Document the 'Contextual' form guard.

core/lib/src/form/context.rs

@@ -4,50 +4,155 @@ use indexmap::{IndexMap, IndexSet};
 use crate::form::prelude::*;
 use crate::http::Status;
 
-/// An infallible form guard that records form fields while parsing any form
+/// An infallible form guard that records form fields and errors during parsing.
-/// type.
 ///
-/// See the [forms guide](https://rocket.rs/master/guide/requests/#context) for
+/// This form guard _never fails_. It should be use _only_ when the form
-/// usage details.
+/// [`Context`] is required. In all other cases, prefer to use `T` directly.
+///
+/// # Usage
+///
+/// `Contextual` acts as a proxy for any form type, recording all submitted form
+/// values and produced errors and associating them with their corresponding
+/// field name. `Contextual` is particularly useful for rendering forms with
+/// previously submitted values and errors associated with form input.
+///
+/// To retrieve the context for a form, use `Form<Contextual<'_, T>>` as a data
+/// guard, where `T` implements `FromForm`. The `context` field contains the
+/// form's [`Context`]:
+///
+/// ```rust
+/// # use rocket::post;
+/// # type T = String;
+/// use rocket::form::{Form, Contextual};
+///
+/// #[post("/submit", data = "<form>")]
+/// fn submit(form: Form<Contextual<'_, T>>) {
+///     if let Some(ref value) = form.value {
+///         // The form parsed successfully. `value` is the `T`.
+///     }
+///
+///     // We can retrieve raw field values and errors.
+///     let raw_id_value = form.context.field_value("id");
+///     let id_errors = form.context.field_errors("id");
+/// }
+/// ```
+///
+/// `Context` serializes as a map, so it can be rendered in templates that
+/// require `Serialize` types. See the [forms guide] for further usage details.
+///
+/// [forms guide]: https://rocket.rs/master/guide/requests/#context
 #[derive(Debug)]
 pub struct Contextual<'v, T> {
-    /// The value, if it could be successfully parsed.
+    /// The value, if it was successfully parsed, or `None` otherwise.
     pub value: Option<T>,
-    /// The context containig all received fields, values, and errors.
+    /// The context with all submitted fields and associated values and errors.
-    pub context: Context<'v>
+    pub context: Context<'v>,
 }
 
 /// A form context containing received fields, values, and encountered errors.
 ///
+/// A value of this type is produced by the [`Contextual`] form guard in its
+/// [`context`](Contextual::context) field. `Context` contains an entry for
+/// every form field submitted by the client regardless of whether the field
+/// parsed or validated successfully.
+///
+/// # Field Values
+///
+/// The original, submitted field value(s) for a _value_ field can be retrieved
+/// via [`Context::field_value()`] or [`Context::field_values()`]. Data fields do not have
+/// their values recorded. All submitted field names, including data field
+/// names, can be retrieved via [`Context::fields()`].
+///
+/// # Field Errors
+///
 /// # Serialization
 ///
 /// When a value of this type is serialized, a `struct` or map with the
 /// following fields is emitted:
 ///
 /// | field         | type                               | description                          |
-/// |---------------|-------------------|------------------------------------------------|
+/// |---------------|------------------------------------|--------------------------------------|
-/// | `errors`      | &str => &[Error]  | map from a field name to errors it encountered |
+/// | `errors`      | map: string to array of [`Error`]s | maps a field name to its errors      |
-/// | `values`      | &str => &[&str]   | map from a field name to its submitted values  |
+/// | `values`      | map: string to array of strings    | maps a field name to its form values |
-/// | `data_values` | &[&str]           | field names of all data fields received        |
+/// | `data_fields` | array of strings                   | field names of all form data fields  |
-/// | `form_errors` | &[Error]          | errors not corresponding to specific fields    |
+/// | `form_errors` | array of [`Error`]s                | errors not associated with a field   |
 ///
-/// See [`Error`] for details on how an `Error` is serialized.
+/// See [`Error`](Error#serialization) for `Error` serialization details.
 #[derive(Debug, Default, Serialize)]
 pub struct Context<'v> {
     errors: IndexMap<NameBuf<'v>, Errors<'v>>,
     values: IndexMap<&'v Name, Vec<&'v str>>,
-    data_values: IndexSet<&'v Name>,
+    data_fields: IndexSet<&'v Name>,
     form_errors: Errors<'v>,
     #[serde(skip)]
     status: Status,
 }
 
 impl<'v> Context<'v> {
-    pub fn value<N: AsRef<Name>>(&self, name: N) -> Option<&'v str> {
+    /// Returns the names of all submitted form fields, both _value_ and _data_
+    /// fields.
+    ///
+    /// # Example
+    ///
+    /// ```rust
+    /// # use rocket::post;
+    /// # type T = String;
+    /// use rocket::form::{Form, Contextual};
+    ///
+    /// #[post("/submit", data = "<form>")]
+    /// fn submit(form: Form<Contextual<'_, T>>) {
+    ///     let field_names = form.context.fields();
+    /// }
+    /// ```
+    pub fn fields(&self) -> impl Iterator<Item = &'v Name> + '_ {
+        self.values.iter()
+            .map(|(name, _)| *name)
+            .chain(self.data_fields.iter().copied())
+    }
+
+    /// Returns the _first_ value, if any, submitted for the _value_ field named
+    /// `name`.
+    ///
+    /// The type of `name` may be `&Name`, `&str`, or `&RawStr`. Lookup is
+    /// case-sensitive but key-seperator (`.` or `[]`) insensitive.
+    ///
+    /// # Example
+    ///
+    /// ```rust
+    /// # use rocket::post;
+    /// # type T = String;
+    /// use rocket::form::{Form, Contextual};
+    ///
+    /// #[post("/submit", data = "<form>")]
+    /// fn submit(form: Form<Contextual<'_, T>>) {
+    ///     let first_value_for_id = form.context.field_value("id");
+    ///     let first_value_for_foo_bar = form.context.field_value("foo.bar");
+    /// }
+    /// ```
+    pub fn field_value<N: AsRef<Name>>(&self, name: N) -> Option<&'v str> {
         self.values.get(name.as_ref())?.get(0).cloned()
     }
 
-    pub fn values<'a, N>(&'a self, name: N) -> impl Iterator<Item = &'v str> + 'a
+    /// Returns the values, if any, submitted for the _value_ field named
+    /// `name`.
+    ///
+    /// The type of `name` may be `&Name`, `&str`, or `&RawStr`. Lookup is
+    /// case-sensitive but key-seperator (`.` or `[]`) insensitive.
+    ///
+    /// # Example
+    ///
+    /// ```rust
+    /// # use rocket::post;
+    /// # type T = String;
+    /// use rocket::form::{Form, Contextual};
+    ///
+    /// #[post("/submit", data = "<form>")]
+    /// fn submit(form: Form<Contextual<'_, T>>) {
+    ///     let values_for_id = form.context.field_values("id");
+    ///     let values_for_foo_bar = form.context.field_values("foo.bar");
+    /// }
+    /// ```
+    pub fn field_values<N>(&self, name: N) -> impl Iterator<Item = &'v str> + '_
         where N: AsRef<Name>
     {
         self.values
@@ -57,27 +162,139 @@ impl<'v> Context<'v> {
             .flatten()
     }
 
-    pub fn has_error<N: AsRef<Name>>(&self, name: &N) -> bool {
+    /// Returns an iterator over all of the errors in the context, including
-        self.errors(name).next().is_some()
+    /// those not associated with any field.
-    }
+    ///
-
+    /// # Example
-    pub fn errors<'a, N>(&'a self, name: &'a N) -> impl Iterator<Item = &Error<'v>> + 'a
+    ///
-        where N: AsRef<Name> + ?Sized
+    /// ```rust
-    {
+    /// # use rocket::post;
-        let name = name.as_ref();
+    /// # type T = String;
-        name.prefixes()
+    /// use rocket::form::{Form, Contextual};
-            .filter_map(move |name| self.errors.get(name))
+    ///
-            .map(|e| e.iter())
+    /// #[post("/submit", data = "<form>")]
-            .flatten()
+    /// fn submit(form: Form<Contextual<'_, T>>) {
-    }
+    ///     let errors = form.context.errors();
-
+    /// }
-    pub fn all_errors(&self) -> impl Iterator<Item = &Error<'v>> {
+    /// ```
+    pub fn errors(&self) -> impl Iterator<Item = &Error<'v>> {
         self.errors.values()
             .map(|e| e.iter())
             .flatten()
             .chain(self.form_errors.iter())
     }
 
+    /// Returns the errors associated with the field `name`. This method is
+    /// roughly equivalent to:
+    ///
+    /// ```rust
+    /// # use rocket::form::{Context, name::Name};
+    /// # let context = Context::default();
+    /// # let name = Name::new("foo");
+    /// context.errors().filter(|e| e.is_for(name))
+    /// # ;
+    /// ```
+    ///
+    /// That is, it uses [`Error::is_for()`] to determine which errors are
+    /// associated with the field named `name`. This considers all errors whose
+    /// associated field name is a prefix of `name` to be an error for the field
+    /// named `name`. In other words, it associates parent field errors with
+    /// their children: `a.b`'s errors apply to `a.b.c`, `a.b.d` and so on but
+    /// not `a.c`.
+    ///
+    /// Lookup is case-sensitive but key-seperator (`.` or `[]`) insensitive.
+    ///
+    /// # Example
+    ///
+    /// ```rust
+    /// # use rocket::post;
+    /// # type T = String;
+    /// use rocket::form::{Form, Contextual};
+    ///
+    /// #[post("/submit", data = "<form>")]
+    /// fn submit(form: Form<Contextual<'_, T>>) {
+    ///     // Get all errors for field `id`.
+    ///     let id = form.context.field_errors("id");
+    ///
+    ///     // Get all errors for `foo.bar` or `foo` if `foo` failed first.
+    ///     let foo_bar = form.context.field_errors("foo.bar");
+    /// }
+    /// ```
+    pub fn field_errors<'a, N>(&'a self, name: N) -> impl Iterator<Item = &Error<'v>> + '_
+        where N: AsRef<Name> + 'a
+    {
+        self.errors.values()
+            .map(|e| e.iter())
+            .flatten()
+            .filter(move |e| e.is_for(&name))
+    }
+
+    /// Returns the errors associated _exactly_ with the field `name`. Prefer
+    /// [`Context::field_errors()`] instead.
+    ///
+    /// This method is roughly equivalent to:
+    ///
+    /// ```rust
+    /// # use rocket::form::{Context, name::Name};
+    /// # let context = Context::default();
+    /// # let name = Name::new("foo");
+    /// context.errors().filter(|e| e.is_for_exactly(name))
+    /// # ;
+    /// ```
+    ///
+    /// That is, it uses [`Error::is_for_exactly()`] to determine which errors
+    /// are associated with the field named `name`. This considers _only_ errors
+    /// whose associated field name is _exactly_ `name` to be an error for the
+    /// field named `name`. This is _not_ what is typically desired as it
+    /// ignores errors that occur in the parent which will result in missing
+    /// errors associated with its chilren. Use [`Context::field_errors()`] in
+    /// almost all cases.
+    ///
+    /// Lookup is case-sensitive but key-seperator (`.` or `[]`) insensitive.
+    ///
+    /// # Example
+    ///
+    /// ```rust
+    /// # use rocket::post;
+    /// # type T = String;
+    /// use rocket::form::{Form, Contextual};
+    ///
+    /// #[post("/submit", data = "<form>")]
+    /// fn submit(form: Form<Contextual<'_, T>>) {
+    ///     // Get all errors for field `id`.
+    ///     let id = form.context.exact_field_errors("id");
+    ///
+    ///     // Get all errors exactly for `foo.bar`. If `foo` failed, we will
+    ///     // this will return no erorrs. Use `Context::field_errors()`.
+    ///     let foo_bar = form.context.exact_field_errors("foo.bar");
+    /// }
+    /// ```
+    pub fn exact_field_errors<'a, N>(&'a self, name: N) -> impl Iterator<Item = &Error<'v>> + '_
+        where N: AsRef<Name> + 'a
+    {
+        self.errors.values()
+            .map(|e| e.iter())
+            .flatten()
+            .filter(move |e| e.is_for_exactly(&name))
+    }
+
+    /// Returns the `max` of the statuses associated with all field errors.
+    ///
+    /// See [`Error::status()`] for details on how an error status is computed.
+    ///
+    /// # Example
+    ///
+    /// ```rust
+    /// # use rocket::post;
+    /// # type T = String;
+    /// use rocket::http::Status;
+    /// use rocket::form::{Form, Contextual};
+    ///
+    /// #[post("/submit", data = "<form>")]
+    /// fn submit(form: Form<Contextual<'_, T>>) -> (Status, &'static str) {
+    ///     (form.context.status(), "Thanks!")
+    /// }
+    /// ```
     pub fn status(&self) -> Status {
         self.status
     }
@@ -119,11 +336,8 @@ impl<'v, T: FromForm<'v>> FromForm<'v> for Contextual<'v, T> {
         T::push_value(val_ctxt, field);
     }
 
-    async fn push_data(
+    async fn push_data((ref mut val_ctxt, ctxt): &mut Self::Context, field: DataField<'v, '_>) {
-        (ref mut val_ctxt, ctxt): &mut Self::Context,
+        ctxt.data_fields.insert(field.name.source());
-        field: DataField<'v, '_>
-    ) {
-        ctxt.data_values.insert(field.name.source());
         T::push_data(val_ctxt, field).await;
     }
 

core/lib/src/form/from_form.rs

@@ -8,7 +8,7 @@ use indexmap::IndexMap;
 use crate::form::prelude::*;
 use crate::http::uncased::AsUncased;
 
-/// Trait for implementing form guards: types parseable from HTTP form fields.
+/// Trait implemented by form guards: types parseable from HTTP forms.
 ///
 /// Only form guards that are _collections_, that is, collect more than one form
 /// field while parsing, should implement `FromForm`. All other types should

core/lib/src/form/name/buf.rs

@@ -178,12 +178,6 @@ impl<N: AsRef<Name> + ?Sized> PartialEq<N> for NameBuf<'_> {
     }
 }
 
-impl PartialEq<Name> for NameBuf<'_> {
-    fn eq(&self, other: &Name) -> bool {
-        self.keys().eq(other.keys())
-    }
-}
-
 impl PartialEq<NameBuf<'_>> for Name {
     fn eq(&self, other: &NameBuf<'_>) -> bool {
         self.keys().eq(other.keys())

core/lib/src/form/name/name.rs

@@ -210,6 +210,12 @@ impl AsRef<Name> for RawStr {
     }
 }
 
+impl AsRef<Name> for Name {
+    fn as_ref(&self) -> &Name {
+        self
+    }
+}
+
 impl Eq for Name { }
 
 impl std::hash::Hash for Name {

core/lib/src/lib.rs

@@ -7,6 +7,7 @@
 #![cfg_attr(nightly, feature(decl_macro))]
 
 #![warn(rust_2018_idioms)]
+#![warn(missing_docs)]
 
 //! # Rocket - Core API Documentation
 //!

examples/forms/src/main.rs

@@ -67,6 +67,9 @@ fn index<'r>() -> Template {
     Template::render("index", &Context::default())
 }
 
+// NOTE: We use `Contextual` here because we want to collect all submitted form
+// fields to re-render forms with submitted values on error. If you have no such
+// need, do not use `Contextual`. Use the equivalent of `Form<Submit<'_>>`.
 #[post("/", data = "<form>")]
 fn submit<'r>(form: Form<Contextual<'r, Submit<'r>>>) -> (Status, Template) {
     let template = match form.value {

site/guide/4-requests.md

@@ -1521,10 +1521,11 @@ map! {
 
 ### Context
 
-The [`Contextual`] type acts as a proxy for any form type, recording all of the
+The [`Contextual`] form guard acts as a proxy for any other form guard,
-submitted form values and produced errors and associating them with their
+recording all submitted form values and produced errors and associating them
-corresponding field name. `Contextual` is particularly useful to render a form
+with their corresponding field name. `Contextual` is particularly useful for
-with previously submitted values and render errors associated with a form input.
+rendering forms with previously submitted values and errors associated with form
+input.
 
 To retrieve the context for a form, use `Form<Contextual<'_, T>>` as a data
 guard, where `T` implements `FromForm`. The `context` field contains the form's
@@ -1542,18 +1543,22 @@ fn submit(form: Form<Contextual<'_, T>>) {
         // The form parsed successfully. `value` is the `T`.
     }
 
-    // In all cases, `form.context` contains the `Context`.
     // We can retrieve raw field values and errors.
-    let raw_id_value = form.context.value("id");
+    let raw_id_value = form.context.field_value("id");
-    let id_errors = form.context.errors("id");
+    let id_errors = form.context.field_errors("id");
 }
 ```
 
+`Context` is nesting-aware for errors. When `Context` is queried for errors for
+a field named `foo.bar`, it returns errors for fields that are a prefix of
+`foo.bar`, namely `foo` and `foo.bar`. Similarly, if queried for errors for a
+field named `foo.bar.baz`, errors for field `foo`, `foo.bar`, and `foo.bar.baz`
+will be returned.
+
 `Context` serializes as a map, so it can be rendered in templates that require
-`Serialize` types. See
+`Serialize` types. See [`Context`] for details about its serialization format.
-[`Context`](@api/rocket/form/struct.Context.html#Serialization) for details
+The [forms example], too, makes use of form contexts, as well as every other
-about its serialization format. The [forms example], too, makes use of form
+forms feature.
-contexts, as well as every other forms feature.
 
 [`Contextual`]: @api/rocket/form/struct.Contextual.html
 [`Context`]: @api/rocket/form/struct.Context.html