diff --git a/.cargo/config.toml b/.cargo/config.toml
index 0cf09f710..72f445d8a 100644
--- a/.cargo/config.toml
+++ b/.cargo/config.toml
@@ -3,6 +3,7 @@ chk = "check --workspace --all-features --tests --examples --bins"
 lint = "clippy --workspace --tests --examples"
 ci-min = "hack check --workspace --no-default-features"
 ci-min-test = "hack check --workspace --no-default-features --tests --examples"
-ci-default = "hack check --workspace"
-ci-full = "check --workspace --bins --examples --tests"
-ci-test = "test --workspace --all-features --no-fail-fast"
+ci-default = "check --workspace --bins --tests --examples"
+ci-full = "check --workspace --all-features --bins --tests --examples"
+ci-test = "test --workspace --all-features --lib --tests --no-fail-fast -- --nocapture"
+ci-doctest = "hack test --workspace --all-features --doc --no-fail-fast -- --nocapture"
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index c57db463a..be595e35c 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -66,43 +66,33 @@ jobs:
 
       - name: check minimal
         uses: actions-rs/cargo@v1
-        with:
-          command: hack
-          args: check --workspace --no-default-features
+        with: { command: ci-min }
 
       - name: check minimal + tests
         uses: actions-rs/cargo@v1
-        with:
-          command: hack
-          args: check --workspace --no-default-features --tests --examples
+        with: { command: ci-min-test }
 
+      - name: check default
+        uses: actions-rs/cargo@v1
+        with: { command: ci-default }
+      
       - name: check full
         uses: actions-rs/cargo@v1
-        with:
-          command: check
-          args: --workspace --bins --examples --tests
+        with: { command: ci-full }
 
       - name: tests
-        uses: actions-rs/cargo@v1
-        with:
-          command: test
-          args: --workspace --all-features --no-fail-fast -- --nocapture
-            --skip=test_h2_content_length
-            --skip=test_reading_deflate_encoding_large_random_rustls
-
-      - name: tests (actix-http)
         uses: actions-rs/cargo@v1
         timeout-minutes: 40
         with:
-          command: test
-          args: --package=actix-http --no-default-features --features=rustls -- --nocapture
+          command: ci-test
+          args: --skip=test_reading_deflate_encoding_large_random_rustls
 
-      - name: tests (awc)
+      - name: doc tests
+        # due to unknown issue with running doc tests on macOS
+        if: matrix.target.os == 'ubuntu-latest'
         uses: actions-rs/cargo@v1
         timeout-minutes: 40
-        with:
-          command: test
-          args: --package=awc --no-default-features --features=rustls -- --nocapture
+        with: { command: ci-doctest }
 
       - name: Generate coverage file
         if: >
diff --git a/.github/workflows/clippy-fmt.yml b/.github/workflows/clippy-fmt.yml
index e966fa4ab..957256d32 100644
--- a/.github/workflows/clippy-fmt.yml
+++ b/.github/workflows/clippy-fmt.yml
@@ -36,4 +36,4 @@ jobs:
         uses: actions-rs/clippy-check@v1
         with:
           token: ${{ secrets.GITHUB_TOKEN }}
-          args: --workspace --tests --all-features
+          args: --workspace --all-features --tests
diff --git a/CHANGES.md b/CHANGES.md
index c5506e04d..32df6ffa5 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -2,8 +2,10 @@
 
 ## Unreleased - 2021-xx-xx
 ### Added
+* Add `ServiceRequest::parts_mut`. [#2177]
 * Add extractors for `Uri` and `Method`. [#2263]
-* Add extractor for `ConnectionInfo` and `PeerAddr`. [#2263]
+* Add extractors for `ConnectionInfo` and `PeerAddr`. [#2263]
+* Add `Route::service` for using hand-written services as handlers. [#2262]
 
 ### Changed
 * Change compression algorithm features flags. [#2250]
@@ -11,8 +13,10 @@
 * Add `#[must_use]` attribute for many public functions/methods. [#2281]
 
 
+[#2177]: https://github.com/actix/actix-web/pull/2177
 [#2250]: https://github.com/actix/actix-web/pull/2250
 [#2271]: https://github.com/actix/actix-web/pull/2271
+[#2262]: https://github.com/actix/actix-web/pull/2262
 [#2263]: https://github.com/actix/actix-web/pull/2263
 [#2281]: https://github.com/actix/actix-web/pull/2281
 
diff --git a/Cargo.toml b/Cargo.toml
index 320751c66..779b52255 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -113,7 +113,6 @@ flate2 = "1.0.13"
 zstd = "0.7"
 rand = "0.8"
 rcgen = "0.8"
-serde_derive = "1.0"
 tls-openssl = { package = "openssl", version = "0.10.9" }
 tls-rustls = { package = "rustls", version = "0.19.0" }
 
diff --git a/awc/src/request.rs b/awc/src/request.rs
index 30be1d678..812c76318 100644
--- a/awc/src/request.rs
+++ b/awc/src/request.rs
@@ -480,6 +480,7 @@ impl ClientRequest {
         // supported, so we cannot guess Accept-Encoding HTTP header.
         if slf.response_decompress {
             // Set Accept-Encoding with compression algorithm awc is built with.
+            #[allow(clippy::vec_init_then_push)]
             #[cfg(feature = "__compress")]
             let accept_encoding = {
                 let mut encoding = vec![];
@@ -498,7 +499,11 @@ impl ClientRequest {
                 #[cfg(feature = "compress-zstd")]
                 encoding.push("zstd");
 
-                assert!(!encoding.is_empty(), "encoding cannot be empty unless __compress feature has been explictily enabled.");
+                assert!(
+                    !encoding.is_empty(),
+                    "encoding can not be empty unless __compress feature has been explicitly enabled"
+                );
+
                 encoding.join(", ")
             };
 
diff --git a/src/app.rs b/src/app.rs
index 20d15f9d2..c6f1058f5 100644
--- a/src/app.rs
+++ b/src/app.rs
@@ -69,36 +69,71 @@ where
         InitError = (),
     >,
 {
-    /// Set application data. Application data could be accessed
-    /// by using `Data<T>` extractor where `T` is data type.
+    /// Set application (root level) data.
     ///
-    /// **Note**: HTTP server accepts an application factory rather than
-    /// an application instance. Http server constructs an application
-    /// instance for each thread, thus application data must be constructed
-    /// multiple times. If you want to share data between different
-    /// threads, a shared object should be used, e.g. `Arc`. Internally `Data` type
-    /// uses `Arc` so data could be created outside of app factory and clones could
-    /// be stored via `App::app_data()` method.
+    /// Application data stored with `App::app_data()` method is available through the
+    /// [`HttpRequest::app_data`](crate::HttpRequest::app_data) method at runtime.
+    ///
+    /// # [`Data<T>`]
+    /// Any [`Data<T>`] type added here can utilize it's extractor implementation in handlers.
+    /// Types not wrapped in `Data<T>` cannot use this extractor. See [its docs](Data<T>) for more
+    /// about its usage and patterns.
     ///
     /// ```
     /// use std::cell::Cell;
-    /// use actix_web::{web, App, HttpResponse, Responder};
+    /// use actix_web::{web, App, HttpRequest, HttpResponse, Responder};
     ///
     /// struct MyData {
-    ///     counter: Cell<usize>,
+    ///     count: std::cell::Cell<usize>,
     /// }
     ///
-    /// async fn index(data: web::Data<MyData>) -> impl Responder {
-    ///     data.counter.set(data.counter.get() + 1);
-    ///     HttpResponse::Ok()
+    /// async fn handler(req: HttpRequest, counter: web::Data<MyData>) -> impl Responder {
+    ///     // note this cannot use the Data<T> extractor because it was not added with it
+    ///     let incr = *req.app_data::<usize>().unwrap();
+    ///     assert_eq!(incr, 3);
+    ///
+    ///     // update counter using other value from app data
+    ///     counter.count.set(counter.count.get() + incr);
+    ///
+    ///     HttpResponse::Ok().body(counter.count.get().to_string())
     /// }
     ///
-    /// let app = App::new()
-    ///     .data(MyData{ counter: Cell::new(0) })
-    ///     .service(
-    ///         web::resource("/index.html").route(
-    ///             web::get().to(index)));
+    /// let app = App::new().service(
+    ///     web::resource("/")
+    ///         .app_data(3usize)
+    ///         .app_data(web::Data::new(MyData { count: Default::default() }))
+    ///         .route(web::get().to(handler))
+    /// );
     /// ```
+    ///
+    /// # Shared Mutable State
+    /// [`HttpServer::new`](crate::HttpServer::new) accepts an application factory rather than an
+    /// application instance; the factory closure is called on each worker thread independently.
+    /// Therefore, if you want to share a data object between different workers, a shareable object
+    /// needs to be created first, outside the `HttpServer::new` closure and cloned into it.
+    /// [`Data<T>`] is an example of such a sharable object.
+    ///
+    /// ```ignore
+    /// let counter = web::Data::new(AppStateWithCounter {
+    ///     counter: Mutex::new(0),
+    /// });
+    ///
+    /// HttpServer::new(move || {
+    ///     // move counter object into the closure and clone for each worker
+    ///
+    ///     App::new()
+    ///         .app_data(counter.clone())
+    ///         .route("/", web::get().to(handler))
+    /// })
+    /// ```
+    pub fn app_data<U: 'static>(mut self, ext: U) -> Self {
+        self.extensions.insert(ext);
+        self
+    }
+
+    /// Add application (root) data after wrapping in `Data<T>`.
+    ///
+    /// Deprecated in favor of [`app_data`](Self::app_data).
     #[deprecated(since = "4.0.0", note = "Use `.app_data(Data::new(val))` instead.")]
     pub fn data<U: 'static>(self, data: U) -> Self {
         self.app_data(Data::new(data))
@@ -139,18 +174,6 @@ where
         self
     }
 
-    /// Set application level arbitrary data item.
-    ///
-    /// Application data stored with `App::app_data()` method is available
-    /// via `HttpRequest::app_data()` method at runtime.
-    ///
-    /// This method could be used for storing `Data<T>` as well, in that case
-    /// data could be accessed by using `Data<T>` extractor.
-    pub fn app_data<U: 'static>(mut self, ext: U) -> Self {
-        self.extensions.insert(ext);
-        self
-    }
-
     /// Run external configuration as part of the application building
     /// process
     ///
diff --git a/src/app_service.rs b/src/app_service.rs
index 95d4f5fb3..205b802d9 100644
--- a/src/app_service.rs
+++ b/src/app_service.rs
@@ -144,7 +144,9 @@ where
     }
 }
 
-/// Service that takes a [`Request`] and delegates to a service that take a [`ServiceRequest`].
+/// The [`Service`] that is passed to `actix-http`'s server builder.
+///
+/// Wraps a service receiving a [`ServiceRequest`] into one receiving a [`Request`].
 pub struct AppInitService<T, B>
 where
     T: Service<ServiceRequest, Response = ServiceResponse<B>, Error = Error>,
@@ -275,6 +277,7 @@ impl ServiceFactory<ServiceRequest> for AppRoutingFactory {
     }
 }
 
+/// The Actix Web router default entry point.
 pub struct AppRouting {
     router: Router<HttpService, Guards>,
     default: HttpService,
diff --git a/src/config.rs b/src/config.rs
index 9e1d78920..4483b7685 100644
--- a/src/config.rs
+++ b/src/config.rs
@@ -63,6 +63,8 @@ impl AppService {
         (self.config, self.services)
     }
 
+    /// Clones inner config and default service, returning new `AppService` with empty service list
+    /// marked as non-root.
     pub(crate) fn clone_config(&self) -> Self {
         AppService {
             config: self.config.clone(),
@@ -72,13 +74,13 @@ impl AppService {
         }
     }
 
-    /// Service configuration
+    /// Returns reference to configuration.
     #[must_use]
     pub fn config(&self) -> &AppConfig {
         &self.config
     }
 
-    /// Default resource
+    /// Returns default handler factory.
     #[must_use]
     pub fn default_service(&self) -> Rc<HttpNewService> {
         self.default.clone()
@@ -119,7 +121,7 @@ impl AppConfig {
         AppConfig { secure, host, addr }
     }
 
-    /// Needed in actix-test crate.
+    /// Needed in actix-test crate. Semver exempt.
     #[doc(hidden)]
     pub fn __priv_test_new(secure: bool, host: String, addr: SocketAddr) -> Self {
         AppConfig::new(secure, host, addr)
@@ -198,6 +200,7 @@ impl ServiceConfig {
     /// Add shared app data item.
     ///
     /// Counterpart to [`App::data()`](crate::App::data).
+    #[deprecated(since = "4.0.0", note = "Use `.app_data(Data::new(val))` instead.")]
     pub fn data<U: 'static>(&mut self, data: U) -> &mut Self {
         self.app_data(Data::new(data));
         self
@@ -263,6 +266,8 @@ mod tests {
     use crate::test::{call_service, init_service, read_body, TestRequest};
     use crate::{web, App, HttpRequest, HttpResponse};
 
+    // allow deprecated `ServiceConfig::data`
+    #[allow(deprecated)]
     #[actix_rt::test]
     async fn test_data() {
         let cfg = |cfg: &mut ServiceConfig| {
diff --git a/src/data.rs b/src/data.rs
index c9c0aca97..ac101282c 100644
--- a/src/data.rs
+++ b/src/data.rs
@@ -36,6 +36,11 @@ pub(crate) type FnDataFactory =
 /// If route data is not set for a handler, using `Data<T>` extractor would cause *Internal
 /// Server Error* response.
 ///
+// TODO: document `dyn T` functionality through converting an Arc
+// TODO: note equivalence of req.app_data<Data<T>> and Data<T> extractor
+// TODO: note that data must be inserted using Data<T> in order to extract it
+///
+/// # Examples
 /// ```
 /// use std::sync::Mutex;
 /// use actix_web::{web, App, HttpResponse, Responder};
diff --git a/src/extract.rs b/src/extract.rs
index d7b67cd90..592f7ab83 100644
--- a/src/extract.rs
+++ b/src/extract.rs
@@ -54,7 +54,7 @@ pub trait FromRequest: Sized {
 /// use actix_web::{web, dev, App, Error, HttpRequest, FromRequest};
 /// use actix_web::error::ErrorBadRequest;
 /// use futures_util::future::{ok, err, Ready};
-/// use serde_derive::Deserialize;
+/// use serde::Deserialize;
 /// use rand;
 ///
 /// #[derive(Debug, Deserialize)]
@@ -145,7 +145,7 @@ where
 /// use actix_web::{web, dev, App, Result, Error, HttpRequest, FromRequest};
 /// use actix_web::error::ErrorBadRequest;
 /// use futures_util::future::{ok, err, Ready};
-/// use serde_derive::Deserialize;
+/// use serde::Deserialize;
 /// use rand;
 ///
 /// #[derive(Debug, Deserialize)]
@@ -265,7 +265,7 @@ impl FromRequest for Method {
 #[doc(hidden)]
 impl FromRequest for () {
     type Error = Infallible;
-    type Future = Ready<Result<(), Infallible>>;
+    type Future = Ready<Result<Self, Self::Error>>;
     type Config = ();
 
     fn from_request(_: &HttpRequest, _: &mut Payload) -> Self::Future {
@@ -376,7 +376,7 @@ mod m {
 mod tests {
     use actix_http::http::header;
     use bytes::Bytes;
-    use serde_derive::Deserialize;
+    use serde::Deserialize;
 
     use super::*;
     use crate::test::TestRequest;
diff --git a/src/lib.rs b/src/lib.rs
index 9d8bf62e7..920abccb6 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -149,7 +149,9 @@ pub mod dev {
     pub use actix_http::{Extensions, Payload, PayloadStream, RequestHead, ResponseHead};
     pub use actix_router::{Path, ResourceDef, ResourcePath, Url};
     pub use actix_server::Server;
-    pub use actix_service::{always_ready, forward_ready, Service, Transform};
+    pub use actix_service::{
+        always_ready, fn_factory, fn_service, forward_ready, Service, Transform,
+    };
 
     pub(crate) fn insert_slash(mut patterns: Vec<String>) -> Vec<String> {
         for path in &mut patterns {
diff --git a/src/request.rs b/src/request.rs
index ba96bc7fc..b61affd7e 100644
--- a/src/request.rs
+++ b/src/request.rs
@@ -361,7 +361,7 @@ impl Drop for HttpRequest {
 /// # Examples
 /// ```
 /// use actix_web::{web, App, HttpRequest};
-/// use serde_derive::Deserialize;
+/// use serde::Deserialize;
 ///
 /// /// extract `Thing` from request
 /// async fn index(req: HttpRequest) -> String {
diff --git a/src/resource.rs b/src/resource.rs
index 85c15efa0..2b44ec0fd 100644
--- a/src/resource.rs
+++ b/src/resource.rs
@@ -169,41 +169,38 @@ where
         self
     }
 
-    /// Provide resource specific data. This method allows to add extractor
-    /// configuration or specific state available via `Data<T>` extractor.
-    /// Provided data is available for all routes registered for the current resource.
-    /// Resource data overrides data registered by `App::data()` method.
-    ///
-    /// ```
-    /// use actix_web::{web, App, FromRequest};
-    ///
-    /// /// extract text data from request
-    /// async fn index(body: String) -> String {
-    ///     format!("Body {}!", body)
-    /// }
-    ///
-    /// fn main() {
-    ///     let app = App::new().service(
-    ///         web::resource("/index.html")
-    ///           // limit size of the payload
-    ///           .data(String::configure(|cfg| {
-    ///                cfg.limit(4096)
-    ///           }))
-    ///           .route(
-    ///               web::get()
-    ///                  // register handler
-    ///                  .to(index)
-    ///           ));
-    /// }
-    /// ```
-    #[deprecated(since = "4.0.0", note = "Use `.app_data(Data::new(val))` instead.")]
-    pub fn data<U: 'static>(self, data: U) -> Self {
-        self.app_data(Data::new(data))
-    }
-
     /// Add resource data.
     ///
-    /// Data of different types from parent contexts will still be accessible.
+    /// Data of different types from parent contexts will still be accessible. Any `Data<T>` types
+    /// set here can be extracted in handlers using the `Data<T>` extractor.
+    ///
+    /// # Examples
+    /// ```
+    /// use std::cell::Cell;
+    /// use actix_web::{web, App, HttpRequest, HttpResponse, Responder};
+    ///
+    /// struct MyData {
+    ///     count: std::cell::Cell<usize>,
+    /// }
+    ///
+    /// async fn handler(req: HttpRequest, counter: web::Data<MyData>) -> impl Responder {
+    ///     // note this cannot use the Data<T> extractor because it was not added with it
+    ///     let incr = *req.app_data::<usize>().unwrap();
+    ///     assert_eq!(incr, 3);
+    ///
+    ///     // update counter using other value from app data
+    ///     counter.count.set(counter.count.get() + incr);
+    ///
+    ///     HttpResponse::Ok().body(counter.count.get().to_string())
+    /// }
+    ///
+    /// let app = App::new().service(
+    ///     web::resource("/")
+    ///         .app_data(3usize)
+    ///         .app_data(web::Data::new(MyData { count: Default::default() }))
+    ///         .route(web::get().to(handler))
+    /// );
+    /// ```
     pub fn app_data<U: 'static>(mut self, data: U) -> Self {
         self.app_data
             .get_or_insert_with(Extensions::new)
@@ -212,6 +209,14 @@ where
         self
     }
 
+    /// Add resource data after wrapping in `Data<T>`.
+    ///
+    /// Deprecated in favor of [`app_data`](Self::app_data).
+    #[deprecated(since = "4.0.0", note = "Use `.app_data(Data::new(val))` instead.")]
+    pub fn data<U: 'static>(self, data: U) -> Self {
+        self.app_data(Data::new(data))
+    }
+
     /// Register a new route and add handler. This route matches all requests.
     ///
     /// ```
@@ -227,7 +232,6 @@ where
     /// This is shortcut for:
     ///
     /// ```
-    /// # extern crate actix_web;
     /// # use actix_web::*;
     /// # fn index(req: HttpRequest) -> HttpResponse { unimplemented!() }
     /// App::new().service(web::resource("/").route(web::route().to(index)));
@@ -695,7 +699,7 @@ mod tests {
         assert_eq!(resp.status(), StatusCode::NO_CONTENT);
     }
 
-    // allow deprecated App::data
+    // allow deprecated `{App, Resource}::data`
     #[allow(deprecated)]
     #[actix_rt::test]
     async fn test_data() {
@@ -729,7 +733,7 @@ mod tests {
         assert_eq!(resp.status(), StatusCode::OK);
     }
 
-    // allow deprecated App::data
+    // allow deprecated `{App, Resource}::data`
     #[allow(deprecated)]
     #[actix_rt::test]
     async fn test_data_default_service() {
diff --git a/src/response/response.rs b/src/response/response.rs
index d27d433a7..fb943c70c 100644
--- a/src/response/response.rs
+++ b/src/response/response.rs
@@ -52,7 +52,10 @@ impl HttpResponse<AnyBody> {
     #[inline]
     #[must_use]
     pub fn from_error(error: impl Into<Error>) -> Self {
-        error.into().as_response_error().error_response()
+        let error = error.into();
+        let mut response = error.as_response_error().error_response();
+        response.error = Some(error);
+        response
     }
 }
 
diff --git a/src/route.rs b/src/route.rs
index f2ef828fb..86ee3038c 100644
--- a/src/route.rs
+++ b/src/route.rs
@@ -5,7 +5,7 @@ use std::{future::Future, rc::Rc};
 use actix_http::http::Method;
 use actix_service::{
     boxed::{self, BoxService, BoxServiceFactory},
-    Service, ServiceFactory,
+    Service, ServiceFactory, ServiceFactoryExt,
 };
 use futures_core::future::LocalBoxFuture;
 
@@ -130,9 +130,10 @@ impl Route {
 
     /// Set handler function, use request extractors for parameters.
     ///
+    /// # Examples
     /// ```
     /// use actix_web::{web, http, App};
-    /// use serde_derive::Deserialize;
+    /// use serde::Deserialize;
     ///
     /// #[derive(Deserialize)]
     /// struct Info {
@@ -156,7 +157,7 @@ impl Route {
     ///
     /// ```
     /// # use std::collections::HashMap;
-    /// # use serde_derive::Deserialize;
+    /// # use serde::Deserialize;
     /// use actix_web::{web, App};
     ///
     /// #[derive(Deserialize)]
@@ -186,6 +187,53 @@ impl Route {
         self.service = boxed::factory(HandlerService::new(handler));
         self
     }
+
+    /// Set raw service to be constructed and called as the request handler.
+    ///
+    /// # Examples
+    /// ```
+    /// # use std::convert::Infallible;
+    /// # use futures_util::future::LocalBoxFuture;
+    /// # use actix_web::{*, dev::*, http::header};
+    /// struct HelloWorld;
+    ///
+    /// impl Service<ServiceRequest> for HelloWorld {
+    ///     type Response = ServiceResponse;
+    ///     type Error = Infallible;
+    ///     type Future = LocalBoxFuture<'static, Result<Self::Response, Self::Error>>;
+    ///
+    ///     always_ready!();
+    ///
+    ///     fn call(&self, req: ServiceRequest) -> Self::Future {
+    ///         let (req, _) = req.into_parts();
+    ///
+    ///         let res = HttpResponse::Ok()
+    ///             .insert_header(header::ContentType::plaintext())
+    ///             .body("Hello world!");
+    ///
+    ///         Box::pin(async move { Ok(ServiceResponse::new(req, res)) })
+    ///     }
+    /// }
+    ///
+    /// App::new().route(
+    ///     "/",
+    ///     web::get().service(fn_factory(|| async { Ok(HelloWorld) })),
+    /// );
+    /// ```
+    pub fn service<S, E>(mut self, service_factory: S) -> Self
+    where
+        S: ServiceFactory<
+                ServiceRequest,
+                Response = ServiceResponse,
+                Error = E,
+                InitError = (),
+                Config = (),
+            > + 'static,
+        E: Into<Error> + 'static,
+    {
+        self.service = boxed::factory(service_factory.map_err(Into::into));
+        self
+    }
 }
 
 #[cfg(test)]
@@ -194,9 +242,12 @@ mod tests {
 
     use actix_rt::time::sleep;
     use bytes::Bytes;
-    use serde_derive::Serialize;
+    use futures_core::future::LocalBoxFuture;
+    use serde::Serialize;
 
-    use crate::http::{Method, StatusCode};
+    use crate::dev::{always_ready, fn_factory, fn_service, Service};
+    use crate::http::{header, Method, StatusCode};
+    use crate::service::{ServiceRequest, ServiceResponse};
     use crate::test::{call_service, init_service, read_body, TestRequest};
     use crate::{error, web, App, HttpResponse};
 
@@ -270,4 +321,65 @@ mod tests {
         let body = read_body(resp).await;
         assert_eq!(body, Bytes::from_static(b"{\"name\":\"test\"}"));
     }
+
+    #[actix_rt::test]
+    async fn test_service_handler() {
+        struct HelloWorld;
+
+        impl Service<ServiceRequest> for HelloWorld {
+            type Response = ServiceResponse;
+            type Error = crate::Error;
+            type Future = LocalBoxFuture<'static, Result<Self::Response, Self::Error>>;
+
+            always_ready!();
+
+            fn call(&self, req: ServiceRequest) -> Self::Future {
+                let (req, _) = req.into_parts();
+
+                let res = HttpResponse::Ok()
+                    .insert_header(header::ContentType::plaintext())
+                    .body("Hello world!");
+
+                Box::pin(async move { Ok(ServiceResponse::new(req, res)) })
+            }
+        }
+
+        let srv = init_service(
+            App::new()
+                .route(
+                    "/hello",
+                    web::get().service(fn_factory(|| async { Ok(HelloWorld) })),
+                )
+                .route(
+                    "/bye",
+                    web::get().service(fn_factory(|| async {
+                        Ok::<_, ()>(fn_service(|req: ServiceRequest| async {
+                            let (req, _) = req.into_parts();
+
+                            let res = HttpResponse::Ok()
+                                .insert_header(header::ContentType::plaintext())
+                                .body("Goodbye, and thanks for all the fish!");
+
+                            Ok::<_, Infallible>(ServiceResponse::new(req, res))
+                        }))
+                    })),
+                ),
+        )
+        .await;
+
+        let req = TestRequest::get().uri("/hello").to_request();
+        let resp = call_service(&srv, req).await;
+        assert_eq!(resp.status(), StatusCode::OK);
+        let body = read_body(resp).await;
+        assert_eq!(body, Bytes::from_static(b"Hello world!"));
+
+        let req = TestRequest::get().uri("/bye").to_request();
+        let resp = call_service(&srv, req).await;
+        assert_eq!(resp.status(), StatusCode::OK);
+        let body = read_body(resp).await;
+        assert_eq!(
+            body,
+            Bytes::from_static(b"Goodbye, and thanks for all the fish!")
+        );
+    }
 }
diff --git a/src/scope.rs b/src/scope.rs
index 7a38fbca1..9b45d1c5e 100644
--- a/src/scope.rs
+++ b/src/scope.rs
@@ -1,28 +1,23 @@
-use std::cell::RefCell;
-use std::fmt;
-use std::future::Future;
-use std::rc::Rc;
+use std::{cell::RefCell, fmt, future::Future, mem, rc::Rc};
 
 use actix_http::Extensions;
 use actix_router::{ResourceDef, Router};
-use actix_service::boxed::{self, BoxService, BoxServiceFactory};
 use actix_service::{
-    apply, apply_fn_factory, IntoServiceFactory, Service, ServiceFactory, ServiceFactoryExt,
-    Transform,
+    apply, apply_fn_factory,
+    boxed::{self, BoxService, BoxServiceFactory},
+    IntoServiceFactory, Service, ServiceFactory, ServiceFactoryExt, Transform,
 };
 use futures_core::future::LocalBoxFuture;
 use futures_util::future::join_all;
 
-use crate::config::ServiceConfig;
-use crate::data::Data;
-use crate::dev::{AppService, HttpServiceFactory};
-use crate::error::Error;
-use crate::guard::Guard;
-use crate::resource::Resource;
-use crate::rmap::ResourceMap;
-use crate::route::Route;
-use crate::service::{
-    AppServiceFactory, ServiceFactoryWrapper, ServiceRequest, ServiceResponse,
+use crate::{
+    config::ServiceConfig,
+    data::Data,
+    dev::{AppService, HttpServiceFactory},
+    guard::Guard,
+    rmap::ResourceMap,
+    service::{AppServiceFactory, ServiceFactoryWrapper, ServiceRequest, ServiceResponse},
+    Error, Resource, Route,
 };
 
 type Guards = Vec<Box<dyn Guard>>;
@@ -72,16 +67,17 @@ impl Scope {
     /// Create a new scope
     #[must_use]
     pub fn new(path: &str) -> Scope {
-        let fref = Rc::new(RefCell::new(None));
+        let factory_ref = Rc::new(RefCell::new(None));
+
         Scope {
-            endpoint: ScopeEndpoint::new(fref.clone()),
+            endpoint: ScopeEndpoint::new(Rc::clone(&factory_ref)),
             rdef: path.to_string(),
             app_data: None,
             guards: Vec::new(),
             services: Vec::new(),
             default: None,
             external: Vec::new(),
-            factory_ref: fref,
+            factory_ref,
         }
     }
 }
@@ -121,40 +117,38 @@ where
         self
     }
 
-    /// Set or override application data. Application data could be accessed
-    /// by using `Data<T>` extractor where `T` is data type.
-    ///
-    /// ```
-    /// use std::cell::Cell;
-    /// use actix_web::{web, App, HttpResponse, Responder};
-    ///
-    /// struct MyData {
-    ///     counter: Cell<usize>,
-    /// }
-    ///
-    /// async fn index(data: web::Data<MyData>) -> impl Responder {
-    ///     data.counter.set(data.counter.get() + 1);
-    ///     HttpResponse::Ok()
-    /// }
-    ///
-    /// fn main() {
-    ///     let app = App::new().service(
-    ///         web::scope("/app")
-    ///             .data(MyData{ counter: Cell::new(0) })
-    ///             .service(
-    ///                 web::resource("/index.html").route(
-    ///                     web::get().to(index)))
-    ///     );
-    /// }
-    /// ```
-    #[deprecated(since = "4.0.0", note = "Use `.app_data(Data::new(val))` instead.")]
-    pub fn data<U: 'static>(self, data: U) -> Self {
-        self.app_data(Data::new(data))
-    }
-
     /// Add scope data.
     ///
-    /// Data of different types from parent contexts will still be accessible.
+    /// Data of different types from parent contexts will still be accessible. Any `Data<T>` types
+    /// set here can be extracted in handlers using the `Data<T>` extractor.
+    ///
+    /// # Examples
+    /// ```
+    /// use std::cell::Cell;
+    /// use actix_web::{web, App, HttpRequest, HttpResponse, Responder};
+    ///
+    /// struct MyData {
+    ///     count: std::cell::Cell<usize>,
+    /// }
+    ///
+    /// async fn handler(req: HttpRequest, counter: web::Data<MyData>) -> impl Responder {
+    ///     // note this cannot use the Data<T> extractor because it was not added with it
+    ///     let incr = *req.app_data::<usize>().unwrap();
+    ///     assert_eq!(incr, 3);
+    ///
+    ///     // update counter using other value from app data
+    ///     counter.count.set(counter.count.get() + incr);
+    ///
+    ///     HttpResponse::Ok().body(counter.count.get().to_string())
+    /// }
+    ///
+    /// let app = App::new().service(
+    ///     web::scope("/app")
+    ///         .app_data(3usize)
+    ///         .app_data(web::Data::new(MyData { count: Default::default() }))
+    ///         .route("/", web::get().to(handler))
+    /// );
+    /// ```
     pub fn app_data<U: 'static>(mut self, data: U) -> Self {
         self.app_data
             .get_or_insert_with(Extensions::new)
@@ -163,15 +157,20 @@ where
         self
     }
 
-    /// Run external configuration as part of the scope building
-    /// process
+    /// Add scope data after wrapping in `Data<T>`.
     ///
-    /// This function is useful for moving parts of configuration to a
-    /// different module or even library. For example,
-    /// some of the resource's configuration could be moved to different module.
+    /// Deprecated in favor of [`app_data`](Self::app_data).
+    #[deprecated(since = "4.0.0", note = "Use `.app_data(Data::new(val))` instead.")]
+    pub fn data<U: 'static>(self, data: U) -> Self {
+        self.app_data(Data::new(data))
+    }
+
+    /// Run external configuration as part of the scope building process.
+    ///
+    /// This function is useful for moving parts of configuration to a different module or library.
+    /// For example, some of the resource's configuration could be moved to different module.
     ///
     /// ```
-    /// # extern crate actix_web;
     /// use actix_web::{web, middleware, App, HttpResponse};
     ///
     /// // this function could be located in different module
@@ -192,18 +191,21 @@ where
     ///         .route("/index.html", web::get().to(|| HttpResponse::Ok()));
     /// }
     /// ```
-    pub fn configure<F>(mut self, f: F) -> Self
+    pub fn configure<F>(mut self, cfg_fn: F) -> Self
     where
         F: FnOnce(&mut ServiceConfig),
     {
         let mut cfg = ServiceConfig::new();
-        f(&mut cfg);
+        cfg_fn(&mut cfg);
+
         self.services.extend(cfg.services);
         self.external.extend(cfg.external);
 
+        // TODO: add Extensions::is_empty check and conditionally insert data
         self.app_data
             .get_or_insert_with(Extensions::new)
             .extend(cfg.app_data);
+
         self
     }
 
@@ -420,7 +422,7 @@ where
         let mut rmap = ResourceMap::new(ResourceDef::root_prefix(&self.rdef));
 
         // external resources
-        for mut rdef in std::mem::take(&mut self.external) {
+        for mut rdef in mem::take(&mut self.external) {
             rmap.add(&mut rdef, None);
         }
 
@@ -992,7 +994,7 @@ mod tests {
         );
     }
 
-    // allow deprecated App::data
+    // allow deprecated {App, Scope}::data
     #[allow(deprecated)]
     #[actix_rt::test]
     async fn test_override_data() {
@@ -1012,7 +1014,7 @@ mod tests {
         assert_eq!(resp.status(), StatusCode::OK);
     }
 
-    // allow deprecated App::data
+    // allow deprecated `{App, Scope}::data`
     #[allow(deprecated)]
     #[actix_rt::test]
     async fn test_override_data_default_service() {
diff --git a/src/service.rs b/src/service.rs
index 259a15870..9e9f9986a 100644
--- a/src/service.rs
+++ b/src/service.rs
@@ -17,9 +17,8 @@ use crate::{
     dev::insert_slash,
     guard::Guard,
     info::ConnectionInfo,
-    request::HttpRequest,
     rmap::ResourceMap,
-    Error, HttpResponse,
+    Error, HttpRequest, HttpResponse,
 };
 
 pub trait HttpServiceFactory {
@@ -81,6 +80,12 @@ impl ServiceRequest {
         (self.req, self.payload)
     }
 
+    /// Get mutable access to inner `HttpRequest` and `Payload`
+    #[inline]
+    pub fn parts_mut(&mut self) -> (&mut HttpRequest, &mut Payload) {
+        (&mut self.req, &mut self.payload)
+    }
+
     /// Construct request from parts.
     #[must_use]
     pub fn from_parts(req: HttpRequest, payload: Payload) -> Self {
diff --git a/src/types/form.rs b/src/types/form.rs
index d4bfa4034..9c852f34f 100644
--- a/src/types/form.rs
+++ b/src/types/form.rs
@@ -81,6 +81,10 @@ use crate::{
 ///     })
 /// }
 /// ```
+///
+/// # Panics
+/// URL encoded forms consist of unordered `key=value` pairs, therefore they cannot be decoded into
+/// any type which depends upon data ordering (eg. tuples). Trying to do so will result in a panic.
 #[derive(PartialEq, Eq, PartialOrd, Ord, Debug)]
 pub struct Form<T>(pub T);
 
diff --git a/src/web.rs b/src/web.rs
index b7f21ca2c..0e27ad6b9 100644
--- a/src/web.rs
+++ b/src/web.rs
@@ -43,7 +43,6 @@ pub use crate::types::*;
 /// the exposed `Params` object:
 ///
 /// ```
-/// # extern crate actix_web;
 /// use actix_web::{web, App, HttpResponse};
 ///
 /// let app = App::new().service(
diff --git a/tests/test_error_propagation.rs b/tests/test_error_propagation.rs
new file mode 100644
index 000000000..3e7320920
--- /dev/null
+++ b/tests/test_error_propagation.rs
@@ -0,0 +1,99 @@
+use std::sync::Arc;
+
+use actix_utils::future::{ok, Ready};
+use actix_web::{
+    dev::{forward_ready, Service, ServiceRequest, ServiceResponse, Transform},
+    get,
+    test::{call_service, init_service, TestRequest},
+    ResponseError,
+};
+use futures_core::future::LocalBoxFuture;
+use futures_util::lock::Mutex;
+
+#[derive(Debug, Clone)]
+pub struct MyError;
+
+impl ResponseError for MyError {}
+
+impl std::fmt::Display for MyError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "A custom error")
+    }
+}
+
+#[get("/test")]
+async fn test() -> Result<actix_web::HttpResponse, actix_web::error::Error> {
+    return Err(MyError.into());
+}
+
+#[derive(Clone)]
+pub struct SpyMiddleware(Arc<Mutex<Option<bool>>>);
+
+impl<S, B> Transform<S, ServiceRequest> for SpyMiddleware
+where
+    S: Service<ServiceRequest, Response = ServiceResponse<B>, Error = actix_web::Error>,
+    S::Future: 'static,
+    B: 'static,
+{
+    type Response = ServiceResponse<B>;
+    type Error = actix_web::Error;
+    type Transform = Middleware<S>;
+    type InitError = ();
+    type Future = Ready<Result<Self::Transform, Self::InitError>>;
+
+    fn new_transform(&self, service: S) -> Self::Future {
+        ok(Middleware {
+            was_error: self.0.clone(),
+            service,
+        })
+    }
+}
+
+#[doc(hidden)]
+pub struct Middleware<S> {
+    was_error: Arc<Mutex<Option<bool>>>,
+    service: S,
+}
+
+impl<S, B> Service<ServiceRequest> for Middleware<S>
+where
+    S: Service<ServiceRequest, Response = ServiceResponse<B>, Error = actix_web::Error>,
+    S::Future: 'static,
+    B: 'static,
+{
+    type Response = ServiceResponse<B>;
+    type Error = actix_web::Error;
+    type Future = LocalBoxFuture<'static, Result<Self::Response, Self::Error>>;
+
+    forward_ready!(service);
+
+    fn call(&self, req: ServiceRequest) -> Self::Future {
+        let lock = self.was_error.clone();
+        let response_future = self.service.call(req);
+        Box::pin(async move {
+            let response = response_future.await;
+            if let Ok(success) = &response {
+                *lock.lock().await = Some(success.response().error().is_some());
+            }
+            response
+        })
+    }
+}
+
+#[actix_rt::test]
+async fn error_cause_should_be_propagated_to_middlewares() {
+    let lock = Arc::new(Mutex::new(None));
+    let spy_middleware = SpyMiddleware(lock.clone());
+
+    let app = init_service(
+        actix_web::App::new()
+            .wrap(spy_middleware.clone())
+            .service(test),
+    )
+    .await;
+
+    call_service(&app, TestRequest::with_uri("/test").to_request()).await;
+
+    let was_error_captured = lock.lock().await.unwrap();
+    assert!(was_error_captured);
+}
diff --git a/tests/test_server.rs b/tests/test_server.rs
index 9131d1f29..afea39dd9 100644
--- a/tests/test_server.rs
+++ b/tests/test_server.rs
@@ -879,7 +879,7 @@ async fn test_brotli_encoding_large_openssl() {
     assert_eq!(bytes, Bytes::from(data));
 }
 
-#[cfg(all(feature = "rustls", feature = "openssl"))]
+#[cfg(feature = "rustls")]
 mod plus_rustls {
     use std::io::BufReader;