Also add label with original type for function pointers

compiler/rustc_lint/messages.ftl

@@ -449,6 +449,7 @@ lint_path_statement_no_effect = path statement with no effect
 
 lint_ptr_null_checks_fn_ptr = function pointers are not nullable, so checking them for null will always return false
     .help = wrap the function pointer inside an `Option` and use `Option::is_none` to check for null pointer value
+    .label = expression has type `{$orig_ty}`
 
 lint_ptr_null_checks_ref = references are not nullable, so checking them for null will always return false
     .label = expression has type `{$orig_ty}`

compiler/rustc_lint/src/lints.rs

@@ -618,7 +618,11 @@ pub struct ExpectationNote {
 pub enum PtrNullChecksDiag<'a> {
     #[diag(lint_ptr_null_checks_fn_ptr)]
     #[help(lint_help)]
-    FnPtr,
+    FnPtr {
+        orig_ty: Ty<'a>,
+        #[label]
+        label: Span,
+    },
     #[diag(lint_ptr_null_checks_ref)]
     Ref {
         orig_ty: Ty<'a>,

compiler/rustc_lint/src/ptr_nulls.rs

@@ -65,7 +65,7 @@ fn incorrect_check<'a>(cx: &LateContext<'a>, expr: &Expr<'_>) -> Option<PtrNullC
 
     let orig_ty = cx.typeck_results().expr_ty(expr);
     if orig_ty.is_fn() {
-        Some(PtrNullChecksDiag::FnPtr)
+        Some(PtrNullChecksDiag::FnPtr { orig_ty, label: expr.span })
     } else if orig_ty.is_ref() {
         Some(PtrNullChecksDiag::Ref { orig_ty, label: expr.span })
     } else {