Rollup merge of rust-lang#42787 - zackmdavis:explain_E0562, r=GuillaumeGomez

add extended information for E0562; impl Trait can only be a return type

r? @GuillaumeGomez

Author: Mark-Simulacrum

src/librustc_typeck/diagnostics.rs

@@ -3989,6 +3989,46 @@ let s = Simba { mother: 1, father: 0 }; // ok!
 ```
 "##,
 
+E0562: r##"
+Abstract return types (written `impl Trait` for some trait `Trait`) are only
+allowed as function return types.
+
+Erroneous code example:
+
+```compile_fail,E0562
+#![feature(conservative_impl_trait)]
+
+fn main() {
+    let count_to_ten: impl Iterator<Item=usize> = 0..10;
+    // error: `impl Trait` not allowed outside of function and inherent method
+    //        return types
+    for i in count_to_ten {
+        println!("{}", i);
+    }
+}
+```
+
+Make sure `impl Trait` only appears in return-type position.
+
+```
+#![feature(conservative_impl_trait)]
+
+fn count_to_n(n: usize) -> impl Iterator<Item=usize> {
+    0..n
+}
+
+fn main() {
+    for i in count_to_n(10) {  // ok!
+        println!("{}", i);
+    }
+}
+```
+
+See [RFC 1522] for more details.
+
+[RFC 1522]: https://github.com/rust-lang/rfcs/blob/master/text/1522-conservative-impl-trait.md
+"##,
+
 E0570: r##"
 The requested ABI is unsupported by the current target.
 
@@ -4591,8 +4631,6 @@ register_diagnostics! {
     E0436, // functional record update requires a struct
     E0521, // redundant default implementations of trait
     E0533, // `{}` does not name a unit variant, unit struct or a constant
-    E0562, // `impl Trait` not allowed outside of function
-           // and inherent method return types
     E0563, // cannot determine a type for this `impl Trait`: {}
     E0564, // only named lifetimes are allowed in `impl Trait`,
            // but `{}` was found in the type `{}`