as_pointer_underscore: don't lint when casting fn item pointers

clippy_lints/src/casts/as_pointer_underscore.rs

@@ -1,10 +1,13 @@
 use rustc_errors::Applicability;
 use rustc_lint::LateContext;
-use rustc_middle::ty::Ty;
+use rustc_middle::ty::{self, Ty};
 
 pub fn check<'tcx>(cx: &LateContext<'tcx>, ty_into: Ty<'_>, cast_to_hir: &'tcx rustc_hir::Ty<'tcx>) {
     if let rustc_hir::TyKind::Ptr(rustc_hir::MutTy { ty, .. }) = cast_to_hir.kind
         && matches!(ty.kind, rustc_hir::TyKind::Infer(()))
+        && let ty::RawPtr(pointee, _) = ty_into.kind()
+        // skip fn item type, since it can not be explicitly expressed with syntax 
+        && !matches!(pointee.kind(), ty::FnDef(..))
     {
         clippy_utils::diagnostics::span_lint_and_sugg(
             cx,

tests/ui/as_pointer_underscore.fixed

@@ -13,3 +13,11 @@ fn g(s: &mut S) -> usize {
     s as *mut S as usize
     //~^ as_pointer_underscore
 }
+
+fn issue_15281() {
+    fn bar(_: usize) {}
+    // pointer to fn item, lint should not trigger
+    let _ = &bar as *const _;
+    let _ = &(bar as fn(usize)) as *const fn(usize);
+    //~^ as_pointer_underscore
+}

tests/ui/as_pointer_underscore.rs

@@ -13,3 +13,11 @@ fn g(s: &mut S) -> usize {
     s as *mut _ as usize
     //~^ as_pointer_underscore
 }
+
+fn issue_15281() {
+    fn bar(_: usize) {}
+    // pointer to fn item, lint should not trigger
+    let _ = &bar as *const _;
+    let _ = &(bar as fn(usize)) as *const _;
+    //~^ as_pointer_underscore
+}

tests/ui/as_pointer_underscore.stderr

@@ -13,5 +13,11 @@ error: using inferred pointer cast
 LL |     s as *mut _ as usize
    |          ^^^^^^ help: use explicit type: `*mut S`
 
-error: aborting due to 2 previous errors
+error: using inferred pointer cast
+  --> tests/ui/as_pointer_underscore.rs:21:36
+   |
+LL |     let _ = &(bar as fn(usize)) as *const _;
+   |                                    ^^^^^^^^ help: use explicit type: `*const fn(usize)`
+
+error: aborting due to 3 previous errors