Typeck: Disallow scalar casts to bare_fn.

Bare functions are another example of a scalar but non-numeric
type (like char) that should be handled separately in casts.
This disallows expressions like `0 as extern "Rust" fn() -> int;`.

It might be advantageous to allow casts between bare functions
and raw pointers in unsafe code in the future, to pass function
pointers between Rust and C.

Closes #8728

src/librustc/middle/ty.rs

@@ -2539,6 +2539,13 @@ pub fn type_is_char(ty: t) -> bool {
     }
 }
 
+pub fn type_is_bare_fn(ty: t) -> bool {
+    match get(ty).sty {
+        ty_bare_fn(*) => true,
+        _ => false
+    }
+}
+
 pub fn type_is_fp(ty: t) -> bool {
     match get(ty).sty {
       ty_infer(FloatVar(_)) | ty_float(_) => true,

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

@@ -2687,10 +2687,13 @@ pub fn check_expr_with_unifier(fcx: @mut FnCtxt,
 
                     let t1 = structurally_resolved_type(fcx, e.span, t_1);
                     let te = structurally_resolved_type(fcx, e.span, t_e);
+                    let t_1_is_scalar = type_is_scalar(fcx, expr.span, t_1);
                     let t_1_is_char = type_is_char(fcx, expr.span, t_1);
+                    let t_1_is_bare_fn = type_is_bare_fn(fcx, expr.span, t_1);
 
-                    // casts to scalars other than `char` are allowed
-                    let t_1_is_trivial = type_is_scalar(fcx, expr.span, t_1) && !t_1_is_char;
+                    // casts to scalars other than `char` and `bare fn` are trivial
+                    let t_1_is_trivial = t_1_is_scalar &&
+                        !t_1_is_char && !t_1_is_bare_fn;
 
                     if type_is_c_like_enum(fcx, expr.span, t_e) && t_1_is_trivial {
                         // casts from C-like enums are allowed
@@ -3448,6 +3451,11 @@ pub fn type_is_char(fcx: @mut FnCtxt, sp: Span, typ: ty::t) -> bool {
     return ty::type_is_char(typ_s);
 }
 
+pub fn type_is_bare_fn(fcx: @mut FnCtxt, sp: Span, typ: ty::t) -> bool {
+    let typ_s = structurally_resolved_type(fcx, sp, typ);
+    return ty::type_is_bare_fn(typ_s);
+}
+
 pub fn type_is_unsafe_ptr(fcx: @mut FnCtxt, sp: Span, typ: ty::t) -> bool {
     let typ_s = structurally_resolved_type(fcx, sp, typ);
     return ty::type_is_unsafe_ptr(typ_s);

src/test/compile-fail/cast-to-bare-fn.rs

@@ -0,0 +1,21 @@
+// Copyright 2013 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 foo(_x: int) { }
+
+#[fixed_stack_segment]
+fn main() {
+    let v: u64 = 5;
+    let x = foo as extern "C" fn() -> int;
+    //~^ ERROR non-scalar cast
+    let y = v as extern "Rust" fn(int) -> (int, int);
+    //~^ ERROR non-scalar cast
+    y(x());
+}