When cast needs a dereference point at full cast

After the fix of #37453 in PR #37369, instead of pointing at only the
cast type, point at the full cast span when a cast needs a dereference:

```
error: casting `&{float}` as `f32` is invalid
  --> ../../../src/test/ui/mismatched_types/cast-rfc0401.rs:81:30
   |
81 |     vec![0.0].iter().map(|s| s as f32).collect::<Vec<f32>>();
   |                              ^^^^^^^^ cannot cast `&{float}` as `f32`
   |
help: did you mean `*s`?
  --> ../../../src/test/ui/mismatched_types/cast-rfc0401.rs:81:30
   |
81 |     vec![0.0].iter().map(|s| s as f32).collect::<Vec<f32>>();
   |                              ^
```

instead of

```
error: casting `&{float}` as `f32` is invalid
  --> ../../../src/test/ui/mismatched_types/cast-rfc0401.rs:81:35
   |
81 |     vec![0.0].iter().map(|s| s as f32).collect::<Vec<f32>>();
   |                              -    ^^^
   |                              |
   |                              |
   |                              did you mean `*s`?
   |                              cannot cast `&{float}` as `f32`
```

src/librustc_typeck/check/cast.rs

@@ -142,20 +142,21 @@ impl<'a, 'gcx, 'tcx> CastCheck<'tcx> {
     fn report_cast_error(&self, fcx: &FnCtxt<'a, 'gcx, 'tcx>, e: CastError) {
         match e {
             CastError::NeedDeref => {
+                let error_span = self.span;
                 let cast_ty = fcx.ty_to_string(self.cast_ty);
-                let mut err = fcx.type_error_struct(self.cast_span,
+                let mut err = fcx.type_error_struct(error_span,
                                        |actual| {
                                            format!("casting `{}` as `{}` is invalid",
                                                    actual,
                                                    cast_ty)
                                        },
                                        self.expr_ty);
-                err.span_label(self.expr.span,
+                err.span_label(error_span,
                                &format!("cannot cast `{}` as `{}`",
                                         fcx.ty_to_string(self.expr_ty),
                                         cast_ty));
                 if let Ok(snippet) = fcx.sess().codemap().span_to_snippet(self.expr.span) {
-                    err.span_label(self.expr.span,
+                    err.span_help(self.expr.span,
                                    &format!("did you mean `*{}`?", snippet));
                 }
                 err.emit();

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

@@ -0,0 +1,82 @@
+// Copyright 2016 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 illegal_cast<U:?Sized,V:?Sized>(u: *const U) -> *const V
+{
+    u as *const V
+}
+
+fn illegal_cast_2<U:?Sized>(u: *const U) -> *const str
+{
+    u as *const str
+}
+
+trait Foo { fn foo(&self) {} }
+impl<T> Foo for T {}
+
+trait Bar { fn foo(&self) {} }
+impl<T> Bar for T {}
+
+enum E {
+    A, B
+}
+
+fn main()
+{
+    let f: f32 = 1.2;
+    let v = 0 as *const u8;
+    let fat_v : *const [u8] = unsafe { &*(0 as *const [u8; 1])};
+    let fat_sv : *const [i8] = unsafe { &*(0 as *const [i8; 1])};
+    let foo: &Foo = &f;
+
+    let _ = v as &u8;
+    let _ = v as E;
+    let _ = v as fn();
+    let _ = v as (u32,);
+    let _ = Some(&v) as *const u8;
+
+    let _ = v as f32;
+    let _ = main as f64;
+    let _ = &v as usize;
+    let _ = f as *const u8;
+    let _ = 3_i32 as bool;
+    let _ = E::A as bool;
+    let _ = 0x61u32 as char;
+
+    let _ = false as f32;
+    let _ = E::A as f32;
+    let _ = 'a' as f32;
+
+    let _ = false as *const u8;
+    let _ = E::A as *const u8;
+    let _ = 'a' as *const u8;
+
+    let _ = 42usize as *const [u8];
+    let _ = v as *const [u8];
+    let _ = fat_v as *const Foo;
+    let _ = foo as *const str;
+    let _ = foo as *mut str;
+    let _ = main as *mut str;
+    let _ = &f as *mut f32;
+    let _ = &f as *const f64;
+    let _ = fat_sv as usize;
+
+    let a : *const str = "hello";
+    let _ = a as *const Foo;
+
+    // check no error cascade
+    let _ = main.f as *const u32;
+
+    let cf: *const Foo = &0;
+    let _ = cf as *const [u16];
+    let _ = cf as *const Bar;
+
+    vec![0.0].iter().map(|s| s as f32).collect::<Vec<f32>>();
+}