Add E0746 explanation to the index

Author: estebank

src/librustc_error_codes/error_codes.rs

@@ -414,6 +414,7 @@ E0742: include_str!("./error_codes/E0742.md"),
 E0743: include_str!("./error_codes/E0743.md"),
 E0744: include_str!("./error_codes/E0744.md"),
 E0745: include_str!("./error_codes/E0745.md"),
+E0746: include_str!("./error_codes/E0746.md"),
 ;
 //  E0006, // merged with E0005
 //  E0008, // cannot bind by-move into a pattern guard
@@ -608,5 +609,4 @@ E0745: include_str!("./error_codes/E0745.md"),
     E0726, // non-explicit (not `'_`) elided lifetime in unsupported position
     E0727, // `async` generators are not yet supported
     E0739, // invalid track_caller application/syntax
-    E0746, // `dyn Trait` return type
 }

src/librustc_error_codes/error_codes/E0746.md

@@ -0,0 +1,137 @@
+Return types cannot be `dyn Trait`s as they must be `Sized`.
+
+Erroneous code example:
+
+```compile_fail,E0746
+trait T {
+    fn bar(&self);
+}
+struct S(usize);
+impl T for S {
+    fn bar(&self) {}
+}
+
+// Having the trait `T` as return type is invalid because bare traits do not
+have a statically known size:
+fn foo() -> dyn T {
+    S(42)
+}
+```
+
+To avoid the error there are a couple of options.
+
+If there is a single type involved, you can use [`impl Trait`]:
+
+```
+# trait T {
+#     fn bar(&self);
+# }
+# struct S(usize);
+# impl T for S {
+#     fn bar(&self) {}
+# }
+// The compiler will select `S(usize)` as the materialized return type of this
+// function, but callers will only be able to access associated items from `T`.
+fn foo() -> impl T {
+    S(42)
+}
+```
+
+If there are multiple types involved, the only way you care to interact with
+them is through the trait's interface and having to rely on dynamic dispatch is
+acceptable, then you can use [trait objects] with `Box`, or other container
+types like `Rc` or `Arc`:
+
+```
+# trait T {
+#     fn bar(&self);
+# }
+# struct S(usize);
+# impl T for S {
+#     fn bar(&self) {}
+# }
+struct O(&'static str);
+impl T for O {
+    fn bar(&self) {}
+}
+
+// This now returns a "trait object" and callers are only be able to access
+// associated items from `T`.
+fn foo(x: bool) -> Box<dyn T> {
+    if x {
+        Box::new(S(42))
+    } else {
+        Box::new(O("val"))
+    }
+}
+```
+
+Finally, if you wish to still be able to access the original type, you can
+create a new `enum` with a variant for each type:
+
+```
+# trait T {
+#     fn bar(&self);
+# }
+# struct S(usize);
+# impl T for S {
+#     fn bar(&self) {}
+# }
+# struct O(&'static str);
+# impl T for O {
+#     fn bar(&self) {}
+# }
+enum E {
+    S(S),
+    O(O),
+}
+
+// The caller can access the original types directly, but it needs to match on
+// the returned `enum E`.
+fn foo(x: bool) -> E {
+    if x {
+        E::S(S(42))
+    } else {
+        E::O(O("val"))
+    }
+}
+```
+
+You can even implement the `trait` on the returned `enum` so the callers
+*don't* have to match on the returned value to invoke the associated items:
+
+```
+# trait T {
+#     fn bar(&self);
+# }
+# struct S(usize);
+# impl T for S {
+#     fn bar(&self) {}
+# }
+# struct O(&'static str);
+# impl T for O {
+#     fn bar(&self) {}
+# }
+# enum E {
+#     S(S),
+#     O(O),
+# }
+impl T for E {
+    fn bar(&self) {
+        match self {
+            E::S(s) => s.bar(),
+            E::O(o) => o.bar(),
+        }
+    }
+}
+```
+
+If you decide to use trait objects, be aware that these rely on
+[dynamic dispatch], which has performance implications, as the compiler needs
+to emit code that will figure out which method to call *at runtime* instead of
+during compilation. Using trait objects we are trading flexibility for
+performance.
+
+[`impl Trait`]: https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits
+[trait objects]: https://doc.rust-lang.org/book/ch17-02-trait-objects.html#using-trait-objects-that-allow-for-values-of-different-types
+[dynamic dispatch]: https://doc.rust-lang.org/book/ch17-02-trait-objects.html#trait-objects-perform-dynamic-dispatch

src/test/ui/error-codes/E0746.stderr

@@ -24,3 +24,4 @@ LL | fn bar() -> impl Trait {
 
 error: aborting due to 2 previous errors
 
+For more information about this error, try `rustc --explain E0746`.

src/test/ui/impl-trait/dyn-trait-return-should-be-impl-trait.stderr

@@ -116,5 +116,5 @@ LL | fn bat() -> impl Trait {
 
 error: aborting due to 9 previous errors
 
-Some errors have detailed explanations: E0277, E0308.
+Some errors have detailed explanations: E0277, E0308, E0746.
 For more information about an error, try `rustc --explain E0277`.