From cf8f3796578b5423d9465ff0d1e8327668753cca Mon Sep 17 00:00:00 2001
From: Victor Korkin <victor.korkin@gmail.com>
Date: Mon, 28 May 2018 23:31:55 +0700
Subject: [PATCH] Add lint on cast Fn to numerical.

---
 clippy_lints/src/types.rs       | 16 ++++++++++++++++
 tests/ui/types_fn_to_int.rs     | 10 ++++++++++
 tests/ui/types_fn_to_int.stderr | 10 ++++++++++
 3 files changed, 36 insertions(+)
 create mode 100644 tests/ui/types_fn_to_int.rs
 create mode 100644 tests/ui/types_fn_to_int.stderr

diff --git a/clippy_lints/src/types.rs b/clippy_lints/src/types.rs
index 168086d574a..f5cbcfb6b00 100644
--- a/clippy_lints/src/types.rs
+++ b/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;
diff --git a/tests/ui/types_fn_to_int.rs b/tests/ui/types_fn_to_int.rs
new file mode 100644
index 00000000000..250307e54b7
--- /dev/null
+++ b/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;
+}
diff --git a/tests/ui/types_fn_to_int.stderr b/tests/ui/types_fn_to_int.stderr
new file mode 100644
index 00000000000..dffcf008302
--- /dev/null
+++ b/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
+