typeck: check casts from pointers to floats, closes rust-lang#15445

Author: fhahn

src/librustc/middle/typeck/check/mod.rs

@@ -1219,6 +1219,13 @@ fn check_cast(fcx: &FnCtxt,
                     actual,
                     fcx.infcx().ty_to_string(t_1))
         }, t_e, None);
+    } else if ty::type_is_unsafe_ptr(t_e) && t_1_is_float {
+        fcx.type_error_message(span, |actual| {
+            format!("cannot cast from pointer to float directly: `{}` as `{}`; cast through an \
+                     integer first",
+                    actual,
+                    fcx.infcx().ty_to_string(t_1))
+        }, t_e, None);
     }
 
     fcx.write_ty(id, t_1);

src/test/compile-fail/typeck-cast-pointer-to-float.rs

@@ -0,0 +1,15 @@
+// Copyright 2014 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() {
+    let x : i16 = 22;
+    ((&x) as *const i16) as f32;
+    //~^ ERROR: cannot cast from pointer to float directly: `*const i16` as `f32`
+}