Rename incorrect_fn_null_checks to useless_ptr_null_checks

compiler/rustc_lint/src/ptr_nulls.rs

@@ -0,0 +1,126 @@
+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]);
+
+fn incorrect_check<'a>(cx: &LateContext<'a>, expr: &Expr<'_>) -> Option<PtrNullChecksDiag<'a>> {
+    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;
+    }
+    if !had_at_least_one_cast {
+        None
+    } else {
+        let orig_ty = cx.typeck_results().expr_ty(expr);
+        if orig_ty.is_fn() {
+            Some(PtrNullChecksDiag::FnPtr)
+        } 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)
+                    },
+
+                    _ => {},
+                }
+            }
+            _ => {}
+        }
+    }
+}