Fix FP in fn_to_numeric_cast_with_truncation

We only want this lint to check casts to numeric, as per the lint title.
Rust already has a built-in check for all other casts
[here][rust_check].

[rust_check]: https://github.com/rust-lang/rust/blob/5472b0718f286266ab89acdf234c3552de7e973c/src/librustc_typeck/check/cast.rs#L430-L433

Author: phansch

clippy_lints/src/types.rs

@@ -1088,6 +1088,11 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for CastPass {
 }
 
 fn lint_fn_to_numeric_cast(cx: &LateContext<'_, '_>, expr: &Expr, cast_expr: &Expr, cast_from: Ty<'_>, cast_to: Ty<'_>) {
+    // We only want to check casts to `ty::Uint` or `ty::Int`
+    match cast_to.sty {
+        ty::Uint(_) | ty::Int(..) => { // continue on },
+        _ => return
+    }
     match cast_from.sty {
         ty::FnDef(..) | ty::FnPtr(_) => {
             let from_snippet = snippet(cx, cast_expr.span, "x");

tests/ui/fn_to_numeric_cast.rs

@@ -31,6 +31,10 @@ fn test_function_to_numeric_cast() {
 
     // Casting to usize is OK and should not warn
     let _ = foo as usize;
+
+    // Cast `f` (a `FnDef`) to `fn()` should not warn
+    fn f() {}
+    let _ = f as fn();
 }
 
 fn test_function_var_to_numeric_cast() {