Auto merge of #111717 - Urgau:uplift_fn_null_check, r=oli-obk

Uplift `clippy::fn_null_check` lint

This PR aims at uplifting the `clippy::fn_null_check` lint into rustc.

## `incorrect_fn_null_checks`

(warn-by-default)

The `incorrect_fn_null_checks` lint checks for expression that checks if a function pointer is null.

### Example

```rust
let fn_ptr: fn() = /* somehow obtained nullable function pointer */

if (fn_ptr as *const ()).is_null() { /* ... */ }
```

### Explanation

Function pointers are assumed to be non-null, checking for their nullity is incorrect.

-----

Mostly followed the instructions for uplifting a clippy lint described here: rust-lang/rust#99696 (review)

`@rustbot` label: +I-lang-nominated
r? compiler

clippy_lints/src/declared_lints.rs

@@ -171,7 +171,6 @@ pub(crate) static LINTS: &[&crate::LintInfo] = &[
     crate::float_literal::LOSSY_FLOAT_LITERAL_INFO,
     crate::floating_point_arithmetic::IMPRECISE_FLOPS_INFO,
     crate::floating_point_arithmetic::SUBOPTIMAL_FLOPS_INFO,
-    crate::fn_null_check::FN_NULL_CHECK_INFO,
     crate::format::USELESS_FORMAT_INFO,
     crate::format_args::FORMAT_IN_FORMAT_ARGS_INFO,
     crate::format_args::TO_STRING_IN_FORMAT_ARGS_INFO,

clippy_lints/src/lib.rs

@@ -131,7 +131,6 @@ mod extra_unused_type_parameters;
 mod fallible_impl_from;
 mod float_literal;
 mod floating_point_arithmetic;
-mod fn_null_check;
 mod format;
 mod format_args;
 mod format_impl;
@@ -1003,7 +1002,6 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
             semicolon_outside_block_ignore_multiline,
         ))
     });
-    store.register_late_pass(|_| Box::new(fn_null_check::FnNullCheck));
     store.register_late_pass(|_| Box::new(permissions_set_readonly_false::PermissionsSetReadonlyFalse));
     store.register_late_pass(|_| Box::new(size_of_ref::SizeOfRef));
     store.register_late_pass(|_| Box::new(multiple_unsafe_ops_per_block::MultipleUnsafeOpsPerBlock));

clippy_lints/src/renamed_lints.rs

@@ -42,6 +42,7 @@ pub static RENAMED_LINTS: &[(&str, &str)] = &[
     ("clippy::for_loops_over_fallibles", "for_loops_over_fallibles"),
     ("clippy::forget_copy", "forgetting_copy_types"),
     ("clippy::forget_ref", "forgetting_references"),
+    ("clippy::fn_null_check", "incorrect_fn_null_checks"),
     ("clippy::into_iter_on_array", "array_into_iter"),
     ("clippy::invalid_atomic_ordering", "invalid_atomic_ordering"),
     ("clippy::invalid_ref", "invalid_value"),

tests/ui/rename.fixed

@@ -37,6 +37,7 @@
 #![allow(for_loops_over_fallibles)]
 #![allow(forgetting_copy_types)]
 #![allow(forgetting_references)]
+#![allow(incorrect_fn_null_checks)]
 #![allow(array_into_iter)]
 #![allow(invalid_atomic_ordering)]
 #![allow(invalid_value)]
@@ -89,6 +90,7 @@
 #![warn(for_loops_over_fallibles)]
 #![warn(forgetting_copy_types)]
 #![warn(forgetting_references)]
+#![warn(incorrect_fn_null_checks)]
 #![warn(array_into_iter)]
 #![warn(invalid_atomic_ordering)]
 #![warn(invalid_value)]

tests/ui/rename.rs

@@ -37,6 +37,7 @@
 #![allow(for_loops_over_fallibles)]
 #![allow(forgetting_copy_types)]
 #![allow(forgetting_references)]
+#![allow(incorrect_fn_null_checks)]
 #![allow(array_into_iter)]
 #![allow(invalid_atomic_ordering)]
 #![allow(invalid_value)]
@@ -89,6 +90,7 @@
 #![warn(clippy::for_loops_over_fallibles)]
 #![warn(clippy::forget_copy)]
 #![warn(clippy::forget_ref)]
+#![warn(clippy::fn_null_check)]
 #![warn(clippy::into_iter_on_array)]
 #![warn(clippy::invalid_atomic_ordering)]
 #![warn(clippy::invalid_ref)]