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: https://github.com/rust-lang/rust/pull/99696#pullrequestreview-1134072751

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

compiler/rustc_lint/messages.ftl

@@ -215,6 +215,9 @@ lint_expectation = this lint expectation is unfulfilled
     .note = the `unfulfilled_lint_expectations` lint can't be expected and will always produce this message
     .rationale = {$rationale}
 
+lint_fn_null_check = 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
+
 lint_for_loops_over_fallibles =
     for loop over {$article} `{$ty}`. This is more readably written as an `if let` statement
     .suggestion = consider using `if let` to clear intent

compiler/rustc_lint/src/fn_null_check.rs

@@ -0,0 +1,112 @@
+use crate::{lints::FnNullCheckDiag, LateContext, LateLintPass, LintContext};
+use rustc_ast::LitKind;
+use rustc_hir::{BinOpKind, Expr, ExprKind, TyKind};
+use rustc_session::{declare_lint, declare_lint_pass};
+use rustc_span::sym;
+
+declare_lint! {
+    /// The `incorrect_fn_null_checks` lint checks for expression that checks if a
+    /// function pointer is null.
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// # fn test() {}
+    /// let fn_ptr: fn() = /* somehow obtained nullable function pointer */
+    /// #   test;
+    ///
+    /// if (fn_ptr as *const ()).is_null() { /* ... */ }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// Function pointers are assumed to be non-null, checking them for null will always
+    /// return false.
+    INCORRECT_FN_NULL_CHECKS,
+    Warn,
+    "incorrect checking of null function pointer"
+}
+
+declare_lint_pass!(IncorrectFnNullChecks => [INCORRECT_FN_NULL_CHECKS]);
+
+fn is_fn_ptr_cast(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
+    let mut expr = expr.peel_blocks();
+    let mut had_at_least_one_cast = false;
+    while let ExprKind::Cast(cast_expr, cast_ty) = expr.kind
+            && let TyKind::Ptr(_) = cast_ty.kind {
+        expr = cast_expr.peel_blocks();
+        had_at_least_one_cast = true;
+    }
+    had_at_least_one_cast && cx.typeck_results().expr_ty_adjusted(expr).is_fn()
+}
+
+impl<'tcx> LateLintPass<'tcx> for IncorrectFnNullChecks {
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
+        match expr.kind {
+            // Catching:
+            // <*<const/mut> <ty>>::is_null(fn_ptr as *<const/mut> <ty>)
+            ExprKind::Call(path, [arg])
+                if let ExprKind::Path(ref qpath) = path.kind
+                    && let Some(def_id) = cx.qpath_res(qpath, path.hir_id).opt_def_id()
+                    && matches!(
+                        cx.tcx.get_diagnostic_name(def_id),
+                        Some(sym::ptr_const_is_null | sym::ptr_is_null)
+                    )
+                    && is_fn_ptr_cast(cx, arg) =>
+            {
+                cx.emit_spanned_lint(INCORRECT_FN_NULL_CHECKS, expr.span, FnNullCheckDiag)
+            }
+
+            // Catching:
+            // (fn_ptr as *<const/mut> <ty>).is_null()
+            ExprKind::MethodCall(_, receiver, _, _)
+                if let Some(def_id) = cx.typeck_results().type_dependent_def_id(expr.hir_id)
+                    && matches!(
+                        cx.tcx.get_diagnostic_name(def_id),
+                        Some(sym::ptr_const_is_null | sym::ptr_is_null)
+                    )
+                    && is_fn_ptr_cast(cx, receiver) =>
+            {
+                cx.emit_spanned_lint(INCORRECT_FN_NULL_CHECKS, expr.span, FnNullCheckDiag)
+            }
+
+            ExprKind::Binary(op, left, right) if matches!(op.node, BinOpKind::Eq) => {
+                let to_check: &Expr<'_>;
+                if is_fn_ptr_cast(cx, left) {
+                    to_check = right;
+                } else if is_fn_ptr_cast(cx, right) {
+                    to_check = left;
+                } else {
+                    return;
+                }
+
+                match to_check.kind {
+                    // Catching:
+                    // (fn_ptr as *<const/mut> <ty>) == (0 as <ty>)
+                    ExprKind::Cast(cast_expr, _)
+                        if let ExprKind::Lit(spanned) = cast_expr.kind
+                            && let LitKind::Int(v, _) = spanned.node && v == 0 =>
+                    {
+                        cx.emit_spanned_lint(INCORRECT_FN_NULL_CHECKS, expr.span, FnNullCheckDiag)
+                    },
+
+                    // Catching:
+                    // (fn_ptr as *<const/mut> <ty>) == std::ptr::null()
+                    ExprKind::Call(path, [])
+                        if let ExprKind::Path(ref qpath) = path.kind
+                            && let Some(def_id) = cx.qpath_res(qpath, path.hir_id).opt_def_id()
+                            && let Some(diag_item) = cx.tcx.get_diagnostic_name(def_id)
+                            && (diag_item == sym::ptr_null || diag_item == sym::ptr_null_mut) =>
+                    {
+                        cx.emit_spanned_lint(INCORRECT_FN_NULL_CHECKS, expr.span, FnNullCheckDiag)
+                    },
+
+                    _ => {},
+                }
+            }
+            _ => {}
+        }
+    }
+}

compiler/rustc_lint/src/lib.rs

@@ -58,6 +58,7 @@
 mod enum_intrinsics_non_enums;
 mod errors;
 mod expect;
+mod fn_null_check;
 mod for_loops_over_fallibles;
 pub mod hidden_unicode_codepoints;
 mod internal;
@@ -102,6 +103,7 @@
 use deref_into_dyn_supertrait::*;
 use drop_forget_useless::*;
 use enum_intrinsics_non_enums::EnumIntrinsicsNonEnums;
+use fn_null_check::*;
 use for_loops_over_fallibles::*;
 use hidden_unicode_codepoints::*;
 use internal::*;
@@ -225,6 +227,7 @@ fn lint_mod(tcx: TyCtxt<'_>, module_def_id: LocalDefId) {
             // Depends on types used in type definitions
             MissingCopyImplementations: MissingCopyImplementations,
             // Depends on referenced function signatures in expressions
+            IncorrectFnNullChecks: IncorrectFnNullChecks,
             MutableTransmutes: MutableTransmutes,
             TypeAliasBounds: TypeAliasBounds,
             TrivialConstraints: TrivialConstraints,

compiler/rustc_lint/src/lints.rs

@@ -613,6 +613,12 @@ pub struct ExpectationNote {
     pub rationale: Symbol,
 }
 
+// fn_null_check.rs
+#[derive(LintDiagnostic)]
+#[diag(lint_fn_null_check)]
+#[help]
+pub struct FnNullCheckDiag;
+
 // for_loops_over_fallibles.rs
 #[derive(LintDiagnostic)]
 #[diag(lint_for_loops_over_fallibles)]

compiler/rustc_span/src/symbol.rs

@@ -1152,8 +1152,10 @@
         profiler_runtime,
         ptr,
         ptr_cast_mut,
+        ptr_const_is_null,
         ptr_from_ref,
         ptr_guaranteed_cmp,
+        ptr_is_null,
         ptr_mask,
         ptr_null,
         ptr_null_mut,

library/core/src/ptr/const_ptr.rs

@@ -30,6 +30,7 @@ impl<T: ?Sized> *const T {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_const_unstable(feature = "const_ptr_is_null", issue = "74939")]
+    #[rustc_diagnostic_item = "ptr_const_is_null"]
     #[inline]
     pub const fn is_null(self) -> bool {
         #[inline]

library/core/src/ptr/mut_ptr.rs

@@ -29,6 +29,7 @@ impl<T: ?Sized> *mut T {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_const_unstable(feature = "const_ptr_is_null", issue = "74939")]
+    #[rustc_diagnostic_item = "ptr_is_null"]
     #[inline]
     pub const fn is_null(self) -> bool {
         #[inline]

src/tools/clippy/clippy_lints/src/declared_lints.rs

@@ -171,7 +171,6 @@
     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,

src/tools/clippy/clippy_lints/src/fn_null_check.rs

@@ -1,102 +0,0 @@
-use clippy_utils::consts::{constant, Constant};
-use clippy_utils::diagnostics::span_lint_and_help;
-use clippy_utils::{is_integer_literal, is_path_diagnostic_item};
-use rustc_hir::{BinOpKind, Expr, ExprKind, TyKind};
-use rustc_lint::{LateContext, LateLintPass};
-use rustc_session::{declare_lint_pass, declare_tool_lint};
-use rustc_span::sym;
-
-declare_clippy_lint! {
-    /// ### What it does
-    /// Checks for comparing a function pointer to null.
-    ///
-    /// ### Why is this bad?
-    /// Function pointers are assumed to not be null.
-    ///
-    /// ### Example
-    /// ```rust,ignore
-    /// let fn_ptr: fn() = /* somehow obtained nullable function pointer */
-    ///
-    /// if (fn_ptr as *const ()).is_null() { ... }
-    /// ```
-    /// Use instead:
-    /// ```rust,ignore
-    /// let fn_ptr: Option<fn()> = /* somehow obtained nullable function pointer */
-    ///
-    /// if fn_ptr.is_none() { ... }
-    /// ```
-    #[clippy::version = "1.68.0"]
-    pub FN_NULL_CHECK,
-    correctness,
-    "`fn()` type assumed to be nullable"
-}
-declare_lint_pass!(FnNullCheck => [FN_NULL_CHECK]);
-
-fn lint_expr(cx: &LateContext<'_>, expr: &Expr<'_>) {
-    span_lint_and_help(
-        cx,
-        FN_NULL_CHECK,
-        expr.span,
-        "function pointer assumed to be nullable, even though it isn't",
-        None,
-        "try wrapping your function pointer type in `Option<T>` instead, and using `is_none` to check for null pointer value",
-    );
-}
-
-fn is_fn_ptr_cast(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
-    if let ExprKind::Cast(cast_expr, cast_ty) = expr.kind
-        && let TyKind::Ptr(_) = cast_ty.kind
-    {
-        cx.typeck_results().expr_ty_adjusted(cast_expr).is_fn()
-    } else {
-        false
-    }
-}
-
-impl<'tcx> LateLintPass<'tcx> for FnNullCheck {
-    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
-        match expr.kind {
-            // Catching:
-            // (fn_ptr as *<const/mut> <ty>).is_null()
-            ExprKind::MethodCall(method_name, receiver, _, _)
-                if method_name.ident.as_str() == "is_null" && is_fn_ptr_cast(cx, receiver) =>
-            {
-                lint_expr(cx, expr);
-            },
-
-            ExprKind::Binary(op, left, right) if matches!(op.node, BinOpKind::Eq) => {
-                let to_check: &Expr<'_>;
-                if is_fn_ptr_cast(cx, left) {
-                    to_check = right;
-                } else if is_fn_ptr_cast(cx, right) {
-                    to_check = left;
-                } else {
-                    return;
-                }
-
-                match to_check.kind {
-                    // Catching:
-                    // (fn_ptr as *<const/mut> <ty>) == (0 as <ty>)
-                    ExprKind::Cast(cast_expr, _) if is_integer_literal(cast_expr, 0) => {
-                        lint_expr(cx, expr);
-                    },
-
-                    // Catching:
-                    // (fn_ptr as *<const/mut> <ty>) == std::ptr::null()
-                    ExprKind::Call(func, []) if is_path_diagnostic_item(cx, func, sym::ptr_null) => {
-                        lint_expr(cx, expr);
-                    },
-
-                    // Catching:
-                    // (fn_ptr as *<const/mut> <ty>) == <const that evaluates to null_ptr>
-                    _ if matches!(constant(cx, cx.typeck_results(), to_check), Some(Constant::RawPtr(0))) => {
-                        lint_expr(cx, expr);
-                    },
-
-                    _ => {},
-                }
-            },
-            _ => {},
-        }
-    }
-}

src/tools/clippy/clippy_lints/src/lib.rs

@@ -131,7 +131,6 @@
 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));

src/tools/clippy/clippy_lints/src/renamed_lints.rs

@@ -42,6 +42,7 @@
     ("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"),

src/tools/clippy/tests/ui/fn_null_check.rs

@@ -1,22 +0,0 @@
-#![allow(unused)]
-#![warn(clippy::fn_null_check)]
-#![allow(clippy::cmp_null)]
-#![allow(clippy::needless_if)]
-#![allow(clippy::ptr_eq)]
-#![allow(clippy::zero_ptr)]
-
-pub const ZPTR: *const () = 0 as *const _;
-pub const NOT_ZPTR: *const () = 1 as *const _;
-
-fn main() {
-    let fn_ptr = main;
-
-    if (fn_ptr as *mut ()).is_null() {}
-    if (fn_ptr as *const u8).is_null() {}
-    if (fn_ptr as *const ()) == std::ptr::null() {}
-    if (fn_ptr as *const ()) == (0 as *const ()) {}
-    if (fn_ptr as *const ()) == ZPTR {}
-
-    // no lint
-    if (fn_ptr as *const ()) == NOT_ZPTR {}
-}

src/tools/clippy/tests/ui/fn_null_check.stderr

@@ -1,43 +0,0 @@
-error: function pointer assumed to be nullable, even though it isn't
-  --> $DIR/fn_null_check.rs:14:8
-   |
-LL |     if (fn_ptr as *mut ()).is_null() {}
-   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
-   = help: try wrapping your function pointer type in `Option<T>` instead, and using `is_none` to check for null pointer value
-   = note: `-D clippy::fn-null-check` implied by `-D warnings`
-
-error: function pointer assumed to be nullable, even though it isn't
-  --> $DIR/fn_null_check.rs:15:8
-   |
-LL |     if (fn_ptr as *const u8).is_null() {}
-   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
-   = help: try wrapping your function pointer type in `Option<T>` instead, and using `is_none` to check for null pointer value
-
-error: function pointer assumed to be nullable, even though it isn't
-  --> $DIR/fn_null_check.rs:16:8
-   |
-LL |     if (fn_ptr as *const ()) == std::ptr::null() {}
-   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
-   = help: try wrapping your function pointer type in `Option<T>` instead, and using `is_none` to check for null pointer value
-
-error: function pointer assumed to be nullable, even though it isn't
-  --> $DIR/fn_null_check.rs:17:8
-   |
-LL |     if (fn_ptr as *const ()) == (0 as *const ()) {}
-   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
-   = help: try wrapping your function pointer type in `Option<T>` instead, and using `is_none` to check for null pointer value
-
-error: function pointer assumed to be nullable, even though it isn't
-  --> $DIR/fn_null_check.rs:18:8
-   |
-LL |     if (fn_ptr as *const ()) == ZPTR {}
-   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
-   = help: try wrapping your function pointer type in `Option<T>` instead, and using `is_none` to check for null pointer value
-
-error: aborting due to 5 previous errors
-

src/tools/clippy/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)]

src/tools/clippy/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)]

src/tools/clippy/tests/ui/rename.stderr

@@ -1,5 +1,5 @@
 error: lint `clippy::almost_complete_letter_range` has been renamed to `clippy::almost_complete_range`
-  --> $DIR/rename.rs:52:9
+  --> $DIR/rename.rs:53:9
    |
 LL | #![warn(clippy::almost_complete_letter_range)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `clippy::almost_complete_range`
@@ -7,310 +7,316 @@ LL | #![warn(clippy::almost_complete_letter_range)]
    = note: `-D renamed-and-removed-lints` implied by `-D warnings`
 
 error: lint `clippy::blacklisted_name` has been renamed to `clippy::disallowed_names`
-  --> $DIR/rename.rs:53:9
+  --> $DIR/rename.rs:54:9
    |
 LL | #![warn(clippy::blacklisted_name)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `clippy::disallowed_names`
 
 error: lint `clippy::block_in_if_condition_expr` has been renamed to `clippy::blocks_in_if_conditions`
-  --> $DIR/rename.rs:54:9
+  --> $DIR/rename.rs:55:9
    |
 LL | #![warn(clippy::block_in_if_condition_expr)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `clippy::blocks_in_if_conditions`
 
 error: lint `clippy::block_in_if_condition_stmt` has been renamed to `clippy::blocks_in_if_conditions`
-  --> $DIR/rename.rs:55:9
+  --> $DIR/rename.rs:56:9
    |
 LL | #![warn(clippy::block_in_if_condition_stmt)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `clippy::blocks_in_if_conditions`
 
 error: lint `clippy::box_vec` has been renamed to `clippy::box_collection`
-  --> $DIR/rename.rs:56:9
+  --> $DIR/rename.rs:57:9
    |
 LL | #![warn(clippy::box_vec)]
    |         ^^^^^^^^^^^^^^^ help: use the new name: `clippy::box_collection`
 
 error: lint `clippy::const_static_lifetime` has been renamed to `clippy::redundant_static_lifetimes`
-  --> $DIR/rename.rs:57:9
+  --> $DIR/rename.rs:58:9
    |
 LL | #![warn(clippy::const_static_lifetime)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `clippy::redundant_static_lifetimes`
 
 error: lint `clippy::cyclomatic_complexity` has been renamed to `clippy::cognitive_complexity`
-  --> $DIR/rename.rs:58:9
+  --> $DIR/rename.rs:59:9
    |
 LL | #![warn(clippy::cyclomatic_complexity)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `clippy::cognitive_complexity`
 
 error: lint `clippy::derive_hash_xor_eq` has been renamed to `clippy::derived_hash_with_manual_eq`
-  --> $DIR/rename.rs:59:9
+  --> $DIR/rename.rs:60:9
    |
 LL | #![warn(clippy::derive_hash_xor_eq)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `clippy::derived_hash_with_manual_eq`
 
 error: lint `clippy::disallowed_method` has been renamed to `clippy::disallowed_methods`
-  --> $DIR/rename.rs:60:9
+  --> $DIR/rename.rs:61:9
    |
 LL | #![warn(clippy::disallowed_method)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `clippy::disallowed_methods`
 
 error: lint `clippy::disallowed_type` has been renamed to `clippy::disallowed_types`
-  --> $DIR/rename.rs:61:9
+  --> $DIR/rename.rs:62:9
    |
 LL | #![warn(clippy::disallowed_type)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `clippy::disallowed_types`
 
 error: lint `clippy::eval_order_dependence` has been renamed to `clippy::mixed_read_write_in_expression`
-  --> $DIR/rename.rs:62:9
+  --> $DIR/rename.rs:63:9
    |
 LL | #![warn(clippy::eval_order_dependence)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `clippy::mixed_read_write_in_expression`
 
 error: lint `clippy::identity_conversion` has been renamed to `clippy::useless_conversion`
-  --> $DIR/rename.rs:63:9
+  --> $DIR/rename.rs:64:9
    |
 LL | #![warn(clippy::identity_conversion)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `clippy::useless_conversion`
 
 error: lint `clippy::if_let_some_result` has been renamed to `clippy::match_result_ok`
-  --> $DIR/rename.rs:64:9
+  --> $DIR/rename.rs:65:9
    |
 LL | #![warn(clippy::if_let_some_result)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `clippy::match_result_ok`
 
 error: lint `clippy::integer_arithmetic` has been renamed to `clippy::arithmetic_side_effects`
-  --> $DIR/rename.rs:65:9
+  --> $DIR/rename.rs:66:9
    |
 LL | #![warn(clippy::integer_arithmetic)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `clippy::arithmetic_side_effects`
 
 error: lint `clippy::logic_bug` has been renamed to `clippy::overly_complex_bool_expr`
-  --> $DIR/rename.rs:66:9
+  --> $DIR/rename.rs:67:9
    |
 LL | #![warn(clippy::logic_bug)]
    |         ^^^^^^^^^^^^^^^^^ help: use the new name: `clippy::overly_complex_bool_expr`
 
 error: lint `clippy::new_without_default_derive` has been renamed to `clippy::new_without_default`
-  --> $DIR/rename.rs:67:9
+  --> $DIR/rename.rs:68:9
    |
 LL | #![warn(clippy::new_without_default_derive)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `clippy::new_without_default`
 
 error: lint `clippy::option_and_then_some` has been renamed to `clippy::bind_instead_of_map`
-  --> $DIR/rename.rs:68:9
+  --> $DIR/rename.rs:69:9
    |
 LL | #![warn(clippy::option_and_then_some)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `clippy::bind_instead_of_map`
 
 error: lint `clippy::option_expect_used` has been renamed to `clippy::expect_used`
-  --> $DIR/rename.rs:69:9
+  --> $DIR/rename.rs:70:9
    |
 LL | #![warn(clippy::option_expect_used)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `clippy::expect_used`
 
 error: lint `clippy::option_map_unwrap_or` has been renamed to `clippy::map_unwrap_or`
-  --> $DIR/rename.rs:70:9
+  --> $DIR/rename.rs:71:9
    |
 LL | #![warn(clippy::option_map_unwrap_or)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `clippy::map_unwrap_or`
 
 error: lint `clippy::option_map_unwrap_or_else` has been renamed to `clippy::map_unwrap_or`
-  --> $DIR/rename.rs:71:9
+  --> $DIR/rename.rs:72:9
    |
 LL | #![warn(clippy::option_map_unwrap_or_else)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `clippy::map_unwrap_or`
 
 error: lint `clippy::option_unwrap_used` has been renamed to `clippy::unwrap_used`
-  --> $DIR/rename.rs:72:9
+  --> $DIR/rename.rs:73:9
    |
 LL | #![warn(clippy::option_unwrap_used)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `clippy::unwrap_used`
 
 error: lint `clippy::ref_in_deref` has been renamed to `clippy::needless_borrow`
-  --> $DIR/rename.rs:73:9
+  --> $DIR/rename.rs:74:9
    |
 LL | #![warn(clippy::ref_in_deref)]
    |         ^^^^^^^^^^^^^^^^^^^^ help: use the new name: `clippy::needless_borrow`
 
 error: lint `clippy::result_expect_used` has been renamed to `clippy::expect_used`
-  --> $DIR/rename.rs:74:9
+  --> $DIR/rename.rs:75:9
    |
 LL | #![warn(clippy::result_expect_used)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `clippy::expect_used`
 
 error: lint `clippy::result_map_unwrap_or_else` has been renamed to `clippy::map_unwrap_or`
-  --> $DIR/rename.rs:75:9
+  --> $DIR/rename.rs:76:9
    |
 LL | #![warn(clippy::result_map_unwrap_or_else)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `clippy::map_unwrap_or`
 
 error: lint `clippy::result_unwrap_used` has been renamed to `clippy::unwrap_used`
-  --> $DIR/rename.rs:76:9
+  --> $DIR/rename.rs:77:9
    |
 LL | #![warn(clippy::result_unwrap_used)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `clippy::unwrap_used`
 
 error: lint `clippy::single_char_push_str` has been renamed to `clippy::single_char_add_str`
-  --> $DIR/rename.rs:77:9
+  --> $DIR/rename.rs:78:9
    |
 LL | #![warn(clippy::single_char_push_str)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `clippy::single_char_add_str`
 
 error: lint `clippy::stutter` has been renamed to `clippy::module_name_repetitions`
-  --> $DIR/rename.rs:78:9
+  --> $DIR/rename.rs:79:9
    |
 LL | #![warn(clippy::stutter)]
    |         ^^^^^^^^^^^^^^^ help: use the new name: `clippy::module_name_repetitions`
 
 error: lint `clippy::to_string_in_display` has been renamed to `clippy::recursive_format_impl`
-  --> $DIR/rename.rs:79:9
+  --> $DIR/rename.rs:80:9
    |
 LL | #![warn(clippy::to_string_in_display)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `clippy::recursive_format_impl`
 
 error: lint `clippy::zero_width_space` has been renamed to `clippy::invisible_characters`
-  --> $DIR/rename.rs:80:9
+  --> $DIR/rename.rs:81:9
    |
 LL | #![warn(clippy::zero_width_space)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `clippy::invisible_characters`
 
 error: lint `clippy::cast_ref_to_mut` has been renamed to `cast_ref_to_mut`
-  --> $DIR/rename.rs:81:9
+  --> $DIR/rename.rs:82:9
    |
 LL | #![warn(clippy::cast_ref_to_mut)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `cast_ref_to_mut`
 
 error: lint `clippy::clone_double_ref` has been renamed to `suspicious_double_ref_op`
-  --> $DIR/rename.rs:82:9
+  --> $DIR/rename.rs:83:9
    |
 LL | #![warn(clippy::clone_double_ref)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `suspicious_double_ref_op`
 
 error: lint `clippy::cmp_nan` has been renamed to `invalid_nan_comparisons`
-  --> $DIR/rename.rs:83:9
+  --> $DIR/rename.rs:84:9
    |
 LL | #![warn(clippy::cmp_nan)]
    |         ^^^^^^^^^^^^^^^ help: use the new name: `invalid_nan_comparisons`
 
 error: lint `clippy::drop_bounds` has been renamed to `drop_bounds`
-  --> $DIR/rename.rs:84:9
+  --> $DIR/rename.rs:85:9
    |
 LL | #![warn(clippy::drop_bounds)]
    |         ^^^^^^^^^^^^^^^^^^^ help: use the new name: `drop_bounds`
 
 error: lint `clippy::drop_copy` has been renamed to `dropping_copy_types`
-  --> $DIR/rename.rs:85:9
+  --> $DIR/rename.rs:86:9
    |
 LL | #![warn(clippy::drop_copy)]
    |         ^^^^^^^^^^^^^^^^^ help: use the new name: `dropping_copy_types`
 
 error: lint `clippy::drop_ref` has been renamed to `dropping_references`
-  --> $DIR/rename.rs:86:9
+  --> $DIR/rename.rs:87:9
    |
 LL | #![warn(clippy::drop_ref)]
    |         ^^^^^^^^^^^^^^^^ help: use the new name: `dropping_references`
 
 error: lint `clippy::for_loop_over_option` has been renamed to `for_loops_over_fallibles`
-  --> $DIR/rename.rs:87:9
+  --> $DIR/rename.rs:88:9
    |
 LL | #![warn(clippy::for_loop_over_option)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `for_loops_over_fallibles`
 
 error: lint `clippy::for_loop_over_result` has been renamed to `for_loops_over_fallibles`
-  --> $DIR/rename.rs:88:9
+  --> $DIR/rename.rs:89:9
    |
 LL | #![warn(clippy::for_loop_over_result)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `for_loops_over_fallibles`
 
 error: lint `clippy::for_loops_over_fallibles` has been renamed to `for_loops_over_fallibles`
-  --> $DIR/rename.rs:89:9
+  --> $DIR/rename.rs:90:9
    |
 LL | #![warn(clippy::for_loops_over_fallibles)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `for_loops_over_fallibles`
 
 error: lint `clippy::forget_copy` has been renamed to `forgetting_copy_types`
-  --> $DIR/rename.rs:90:9
+  --> $DIR/rename.rs:91:9
    |
 LL | #![warn(clippy::forget_copy)]
    |         ^^^^^^^^^^^^^^^^^^^ help: use the new name: `forgetting_copy_types`
 
 error: lint `clippy::forget_ref` has been renamed to `forgetting_references`
-  --> $DIR/rename.rs:91:9
+  --> $DIR/rename.rs:92:9
    |
 LL | #![warn(clippy::forget_ref)]
    |         ^^^^^^^^^^^^^^^^^^ help: use the new name: `forgetting_references`
 
+error: lint `clippy::fn_null_check` has been renamed to `incorrect_fn_null_checks`
+  --> $DIR/rename.rs:93:9
+   |
+LL | #![warn(clippy::fn_null_check)]
+   |         ^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `incorrect_fn_null_checks`
+
 error: lint `clippy::into_iter_on_array` has been renamed to `array_into_iter`
-  --> $DIR/rename.rs:92:9
+  --> $DIR/rename.rs:94:9
    |
 LL | #![warn(clippy::into_iter_on_array)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `array_into_iter`
 
 error: lint `clippy::invalid_atomic_ordering` has been renamed to `invalid_atomic_ordering`
-  --> $DIR/rename.rs:93:9
+  --> $DIR/rename.rs:95:9
    |
 LL | #![warn(clippy::invalid_atomic_ordering)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `invalid_atomic_ordering`
 
 error: lint `clippy::invalid_ref` has been renamed to `invalid_value`
-  --> $DIR/rename.rs:94:9
+  --> $DIR/rename.rs:96:9
    |
 LL | #![warn(clippy::invalid_ref)]
    |         ^^^^^^^^^^^^^^^^^^^ help: use the new name: `invalid_value`
 
 error: lint `clippy::invalid_utf8_in_unchecked` has been renamed to `invalid_from_utf8_unchecked`
-  --> $DIR/rename.rs:95:9
+  --> $DIR/rename.rs:97:9
    |
 LL | #![warn(clippy::invalid_utf8_in_unchecked)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `invalid_from_utf8_unchecked`
 
 error: lint `clippy::let_underscore_drop` has been renamed to `let_underscore_drop`
-  --> $DIR/rename.rs:96:9
+  --> $DIR/rename.rs:98:9
    |
 LL | #![warn(clippy::let_underscore_drop)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `let_underscore_drop`
 
 error: lint `clippy::mem_discriminant_non_enum` has been renamed to `enum_intrinsics_non_enums`
-  --> $DIR/rename.rs:97:9
+  --> $DIR/rename.rs:99:9
    |
 LL | #![warn(clippy::mem_discriminant_non_enum)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `enum_intrinsics_non_enums`
 
 error: lint `clippy::panic_params` has been renamed to `non_fmt_panics`
-  --> $DIR/rename.rs:98:9
+  --> $DIR/rename.rs:100:9
    |
 LL | #![warn(clippy::panic_params)]
    |         ^^^^^^^^^^^^^^^^^^^^ help: use the new name: `non_fmt_panics`
 
 error: lint `clippy::positional_named_format_parameters` has been renamed to `named_arguments_used_positionally`
-  --> $DIR/rename.rs:99:9
+  --> $DIR/rename.rs:101:9
    |
 LL | #![warn(clippy::positional_named_format_parameters)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `named_arguments_used_positionally`
 
 error: lint `clippy::temporary_cstring_as_ptr` has been renamed to `temporary_cstring_as_ptr`
-  --> $DIR/rename.rs:100:9
+  --> $DIR/rename.rs:102:9
    |
 LL | #![warn(clippy::temporary_cstring_as_ptr)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `temporary_cstring_as_ptr`
 
 error: lint `clippy::undropped_manually_drops` has been renamed to `undropped_manually_drops`
-  --> $DIR/rename.rs:101:9
+  --> $DIR/rename.rs:103:9
    |
 LL | #![warn(clippy::undropped_manually_drops)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `undropped_manually_drops`
 
 error: lint `clippy::unknown_clippy_lints` has been renamed to `unknown_lints`
-  --> $DIR/rename.rs:102:9
+  --> $DIR/rename.rs:104:9
    |
 LL | #![warn(clippy::unknown_clippy_lints)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `unknown_lints`
 
 error: lint `clippy::unused_label` has been renamed to `unused_labels`
-  --> $DIR/rename.rs:103:9
+  --> $DIR/rename.rs:105:9
    |
 LL | #![warn(clippy::unused_label)]
    |         ^^^^^^^^^^^^^^^^^^^^ help: use the new name: `unused_labels`
 
-error: aborting due to 52 previous errors
+error: aborting due to 53 previous errors
 

tests/ui/lint/fn_null_check.rs

@@ -0,0 +1,30 @@
+// check-pass
+
+fn main() {
+    let fn_ptr = main;
+
+    if (fn_ptr as *mut ()).is_null() {}
+    //~^ WARN function pointers are not nullable
+    if (fn_ptr as *const u8).is_null() {}
+    //~^ WARN function pointers are not nullable
+    if (fn_ptr as *const ()) == std::ptr::null() {}
+    //~^ WARN function pointers are not nullable
+    if (fn_ptr as *mut ()) == std::ptr::null_mut() {}
+    //~^ WARN function pointers are not nullable
+    if (fn_ptr as *const ()) == (0 as *const ()) {}
+    //~^ WARN function pointers are not nullable
+    if <*const _>::is_null(fn_ptr as *const ()) {}
+    //~^ WARN function pointers are not nullable
+    if (fn_ptr as *mut fn() as *const fn() as *const ()).is_null() {}
+    //~^ WARN function pointers are not nullable
+    if (fn_ptr as fn() as *const ()).is_null() {}
+    //~^ WARN function pointers are not nullable
+
+    const ZPTR: *const () = 0 as *const _;
+    const NOT_ZPTR: *const () = 1 as *const _;
+
+    // unlike the uplifted clippy::fn_null_check lint we do
+    // not lint on them
+    if (fn_ptr as *const ()) == ZPTR {}
+    if (fn_ptr as *const ()) == NOT_ZPTR {}
+}

tests/ui/lint/fn_null_check.stderr

@@ -0,0 +1,67 @@
+warning: function pointers are not nullable, so checking them for null will always return false
+  --> $DIR/fn_null_check.rs:6:8
+   |
+LL |     if (fn_ptr as *mut ()).is_null() {}
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: wrap the function pointer inside an `Option` and use `Option::is_none` to check for null pointer value
+   = note: `#[warn(incorrect_fn_null_checks)]` on by default
+
+warning: function pointers are not nullable, so checking them for null will always return false
+  --> $DIR/fn_null_check.rs:8:8
+   |
+LL |     if (fn_ptr as *const u8).is_null() {}
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: wrap the function pointer inside an `Option` and use `Option::is_none` to check for null pointer value
+
+warning: function pointers are not nullable, so checking them for null will always return false
+  --> $DIR/fn_null_check.rs:10:8
+   |
+LL |     if (fn_ptr as *const ()) == std::ptr::null() {}
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: wrap the function pointer inside an `Option` and use `Option::is_none` to check for null pointer value
+
+warning: function pointers are not nullable, so checking them for null will always return false
+  --> $DIR/fn_null_check.rs:12:8
+   |
+LL |     if (fn_ptr as *mut ()) == std::ptr::null_mut() {}
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: wrap the function pointer inside an `Option` and use `Option::is_none` to check for null pointer value
+
+warning: function pointers are not nullable, so checking them for null will always return false
+  --> $DIR/fn_null_check.rs:14:8
+   |
+LL |     if (fn_ptr as *const ()) == (0 as *const ()) {}
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: wrap the function pointer inside an `Option` and use `Option::is_none` to check for null pointer value
+
+warning: function pointers are not nullable, so checking them for null will always return false
+  --> $DIR/fn_null_check.rs:16:8
+   |
+LL |     if <*const _>::is_null(fn_ptr as *const ()) {}
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: wrap the function pointer inside an `Option` and use `Option::is_none` to check for null pointer value
+
+warning: function pointers are not nullable, so checking them for null will always return false
+  --> $DIR/fn_null_check.rs:18:8
+   |
+LL |     if (fn_ptr as *mut fn() as *const fn() as *const ()).is_null() {}
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: wrap the function pointer inside an `Option` and use `Option::is_none` to check for null pointer value
+
+warning: function pointers are not nullable, so checking them for null will always return false
+  --> $DIR/fn_null_check.rs:20:8
+   |
+LL |     if (fn_ptr as fn() as *const ()).is_null() {}
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: wrap the function pointer inside an `Option` and use `Option::is_none` to check for null pointer value
+
+warning: 8 warnings emitted
+