client-api: Make from param optional in get_message_events

… and provide some additional convenience constructors.

crates/ruma-client-api/src/message/get_message_events.rs

@@ -38,8 +38,18 @@ pub mod v3 {
             /// This token can be obtained from a `prev_batch` token returned for each room by the
             /// sync endpoint, or from a `start` or `end` token returned by a previous request to
             /// this endpoint.
+            ///
+            /// If this is `None`, the server will return messages from the start or end of the
+            /// history visible to the user, depending on the value of [`dir`][Self::dir].
+            ///
+            /// *Note: This field is marked required in v1.2 of the specification, but that is
+            /// changing, most likely as part of v1.3. To frontload the breaking change, this field
+            /// is optional already even though v1.2 is the latest version at the time of writing.
+            /// The specification change can be found [here].*
+            ///
+            /// [here]: https://github.com/matrix-org/matrix-spec/pull/1002
             #[ruma_api(query)]
-            pub from: &'a str,
+            pub from: Option<&'a str>,
 
             /// The token to stop returning events at.
             ///
@@ -61,7 +71,7 @@ pub mod v3 {
             #[serde(default = "default_limit", skip_serializing_if = "is_default_limit")]
             pub limit: UInt,
 
-            /// A `RoomEventFilter` to filter returned events with.
+            /// A [`RoomEventFilter`] to filter returned events with.
             #[ruma_api(query)]
             #[serde(
                 with = "ruma_common::serde::json_string",
@@ -99,7 +109,7 @@ pub mod v3 {
         pub fn new(room_id: &'a RoomId, from: &'a str, dir: Direction) -> Self {
             Self {
                 room_id,
-                from,
+                from: Some(from),
                 to: None,
                 dir,
                 limit: default_limit(),
@@ -118,6 +128,32 @@ pub mod v3 {
         pub fn forward(room_id: &'a RoomId, from: &'a str) -> Self {
             Self::new(room_id, from, Direction::Forward)
         }
+
+        /// Creates a new `Request` to fetch messages from the very start of the available history
+        /// for a given room.
+        pub fn from_start(room_id: &'a RoomId) -> Self {
+            Self {
+                room_id,
+                from: None,
+                to: None,
+                dir: Direction::Forward,
+                limit: default_limit(),
+                filter: RoomEventFilter::default(),
+            }
+        }
+
+        /// Creates a new `Request` to fetch messages from the very end of the available history for
+        /// a given room.
+        pub fn from_end(room_id: &'a RoomId) -> Self {
+            Self {
+                room_id,
+                from: None,
+                to: None,
+                dir: Direction::Backward,
+                limit: default_limit(),
+                filter: RoomEventFilter::default(),
+            }
+        }
     }
 
     impl Response {
@@ -177,7 +213,7 @@ pub mod v3 {
             };
             let req = Request {
                 room_id,
-                from: "token",
+                from: Some("token"),
                 to: Some("token2"),
                 dir: Direction::Backward,
                 limit: uint!(0),
@@ -206,7 +242,7 @@ pub mod v3 {
             let room_id = room_id!("!roomid:example.org");
             let req = Request {
                 room_id,
-                from: "token",
+                from: Some("token"),
                 to: Some("token2"),
                 dir: Direction::Backward,
                 limit: uint!(0),