Add E0604

src/librustc_typeck/check/cast.rs

@@ -205,12 +205,8 @@ fn report_cast_error(&self, fcx: &FnCtxt<'a, 'gcx, 'tcx>, e: CastError) {
                     .emit();
             }
             CastError::CastToChar => {
-                fcx.type_error_message(self.span,
-                                       |actual| {
-                                           format!("only `u8` can be cast as `char`, not `{}`",
-                                                   actual)
-                                       },
-                                       self.expr_ty);
+                struct_span_err!(fcx.tcx.sess, self.span, E0604,
+                                 "only `u8` can be cast as `char`, not `{}`", self.expr_ty).emit();
             }
             CastError::NonScalar => {
                 fcx.type_error_message(self.span,

src/librustc_typeck/diagnostics.rs

@@ -4208,6 +4208,23 @@ trait was performed.
 ```
 "##,
 
+E0604: r##"
+A cast to `char` was attempted on another type than `u8`.
+
+Erroneous code example:
+
+```compile_fail,E0604
+0u32 as char; // error: only `u8` can be cast as `char`, not `u32`
+```
+
+As the error message indicates, only `u8` can be casted into `char`. Example:
+
+```
+let c = 86u8 as char; // ok!
+assert!(c, 'V');
+```
+"##,
+
 E0609: r##"
 Attempted to access a non-existent field in a struct.
 

src/test/compile-fail/E0604.rs

@@ -0,0 +1,13 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+fn main() {
+    1u32 as char; //~ ERROR E0604
+}

src/test/ui/mismatched_types/cast-rfc0401.stderr

@@ -92,7 +92,7 @@ error[E0054]: cannot cast as `bool`
    |
    = help: compare with zero instead
 
-error: only `u8` can be cast as `char`, not `u32`
+error[E0604]: only `u8` can be cast as `char`, not `u32`
   --> $DIR/cast-rfc0401.rs:51:13
    |
 51 |     let _ = 0x61u32 as char;