Add lint on cast Fn to numerical.

clippy_lints/src/types.rs

@@ -975,6 +975,22 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for CastPass {
                     },
                 }
             }
+
+            match &cast_from.sty {
+                ty::TyFnDef(..) |
+                ty::TyFnPtr(..) => {
+                    if cast_to.is_numeric() && cast_to.sty != ty::TyUint(UintTy::Usize){
+                        span_lint(
+                            cx,
+                            UNNECESSARY_CAST,
+                            expr.span,
+                            "casting Fn not to usize may truncate the value",
+                        );
+                    }
+                }
+                _ => ()
+            }
+
             if_chain!{
                 if let ty::TyRawPtr(from_ptr_ty) = &cast_from.sty;
                 if let ty::TyRawPtr(to_ptr_ty) = &cast_to.sty;

tests/ui/types_fn_to_int.rs

@@ -0,0 +1,10 @@
+#![feature(tool_attributes)]
+enum Foo {
+    A(usize),
+    B
+}
+
+fn main() {
+    let x = Foo::A;
+    let y = x as i32;
+}

tests/ui/types_fn_to_int.stderr

@@ -0,0 +1,10 @@
+error: casting Fn not to usize may truncate the value
+ --> $DIR/types_fn_to_int.rs:9:13
+  |
+9 |     let y = x as i32;
+  |             ^^^^^^^^
+  |
+  = note: `-D unnecessary-cast` implied by `-D warnings`
+
+error: aborting due to previous error
+