client-api: Add support for using an existing session to log in another

According to MSC3882

crates/ruma-client-api/CHANGELOG.md

@@ -9,6 +9,7 @@ Breaking changes:
 Improvements:
 
 - Add convenience constructors for enabling lazy-loading in filters 
+- Add support for using an existing session to log in another (MSC3882 / Matrix 1.7)
 
 # 0.16.2
 

crates/ruma-client-api/src/session.rs

@@ -1,5 +1,6 @@
 //! Endpoints for user session management.
 
+pub mod get_login_token;
 pub mod get_login_types;
 pub mod login;
 pub mod login_fallback;

crates/ruma-client-api/src/session/get_login_token.rs

@@ -0,0 +1,75 @@
+//! `GET /_matrix/client/*/login/get_token`
+//!
+//! Generate a single-use, time-limited, `m.login.token` token.
+
+pub mod v1 {
+    //! `/v1/` ([spec])
+    //!
+    //! [spec]: https://spec.matrix.org/latest/client-server-api/#post_matrixclientv1loginget_token
+
+    use std::time::Duration;
+
+    use ruma_common::{
+        api::{request, response, Metadata},
+        metadata,
+    };
+
+    use crate::uiaa::{AuthData, UiaaResponse};
+
+    const METADATA: Metadata = metadata! {
+        method: POST,
+        rate_limited: true,
+        authentication: AccessToken,
+        history: {
+            unstable => "/_matrix/client/unstable/org.matrix.msc3882/login/get_token",
+            1.7 => "/_matrix/client/v1/login/get_token",
+        }
+    };
+
+    /// Request type for the `login` endpoint.
+    #[request(error = UiaaResponse)]
+    #[derive(Default)]
+    pub struct Request {
+        /// Additional authentication information for the user-interactive authentication API.
+        #[serde(skip_serializing_if = "Option::is_none")]
+        pub auth: Option<AuthData>,
+    }
+
+    /// Response type for the `login` endpoint.
+    #[response(error = UiaaResponse)]
+    pub struct Response {
+        /// The time remaining in milliseconds until the homeserver will no longer accept the
+        /// token.
+        ///
+        /// 2 minutes is recommended as a default.
+        #[serde(with = "ruma_common::serde::duration::ms", rename = "expires_in_ms")]
+        pub expires_in: Duration,
+
+        /// The login token for the `m.login.token` login flow.
+        pub login_token: String,
+    }
+
+    impl Request {
+        /// Creates a new empty `Request`.
+        pub fn new() -> Self {
+            Self::default()
+        }
+    }
+
+    impl Response {
+        /// Creates a new `Response` with the given expiration duration and login token.
+        pub fn new(expires_in: Duration, login_token: String) -> Self {
+            Self { expires_in, login_token }
+        }
+
+        /// Creates a new `Response` with the default expiration duration and the given login token.
+        pub fn with_default_expiration_duration(login_token: String) -> Self {
+            Self::new(Self::default_expiration_duration(), login_token)
+        }
+
+        fn default_expiration_duration() -> Duration {
+            // 2 minutes.
+            Duration::from_secs(2 * 60)
+        }
+    }
+}

crates/ruma-client-api/src/session/get_login_types.rs

@@ -151,12 +151,16 @@ pub mod v3 {
     #[derive(Clone, Debug, Default, Deserialize, Serialize)]
     #[cfg_attr(not(feature = "unstable-exhaustive-types"), non_exhaustive)]
     #[serde(tag = "type", rename = "m.login.token")]
-    pub struct TokenLoginType {}
+    pub struct TokenLoginType {
+        /// Whether the homeserver supports the `POST /login/get_token` endpoint.
+        #[serde(default, skip_serializing_if = "ruma_common::serde::is_default")]
+        pub get_login_token: bool,
+    }
 
     impl TokenLoginType {
         /// Creates a new `TokenLoginType`.
         pub fn new() -> Self {
-            Self {}
+            Self { get_login_token: false }
         }
     }
 
@@ -409,7 +413,7 @@ pub mod v3 {
         fn serialize_sso_login_type() {
             let wrapper = to_json_value(Wrapper {
                 flows: vec![
-                    LoginType::Token(TokenLoginType {}),
+                    LoginType::Token(TokenLoginType::new()),
                     LoginType::Sso(SsoLoginType {
                         identity_providers: vec![IdentityProvider {
                             id: "oidc-github".into(),