Illustrating rust-lang/rust#46711

cargo.out

@@ -1,24 +1,16 @@
    Compiling futuretest v0.1.0 (file:///mnt/c/Users/Mahmoud/git/futuretest)
-error[E0271]: type mismatch resolving `<futures::FutureResult<(), std::string::String> as futures::IntoFuture>::Error == &str`
-  --> src/main.rs:13:10
+error[E0277]: the trait bound `(): futures::Future` is not satisfied
+  --> src/main.rs:17:1
    |
-13 |         .and_then(|_|
-   |          ^^^^^^^^ expected struct `std::string::String`, found &str
+17 | / fn test() -> MapErr<(), ()> {
+18 | |     future::ok(())
+19 | |         .map_err(|String| ())
+20 | | }
+   | |_^ the trait `futures::Future` is not implemented for `()`
    |
-   = note: expected type `std::string::String`
-              found type `&str`
+   = note: required by `futures::MapErr`
 
-error[E0271]: type mismatch resolving `<futures::FutureResult<(), std::string::String> as futures::IntoFuture>::Error == &str`
-  --> src/main.rs:20:10
-   |
-20 |     core.run(f).unwrap();
-   |          ^^^ expected struct `std::string::String`, found &str
-   |
-   = note: expected type `std::string::String`
-              found type `&str`
-   = note: required because of the requirements on the impl of `futures::Future` for `futures::AndThen<futures::Map<futures::MapErr<futures::FutureResult<(), ()>, [closure@src/main.rs:11:18: 11:43]>, [closure@src/main.rs:12:14: 12:67]>, futures::FutureResult<(), std::string::String>, [closure@src/main.rs:13:19: 16:14]>`
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
 error: Could not compile `futuretest`.
 

src/main.rs

@@ -1,3 +1,5 @@
+#![feature(conservative_impl_trait)]
+
 extern crate futures;
 extern crate tokio_core;
 use futures::future::{self};
@@ -7,15 +9,12 @@ use tokio_core::reactor::Core;
 fn main() {
     let mut core = Core::new().expect("Failed to initialize tokio_core reactor!");
 
-    let f = future::result(Ok(()))
-        .map_err(|()| "&'static str error")
-        .map(|_| future::result(Err("another &'static str error")))
-        .and_then(|_|
-             future::result(Ok(())
-                .map_err(|()| "String error".to_owned())
-            )
-        )
-    ;
+    let f = test();
 
     core.run(f).unwrap();
 }
+
+fn test() -> MapErr<(), ()> {
+    future::ok(())
+        .map_err(|String| ())
+}