MiddlewareNext iterator without Box

Thanks @jsha for showing me the way.

src/agent.rs

@@ -43,7 +43,7 @@ pub struct AgentBuilder {
     #[cfg(feature = "cookies")]
     cookie_store: Option<CookieStore>,
     resolver: ArcResolver,
-    middleware: Vec<Arc<dyn Middleware + Send + Sync>>,
+    middleware: Vec<Box<dyn Middleware + Send + Sync>>,
 }
 
 /// Config as built by AgentBuilder and then static for the lifetime of the Agent.
@@ -111,7 +111,7 @@ pub(crate) struct AgentState {
     #[cfg(feature = "cookies")]
     pub(crate) cookie_tin: CookieTin,
     pub(crate) resolver: ArcResolver,
-    pub(crate) middleware: Vec<Arc<dyn Middleware + Send + Sync>>,
+    pub(crate) middleware: Vec<Box<dyn Middleware + Send + Sync>>,
 }
 
 impl Agent {
@@ -602,7 +602,7 @@ impl AgentBuilder {
     /// All requests made by the agent will use this middleware. Middleware is invoked
     /// in the order they are added to the builder.
     pub fn middleware(mut self, m: impl Middleware + Send + Sync + 'static) -> Self {
-        self.middleware.push(Arc::new(m));
+        self.middleware.push(Box::new(m));
         self
     }
 }

src/middleware.rs

@@ -1,5 +1,3 @@
-use std::sync::Arc;
-
 use crate::{Error, Request, Response};
 
 /// Chained processing of request (and response).
@@ -135,13 +133,21 @@ pub trait Middleware {
 
 /// Continuation of a [`Middleware`] chain.
 pub struct MiddlewareNext<'a> {
-    pub(crate) chain: Box<dyn Iterator<Item = Arc<dyn Middleware + Send + Sync + 'static>>>,
+    pub(crate) chain: &'a mut (dyn Iterator<Item = &'a dyn Middleware>),
+    // Since request_fn consumes the Payload<'a>, we must have an FnOnce.
+    //
+    // It's possible to get rid of this Box if we make MiddlewareNext generic
+    // over some type variable, i.e. MiddlewareNext<'a, R> where R: FnOnce...
+    // however that would "leak" to Middleware::handle introducing a complicated
+    // type signature that is totally irrelevant for someone implementing a middleware.
+    //
+    // So in the name of having a sane external API, we accept this Box.
     pub(crate) request_fn: Box<dyn FnOnce(Request) -> Result<Response, Error> + 'a>,
 }
 
 impl<'a> MiddlewareNext<'a> {
     /// Continue the middleware chain by providing (a possibly amended) [`Request`].
-    pub fn handle(mut self, request: Request) -> Result<Response, Error> {
+    pub fn handle(self, request: Request) -> Result<Response, Error> {
         if let Some(step) = self.chain.next() {
             step.handle(request, self)
         } else {

src/request.rs

@@ -7,8 +7,8 @@ use crate::body::Payload;
 use crate::header::{self, Header};
 use crate::middleware::MiddlewareNext;
 use crate::unit::{self, Unit};
-use crate::Response;
 use crate::{agent::Agent, error::Error};
+use crate::{Middleware, Response};
 
 pub type Result<T> = std::result::Result<T, Error>;
 
@@ -141,9 +141,13 @@ impl Request {
         };
 
         let response = if !self.agent.state.middleware.is_empty() {
-            let middleware = self.agent.state.middleware.clone();
-
-            let chain = Box::new(middleware.into_iter());
+            // Clone agent to get a local copy with same lifetime as Payload
+            let agent = self.agent.clone();
+            let chain = &mut agent
+                .state
+                .middleware
+                .iter()
+                .map(|mw| mw.as_ref() as &dyn Middleware);
 
             let request_fn = Box::new(request_fn);