Add error explanations for E0066 and E0069.

This also updates the error messages for both. For E0066, it removes mention
of "managed heap", which was removed in 8a91d33. For E0069, I just tweaked
the wording to make it a bit more explicit.

src/librustc_typeck/check/mod.rs

@@ -3082,8 +3082,8 @@ fn check_expr_with_unifier<'a, 'tcx, F>(fcx: &FnCtxt<'a, 'tcx>,
           let mut checked = false;
           opt_place.as_ref().map(|place| match place.node {
               ast::ExprPath(None, ref path) => {
-                  // FIXME(pcwalton): For now we hardcode the two permissible
-                  // places: the exchange heap and the managed heap.
+                  // FIXME(pcwalton): For now we hardcode the only permissible
+                  // place: the exchange heap.
                   let definition = lookup_full_def(tcx, path.span, place.id);
                   let def_id = definition.def_id();
                   let referent_ty = fcx.expr_ty(&**subexpr);
@@ -3097,7 +3097,7 @@ fn check_expr_with_unifier<'a, 'tcx, F>(fcx: &FnCtxt<'a, 'tcx>,
 
           if !checked {
               span_err!(tcx.sess, expr.span, E0066,
-                  "only the managed heap and exchange heap are currently supported");
+                  "only the exchange heap is currently supported");
               fcx.write_ty(id, tcx.types.err);
           }
       }
@@ -3317,7 +3317,8 @@ fn check_expr_with_unifier<'a, 'tcx, F>(fcx: &FnCtxt<'a, 'tcx>,
                         if let Err(_) = fcx.mk_eqty(false, infer::Misc(expr.span),
                                                     result_type, ty::mk_nil(fcx.tcx())) {
                             span_err!(tcx.sess, expr.span, E0069,
-                                "`return;` in function returning non-nil");
+                                "`return;` in a function whose return type is \
+                                 not `()`");
                         },
                     Some(ref e) => {
                         check_expr_coercable_to_type(fcx, &**e, result_type);

src/librustc_typeck/diagnostics.rs

@@ -91,6 +91,16 @@ enum variant, one of the fields was not provided. Each field should be specified
 exactly once.
 "##,
 
+E0066: r##"
+Box placement expressions (like C++'s "placement new") do not support any
+place expression except the exchange heap (i.e. `std::boxed::HEAP`).
+Furthermore, the syntax is changing to use `in` instead of `box`. See [RFC
+470][rfc470] and [RFC 809][rfc809] for more details.
+
+[rfc470]: https://github.com/rust-lang/rfcs/pull/470
+[rfc809]: https://github.com/rust-lang/rfcs/pull/809
+"##,
+
 E0067: r##"
 The left-hand side of an assignment operator must be an lvalue expression. An
 lvalue expression represents a memory location and includes item paths (ie,
@@ -108,6 +118,21 @@ LinkedList::new() += 1;
 ```
 "##,
 
+E0069: r##"
+The compiler found a function whose body contains a `return;` statement but
+whose return type is not `()`. An example of this is:
+
+```
+// error
+fn foo() -> u8 {
+    return;
+}
+```
+
+Since `return;` is just like `return ();`, there is a mismatch between the
+function's return type and the value being returned.
+"##,
+
 E0081: r##"
 Enum discriminants are used to differentiate enum variants stored in memory.
 This error indicates that the same value was used for two or more variants,
@@ -484,9 +509,7 @@ register_diagnostics! {
     E0059,
     E0060,
     E0061,
-    E0066,
     E0068,
-    E0069,
     E0070,
     E0071,
     E0072,

src/test/compile-fail/issue-14084.rs

@@ -12,5 +12,5 @@
 
 fn main() {
     box ( () ) 0;
-    //~^ ERROR: only the managed heap and exchange heap are currently supported
+    //~^ ERROR: only the exchange heap is currently supported
 }

src/test/compile-fail/ret-non-nil.rs

@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// error-pattern: `return;` in function returning non-nil
+// error-pattern: `return;` in a function whose return type is not `()`
 
 fn f() { return; }