Rollup merge of rust-lang#113657 - Urgau:expand-incorrect_fn_null_check-lint, r=cjgillot

Expand, rename and improve `incorrect_fn_null_checks` lint

This PR,

 - firstly, expand the lint by now linting on references
 - secondly, it renames the lint `incorrect_fn_null_checks` -> `useless_ptr_null_checks`
 - and thirdly it improves the lint by catching `ptr::from_mut`, `ptr::from_ref`, as well as `<*mut _>::cast` and `<*const _>::cast_mut`

Fixes rust-lang#113601
cc `@est31`

Author: matthiaskrgr

Diff for: compiler/rustc_lint/messages.ftl

@@ -213,9 +213,6 @@ 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
@@ -454,6 +451,13 @@ lint_path_statement_drop = path statement drops value
 
 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}`
+
 lint_query_instability = using `{$query}` can result in unstable query results
     .note = if you believe this case to be fine, allow this lint and add a comment explaining your rationale
 

Diff for: compiler/rustc_lint/src/fn_null_check.rs

@@ -57,7 +57,6 @@ mod early;
 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;
@@ -76,6 +75,7 @@ mod noop_method_call;
 mod opaque_hidden_inferred_bound;
 mod pass_by_value;
 mod passes;
+mod ptr_nulls;
 mod redundant_semicolon;
 mod reference_casting;
 mod traits;
@@ -102,7 +102,6 @@ use builtin::*;
 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::*;
@@ -117,6 +116,7 @@ use nonstandard_style::*;
 use noop_method_call::*;
 use opaque_hidden_inferred_bound::*;
 use pass_by_value::*;
+use ptr_nulls::*;
 use redundant_semicolon::*;
 use reference_casting::*;
 use traits::*;
@@ -227,7 +227,7 @@ late_lint_methods!(
             // Depends on types used in type definitions
             MissingCopyImplementations: MissingCopyImplementations,
             // Depends on referenced function signatures in expressions
-            IncorrectFnNullChecks: IncorrectFnNullChecks,
+            PtrNullChecks: PtrNullChecks,
             MutableTransmutes: MutableTransmutes,
             TypeAliasBounds: TypeAliasBounds,
             TrivialConstraints: TrivialConstraints,

Diff for: compiler/rustc_lint/src/lib.rs

@@ -613,11 +613,23 @@ pub struct ExpectationNote {
     pub rationale: Symbol,
 }
 
-// fn_null_check.rs
+// ptr_nulls.rs
 #[derive(LintDiagnostic)]
-#[diag(lint_fn_null_check)]
-#[help]
-pub struct FnNullCheckDiag;
+pub enum PtrNullChecksDiag<'a> {
+    #[diag(lint_ptr_null_checks_fn_ptr)]
+    #[help(lint_help)]
+    FnPtr {
+        orig_ty: Ty<'a>,
+        #[label]
+        label: Span,
+    },
+    #[diag(lint_ptr_null_checks_ref)]
+    Ref {
+        orig_ty: Ty<'a>,
+        #[label]
+        label: Span,
+    },
+}
 
 // for_loops_over_fallibles.rs
 #[derive(LintDiagnostic)]

Diff for: compiler/rustc_lint/src/lints.rs

@@ -0,0 +1,146 @@
+use crate::{lints::PtrNullChecksDiag, 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 `useless_ptr_null_checks` lint checks for useless null checks against pointers
+    /// obtained from non-null types.
+    ///
+    /// ### 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 and references are assumed to be non-null, checking them for null
+    /// will always return false.
+    USELESS_PTR_NULL_CHECKS,
+    Warn,
+    "useless checking of non-null-typed pointer"
+}
+
+declare_lint_pass!(PtrNullChecks => [USELESS_PTR_NULL_CHECKS]);
+
+/// This function detects and returns the original expression from a series of consecutive casts,
+/// ie. `(my_fn as *const _ as *mut _).cast_mut()` would return the expression for `my_fn`.
+fn ptr_cast_chain<'a>(cx: &'a LateContext<'_>, mut e: &'a Expr<'a>) -> Option<&'a Expr<'a>> {
+    let mut had_at_least_one_cast = false;
+    loop {
+        e = e.peel_blocks();
+        e = if let ExprKind::Cast(expr, t) = e.kind
+            && let TyKind::Ptr(_) = t.kind {
+            had_at_least_one_cast = true;
+            expr
+        } else if let ExprKind::MethodCall(_, expr, [], _) = e.kind
+            && let Some(def_id) = cx.typeck_results().type_dependent_def_id(e.hir_id)
+            && matches!(cx.tcx.get_diagnostic_name(def_id), Some(sym::ptr_cast | sym::ptr_cast_mut)) {
+            had_at_least_one_cast = true;
+            expr
+        } else if let ExprKind::Call(path, [arg]) = e.kind
+            && 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_from_ref | sym::ptr_from_mut)) {
+            had_at_least_one_cast = true;
+            arg
+        } else if had_at_least_one_cast {
+            return Some(e);
+        } else {
+            return None;
+        };
+    }
+}
+
+fn incorrect_check<'a>(cx: &LateContext<'a>, expr: &Expr<'_>) -> Option<PtrNullChecksDiag<'a>> {
+    let expr = ptr_cast_chain(cx, expr)?;
+
+    let orig_ty = cx.typeck_results().expr_ty(expr);
+    if orig_ty.is_fn() {
+        Some(PtrNullChecksDiag::FnPtr { orig_ty, label: expr.span })
+    } else if orig_ty.is_ref() {
+        Some(PtrNullChecksDiag::Ref { orig_ty, label: expr.span })
+    } else {
+        None
+    }
+}
+
+impl<'tcx> LateLintPass<'tcx> for PtrNullChecks {
+    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)
+                    )
+                    && let Some(diag) = incorrect_check(cx, arg) =>
+            {
+                cx.emit_spanned_lint(USELESS_PTR_NULL_CHECKS, expr.span, diag)
+            }
+
+            // 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)
+                    )
+                    && let Some(diag) = incorrect_check(cx, receiver) =>
+            {
+                cx.emit_spanned_lint(USELESS_PTR_NULL_CHECKS, expr.span, diag)
+            }
+
+            ExprKind::Binary(op, left, right) if matches!(op.node, BinOpKind::Eq) => {
+                let to_check: &Expr<'_>;
+                let diag: PtrNullChecksDiag<'_>;
+                if let Some(ddiag) = incorrect_check(cx, left) {
+                    to_check = right;
+                    diag = ddiag;
+                } else if let Some(ddiag) = incorrect_check(cx, right) {
+                    to_check = left;
+                    diag = ddiag;
+                } 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(USELESS_PTR_NULL_CHECKS, expr.span, diag)
+                    },
+
+                    // 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(USELESS_PTR_NULL_CHECKS, expr.span, diag)
+                    },
+
+                    _ => {},
+                }
+            }
+            _ => {}
+        }
+    }
+}

Diff for: compiler/rustc_lint/src/ptr_nulls.rs

@@ -1155,8 +1155,10 @@ symbols! {
         profiler_builtins,
         profiler_runtime,
         ptr,
+        ptr_cast,
         ptr_cast_mut,
         ptr_const_is_null,
+        ptr_from_mut,
         ptr_from_ref,
         ptr_guaranteed_cmp,
         ptr_is_null,

Diff for: compiler/rustc_span/src/symbol.rs

@@ -710,6 +710,7 @@ pub const fn from_ref<T: ?Sized>(r: &T) -> *const T {
 #[inline(always)]
 #[must_use]
 #[unstable(feature = "ptr_from_ref", issue = "106116")]
+#[rustc_diagnostic_item = "ptr_from_mut"]
 pub const fn from_mut<T: ?Sized>(r: &mut T) -> *mut T {
     r
 }

Diff for: library/core/src/ptr/mod.rs

@@ -54,6 +54,7 @@ impl<T: ?Sized> *mut T {
     /// Casts to a pointer of another type.
     #[stable(feature = "ptr_cast", since = "1.38.0")]
     #[rustc_const_stable(feature = "const_ptr_cast", since = "1.38.0")]
+    #[rustc_diagnostic_item = "ptr_cast"]
     #[inline(always)]
     pub const fn cast<U>(self) -> *mut U {
         self as _

Diff for: library/core/src/ptr/mut_ptr.rs

@@ -43,7 +43,7 @@ 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::fn_null_check", "useless_ptr_null_checks"),
     ("clippy::into_iter_on_array", "array_into_iter"),
     ("clippy::invalid_atomic_ordering", "invalid_atomic_ordering"),
     ("clippy::invalid_ref", "invalid_value"),