use crate::utils::{get_parent_expr, path_to_local, path_to_local_id}; use clippy_utils::diagnostics::{span_lint, span_lint_and_note}; use rustc_hir::intravisit::{walk_expr, NestedVisitorMap, Visitor}; use rustc_hir::{BinOpKind, Block, Expr, ExprKind, Guard, HirId, Local, Node, Stmt, StmtKind}; use rustc_lint::{LateContext, LateLintPass}; use rustc_middle::hir::map::Map; use rustc_middle::ty; use rustc_session::{declare_lint_pass, declare_tool_lint}; declare_clippy_lint! { /// **What it does:** Checks for a read and a write to the same variable where /// whether the read occurs before or after the write depends on the evaluation /// order of sub-expressions. /// /// **Why is this bad?** It is often confusing to read. In addition, the /// sub-expression evaluation order for Rust is not well documented. /// /// **Known problems:** Code which intentionally depends on the evaluation /// order, or which is correct for any evaluation order. /// /// **Example:** /// ```rust /// let mut x = 0; /// /// // Bad /// let a = { /// x = 1; /// 1 /// } + x; /// // Unclear whether a is 1 or 2. /// /// // Good /// let tmp = { /// x = 1; /// 1 /// }; /// let a = tmp + x; /// ``` pub EVAL_ORDER_DEPENDENCE, complexity, "whether a variable read occurs before a write depends on sub-expression evaluation order" } declare_clippy_lint! { /// **What it does:** Checks for diverging calls that are not match arms or /// statements. /// /// **Why is this bad?** It is often confusing to read. In addition, the /// sub-expression evaluation order for Rust is not well documented. /// /// **Known problems:** Someone might want to use `some_bool || panic!()` as a /// shorthand. /// /// **Example:** /// ```rust,no_run /// # fn b() -> bool { true } /// # fn c() -> bool { true } /// let a = b() || panic!() || c(); /// // `c()` is dead, `panic!()` is only called if `b()` returns `false` /// let x = (a, b, c, panic!()); /// // can simply be replaced by `panic!()` /// ``` pub DIVERGING_SUB_EXPRESSION, complexity, "whether an expression contains a diverging sub expression" } declare_lint_pass!(EvalOrderDependence => [EVAL_ORDER_DEPENDENCE, DIVERGING_SUB_EXPRESSION]); impl<'tcx> LateLintPass<'tcx> for EvalOrderDependence { fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) { // Find a write to a local variable. match expr.kind { ExprKind::Assign(ref lhs, ..) | ExprKind::AssignOp(_, ref lhs, _) => { if let Some(var) = path_to_local(lhs) { let mut visitor = ReadVisitor { cx, var, write_expr: expr, last_expr: expr, }; check_for_unsequenced_reads(&mut visitor); } }, _ => {}, } } fn check_stmt(&mut self, cx: &LateContext<'tcx>, stmt: &'tcx Stmt<'_>) { match stmt.kind { StmtKind::Local(ref local) => { if let Local { init: Some(ref e), .. } = **local { DivergenceVisitor { cx }.visit_expr(e); } }, StmtKind::Expr(ref e) | StmtKind::Semi(ref e) => DivergenceVisitor { cx }.maybe_walk_expr(e), StmtKind::Item(..) => {}, } } } struct DivergenceVisitor<'a, 'tcx> { cx: &'a LateContext<'tcx>, } impl<'a, 'tcx> DivergenceVisitor<'a, 'tcx> { fn maybe_walk_expr(&mut self, e: &'tcx Expr<'_>) { match e.kind { ExprKind::Closure(..) => {}, ExprKind::Match(ref e, arms, _) => { self.visit_expr(e); for arm in arms { if let Some(Guard::If(if_expr)) = arm.guard { self.visit_expr(if_expr) } // make sure top level arm expressions aren't linted self.maybe_walk_expr(&*arm.body); } }, _ => walk_expr(self, e), } } fn report_diverging_sub_expr(&mut self, e: &Expr<'_>) { span_lint(self.cx, DIVERGING_SUB_EXPRESSION, e.span, "sub-expression diverges"); } } impl<'a, 'tcx> Visitor<'tcx> for DivergenceVisitor<'a, 'tcx> { type Map = Map<'tcx>; fn visit_expr(&mut self, e: &'tcx Expr<'_>) { match e.kind { ExprKind::Continue(_) | ExprKind::Break(_, _) | ExprKind::Ret(_) => self.report_diverging_sub_expr(e), ExprKind::Call(ref func, _) => { let typ = self.cx.typeck_results().expr_ty(func); match typ.kind() { ty::FnDef(..) | ty::FnPtr(_) => { let sig = typ.fn_sig(self.cx.tcx); if let ty::Never = self.cx.tcx.erase_late_bound_regions(sig).output().kind() { self.report_diverging_sub_expr(e); } }, _ => {}, } }, ExprKind::MethodCall(..) => { let borrowed_table = self.cx.typeck_results(); if borrowed_table.expr_ty(e).is_never() { self.report_diverging_sub_expr(e); } }, _ => { // do not lint expressions referencing objects of type `!`, as that required a // diverging expression // to begin with }, } self.maybe_walk_expr(e); } fn visit_block(&mut self, _: &'tcx Block<'_>) { // don't continue over blocks, LateLintPass already does that } fn nested_visit_map(&mut self) -> NestedVisitorMap { NestedVisitorMap::None } } /// Walks up the AST from the given write expression (`vis.write_expr`) looking /// for reads to the same variable that are unsequenced relative to the write. /// /// This means reads for which there is a common ancestor between the read and /// the write such that /// /// * evaluating the ancestor necessarily evaluates both the read and the write (for example, `&x` /// and `|| x = 1` don't necessarily evaluate `x`), and /// /// * which one is evaluated first depends on the order of sub-expression evaluation. Blocks, `if`s, /// loops, `match`es, and the short-circuiting logical operators are considered to have a defined /// evaluation order. /// /// When such a read is found, the lint is triggered. fn check_for_unsequenced_reads(vis: &mut ReadVisitor<'_, '_>) { let map = &vis.cx.tcx.hir(); let mut cur_id = vis.write_expr.hir_id; loop { let parent_id = map.get_parent_node(cur_id); if parent_id == cur_id { break; } let parent_node = match map.find(parent_id) { Some(parent) => parent, None => break, }; let stop_early = match parent_node { Node::Expr(expr) => check_expr(vis, expr), Node::Stmt(stmt) => check_stmt(vis, stmt), Node::Item(_) => { // We reached the top of the function, stop. break; }, _ => StopEarly::KeepGoing, }; match stop_early { StopEarly::Stop => break, StopEarly::KeepGoing => {}, } cur_id = parent_id; } } /// Whether to stop early for the loop in `check_for_unsequenced_reads`. (If /// `check_expr` weren't an independent function, this would be unnecessary and /// we could just use `break`). enum StopEarly { KeepGoing, Stop, } fn check_expr<'a, 'tcx>(vis: &mut ReadVisitor<'a, 'tcx>, expr: &'tcx Expr<'_>) -> StopEarly { if expr.hir_id == vis.last_expr.hir_id { return StopEarly::KeepGoing; } match expr.kind { ExprKind::Array(_) | ExprKind::Tup(_) | ExprKind::MethodCall(..) | ExprKind::Call(_, _) | ExprKind::Assign(..) | ExprKind::Index(_, _) | ExprKind::Repeat(_, _) | ExprKind::Struct(_, _, _) => { walk_expr(vis, expr); }, ExprKind::Binary(op, _, _) | ExprKind::AssignOp(op, _, _) => { if op.node == BinOpKind::And || op.node == BinOpKind::Or { // x && y and x || y always evaluate x first, so these are // strictly sequenced. } else { walk_expr(vis, expr); } }, ExprKind::Closure(_, _, _, _, _) => { // Either // // * `var` is defined in the closure body, in which case we've reached the top of the enclosing // function and can stop, or // // * `var` is captured by the closure, in which case, because evaluating a closure does not evaluate // its body, we don't necessarily have a write, so we need to stop to avoid generating false // positives. // // This is also the only place we need to stop early (grrr). return StopEarly::Stop; }, // All other expressions either have only one child or strictly // sequence the evaluation order of their sub-expressions. _ => {}, } vis.last_expr = expr; StopEarly::KeepGoing } fn check_stmt<'a, 'tcx>(vis: &mut ReadVisitor<'a, 'tcx>, stmt: &'tcx Stmt<'_>) -> StopEarly { match stmt.kind { StmtKind::Expr(ref expr) | StmtKind::Semi(ref expr) => check_expr(vis, expr), // If the declaration is of a local variable, check its initializer // expression if it has one. Otherwise, keep going. StmtKind::Local(ref local) => local .init .as_ref() .map_or(StopEarly::KeepGoing, |expr| check_expr(vis, expr)), _ => StopEarly::KeepGoing, } } /// A visitor that looks for reads from a variable. struct ReadVisitor<'a, 'tcx> { cx: &'a LateContext<'tcx>, /// The ID of the variable we're looking for. var: HirId, /// The expressions where the write to the variable occurred (for reporting /// in the lint). write_expr: &'tcx Expr<'tcx>, /// The last (highest in the AST) expression we've checked, so we know not /// to recheck it. last_expr: &'tcx Expr<'tcx>, } impl<'a, 'tcx> Visitor<'tcx> for ReadVisitor<'a, 'tcx> { type Map = Map<'tcx>; fn visit_expr(&mut self, expr: &'tcx Expr<'_>) { if expr.hir_id == self.last_expr.hir_id { return; } if path_to_local_id(expr, self.var) { // Check that this is a read, not a write. if !is_in_assignment_position(self.cx, expr) { span_lint_and_note( self.cx, EVAL_ORDER_DEPENDENCE, expr.span, "unsequenced read of a variable", Some(self.write_expr.span), "whether read occurs before this write depends on evaluation order", ); } } match expr.kind { // We're about to descend a closure. Since we don't know when (or // if) the closure will be evaluated, any reads in it might not // occur here (or ever). Like above, bail to avoid false positives. ExprKind::Closure(_, _, _, _, _) | // We want to avoid a false positive when a variable name occurs // only to have its address taken, so we stop here. Technically, // this misses some weird cases, eg. // // ```rust // let mut x = 0; // let a = foo(&{x = 1; x}, x); // ``` // // TODO: fix this ExprKind::AddrOf(_, _, _) => { return; } _ => {} } walk_expr(self, expr); } fn nested_visit_map(&mut self) -> NestedVisitorMap { NestedVisitorMap::None } } /// Returns `true` if `expr` is the LHS of an assignment, like `expr = ...`. fn is_in_assignment_position(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool { if let Some(parent) = get_parent_expr(cx, expr) { if let ExprKind::Assign(ref lhs, ..) = parent.kind { return lhs.hir_id == expr.hir_id; } } false }