", false, &plain());
        assert_eq!(
            errors.first("address.city"),
            Some("The city field is required.")
        );
    }

    #[derive(Deserialize, Debug)]
    #[allow(dead_code)]
    struct Address {
        city: String,
        zip: i64,
    }

    #[derive(Deserialize, Debug)]
    #[allow(dead_code)]
    struct Shipping {
        address: Address,
    }

    #[test]
    fn json_errors_inside_an_object_name_the_nested_field() {
        let (parsed, _) =
            parse_json::<Shipping>(br#"{"address": {"city": "Bandung", "zip": "x"}}"#, &plain())
                .unwrap();
        let errors = errors(parsed);
        assert!(errors.has("address.zip"), "{errors:?}");
        // A field missing inside an object can't be put back at the top:
        // it's reported, not retried forever.
        let (parsed, _) = parse_json::<Shipping>(br#"{"address": {"zip": 1}}"#, &plain()).unwrap();
        assert!(!errors_of_parsed_is_empty(parsed));
    }

    fn errors_of_parsed_is_empty<T>(parsed: Parsed<T>) -> bool {
        match parsed {
            Parsed::Ok(_, errors) => errors.is_empty(),
            Parsed::Invalid(_) => false,
        }
    }

    #[derive(Deserialize, Debug)]
    #[allow(dead_code)]
    struct Line {
        name: String,
        qty: i64,
    }

    #[derive(Deserialize, Debug)]
    #[allow(dead_code)]
    struct Order {
        title: String,
        lines: Vec<Line>,
    }

    fn nested_errors<T: DeserializeOwned>(body: &str) -> Errors {
        let pairs = form_urlencoded::parse(body.as_bytes())
            .into_owned()
            .collect();
        errors(parse_pairs::<T>(pairs, &HashMap::new(), &plain()).0)
    }

    #[test]
    fn nested_forms_put_back_missing_fields_at_every_level() {
        // `title` (top level) and `lines[0][qty]` are missing: both are put
        // back empty, so text reaches the rules as "" and a number is
        // "required".
        let errors = nested_errors::<Order>("lines%5B0%5D%5Bname%5D=Tea");
        assert!(!errors.has("title"), "{errors:?}");
        assert_eq!(
            errors.first("lines.0.qty"),
            Some("The qty field is required.")
        );
        // A wrong type deep down is named by its full path.
        let errors =
            nested_errors::<Order>("title=T&lines%5B0%5D%5Bname%5D=Tea&lines%5B0%5D%5Bqty%5D=lots");
        assert_eq!(
            errors.first("lines.0.qty"),
            Some("The qty must be a number.")
        );
    }

    #[derive(Deserialize, Debug)]
    #[serde(deny_unknown_fields)]
    #[allow(dead_code)]
    struct StrictOrder {
        lines: Vec<Line>,
    }

    #[test]
    fn an_unknown_nested_field_is_reported_under_its_name() {
        let errors = nested_errors::<StrictOrder>(
            "lines%5B0%5D%5Bname%5D=Tea&lines%5B0%5D%5Bqty%5D=1&extra%5Bx%5D=1",
        );
        assert_eq!(errors.first("extra"), Some("The extra is invalid."));
    }

    #[test]
    fn a_nested_form_can_be_a_list_at_its_root() {
        // Every name starts with a number: the form is a list, and there is
        // no object to keep as old input.
        let pairs =
            form_urlencoded::parse(b"0%5Bname%5D=Tea&0%5Bqty%5D=1&1%5Bname%5D=Coffee&1%5Bqty%5D=2")
                .into_owned()
                .collect();
        let (parsed, input) = parse_pairs::<Vec<Line>>(pairs, &HashMap::new(), &plain());
        match parsed {
            Parsed::Ok(lines, errors) => {
                assert!(errors.is_empty(), "{errors:?}");
                assert_eq!(lines.len(), 2);
                assert_eq!(lines[1].name, "Coffee");
            }
            Parsed::Invalid(errors) => panic!("{errors:?}"),
        }
        assert!(input.is_empty());
    }

    #[derive(Deserialize, Debug)]
    #[allow(dead_code)]
    struct Visit {
        title: String,
        slots: Vec<Slot>,
    }

    #[derive(Deserialize, Debug)]
    #[allow(dead_code)]
    struct Slot {
        starts_at: chrono::NaiveDateTime,
    }

    #[test]
    fn nested_forms_rewrite_what_browsers_send_and_try_every_placeholder() {
        // `datetime-local` leaves out the seconds: put back, deep down too.
        let pairs =
            form_urlencoded::parse(b"title=T&slots%5B0%5D%5Bstarts_at%5D=2026-10-01T10%3A30")
                .into_owned()
                .collect();
        let (parsed, _) = parse_pairs::<Visit>(pairs, &HashMap::new(), &plain());
        assert!(errors_of_parsed_is_empty(parsed));
        // A month 13 still fails once rewritten; it isn't rewritten twice,
        // and no placeholder ("0", "false") reads as a date either, so the
        // form is invalid with one error for the field.
        let errors =
            nested_errors::<Visit>("title=T&slots%5B0%5D%5Bstarts_at%5D=2026-13-01T10%3A30");
        let messages: Vec<_> = errors.iter().collect();
        assert_eq!(messages.len(), 1, "{errors:?}");
        assert_eq!(messages[0].0, "slots.0.starts_at");
        assert_eq!(messages[0].1.len(), 1, "{errors:?}");
    }

    #[test]
    fn a_plain_form_rewrites_a_value_once() {
        let errors = parse_browser("agree=on&news=off&starts_at=2026-13-01T10%3A30").unwrap_err();
        assert!(errors.has("starts_at"), "{errors:?}");
        assert!(!errors.has("agree") && !errors.has("news"), "{errors:?}");
    }

    #[test]
    fn a_json_enum_of_an_unknown_variant_gets_a_valid_one_as_placeholder() {
        #[derive(Deserialize, Debug)]
        #[serde(rename_all = "lowercase")]
        #[allow(dead_code)]
        enum Size {
            Small,
            Large,
        }
        #[derive(Deserialize, Debug)]
        #[allow(dead_code)]
        struct Cup {
            size: Size,
            name: String,
        }
        let (parsed, _) = parse_json::<Cup>(br#"{"size": "huge", "name": 5}"#, &plain()).unwrap();
        let errors = errors(parsed);
        // Both fields are reported: the placeholder variant let the rest
        // of the body be read.
        assert!(errors.has("size"), "{errors:?}");
        assert!(errors.has("name"), "{errors:?}");
    }

    #[tokio::test]
    async fn a_multipart_part_without_a_name_is_skipped() {
        let body = "--XX\r\nContent-Disposition: form-data\r\n\r\nlost\r\n\
                    --XX\r\nContent-Disposition: form-data; name=\"name\"\r\n\r\nCoffee\r\n\
                    --XX--\r\n";
        let req = Request::builder()
            .method(Method::POST)
            .header(CONTENT_TYPE, "multipart/form-data; boundary=XX")
            .body(axum::body::Body::from(body))
            .unwrap();
        let multipart = Multipart::from_request(req, &()).await.unwrap();
        let (pairs, uploads) = read_multipart(multipart).await.unwrap();
        assert_eq!(pairs, [("name".to_owned(), "Coffee".to_owned())]);
        assert!(uploads.is_empty());
    }
}