1 use crate::utils::{get_parent_expr, path_to_local, path_to_local_id, span_lint, span_lint_and_note};
2 use rustc_hir::intravisit::{walk_expr, NestedVisitorMap, Visitor};
3 use rustc_hir::{BinOpKind, Block, Expr, ExprKind, Guard, HirId, Local, Node, Stmt, StmtKind};
4 use rustc_lint::{LateContext, LateLintPass};
5 use rustc_middle::hir::map::Map;
7 use rustc_session::{declare_lint_pass, declare_tool_lint};
10 /// **What it does:** Checks for a read and a write to the same variable where
11 /// whether the read occurs before or after the write depends on the evaluation
12 /// order of sub-expressions.
14 /// **Why is this bad?** It is often confusing to read. In addition, the
15 /// sub-expression evaluation order for Rust is not well documented.
17 /// **Known problems:** Code which intentionally depends on the evaluation
18 /// order, or which is correct for any evaluation order.
29 /// // Unclear whether a is 1 or 2.
38 pub EVAL_ORDER_DEPENDENCE,
40 "whether a variable read occurs before a write depends on sub-expression evaluation order"
43 declare_clippy_lint! {
44 /// **What it does:** Checks for diverging calls that are not match arms or
47 /// **Why is this bad?** It is often confusing to read. In addition, the
48 /// sub-expression evaluation order for Rust is not well documented.
50 /// **Known problems:** Someone might want to use `some_bool || panic!()` as a
55 /// # fn b() -> bool { true }
56 /// # fn c() -> bool { true }
57 /// let a = b() || panic!() || c();
58 /// // `c()` is dead, `panic!()` is only called if `b()` returns `false`
59 /// let x = (a, b, c, panic!());
60 /// // can simply be replaced by `panic!()`
62 pub DIVERGING_SUB_EXPRESSION,
64 "whether an expression contains a diverging sub expression"
67 declare_lint_pass!(EvalOrderDependence => [EVAL_ORDER_DEPENDENCE, DIVERGING_SUB_EXPRESSION]);
69 impl<'tcx> LateLintPass<'tcx> for EvalOrderDependence {
70 fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
71 // Find a write to a local variable.
73 ExprKind::Assign(ref lhs, ..) | ExprKind::AssignOp(_, ref lhs, _) => {
74 if let Some(var) = path_to_local(lhs) {
75 let mut visitor = ReadVisitor {
81 check_for_unsequenced_reads(&mut visitor);
87 fn check_stmt(&mut self, cx: &LateContext<'tcx>, stmt: &'tcx Stmt<'_>) {
89 StmtKind::Local(ref local) => {
90 if let Local { init: Some(ref e), .. } = **local {
91 DivergenceVisitor { cx }.visit_expr(e);
94 StmtKind::Expr(ref e) | StmtKind::Semi(ref e) => DivergenceVisitor { cx }.maybe_walk_expr(e),
95 StmtKind::Item(..) => {},
100 struct DivergenceVisitor<'a, 'tcx> {
101 cx: &'a LateContext<'tcx>,
104 impl<'a, 'tcx> DivergenceVisitor<'a, 'tcx> {
105 fn maybe_walk_expr(&mut self, e: &'tcx Expr<'_>) {
107 ExprKind::Closure(..) => {},
108 ExprKind::Match(ref e, arms, _) => {
111 if let Some(Guard::If(if_expr)) = arm.guard {
112 self.visit_expr(if_expr)
114 // make sure top level arm expressions aren't linted
115 self.maybe_walk_expr(&*arm.body);
118 _ => walk_expr(self, e),
121 fn report_diverging_sub_expr(&mut self, e: &Expr<'_>) {
122 span_lint(self.cx, DIVERGING_SUB_EXPRESSION, e.span, "sub-expression diverges");
126 impl<'a, 'tcx> Visitor<'tcx> for DivergenceVisitor<'a, 'tcx> {
127 type Map = Map<'tcx>;
129 fn visit_expr(&mut self, e: &'tcx Expr<'_>) {
131 ExprKind::Continue(_) | ExprKind::Break(_, _) | ExprKind::Ret(_) => self.report_diverging_sub_expr(e),
132 ExprKind::Call(ref func, _) => {
133 let typ = self.cx.typeck_results().expr_ty(func);
135 ty::FnDef(..) | ty::FnPtr(_) => {
136 let sig = typ.fn_sig(self.cx.tcx);
137 if let ty::Never = self.cx.tcx.erase_late_bound_regions(sig).output().kind() {
138 self.report_diverging_sub_expr(e);
144 ExprKind::MethodCall(..) => {
145 let borrowed_table = self.cx.typeck_results();
146 if borrowed_table.expr_ty(e).is_never() {
147 self.report_diverging_sub_expr(e);
151 // do not lint expressions referencing objects of type `!`, as that required a
152 // diverging expression
156 self.maybe_walk_expr(e);
158 fn visit_block(&mut self, _: &'tcx Block<'_>) {
159 // don't continue over blocks, LateLintPass already does that
161 fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
162 NestedVisitorMap::None
166 /// Walks up the AST from the given write expression (`vis.write_expr`) looking
167 /// for reads to the same variable that are unsequenced relative to the write.
169 /// This means reads for which there is a common ancestor between the read and
170 /// the write such that
172 /// * evaluating the ancestor necessarily evaluates both the read and the write (for example, `&x`
173 /// and `|| x = 1` don't necessarily evaluate `x`), and
175 /// * which one is evaluated first depends on the order of sub-expression evaluation. Blocks, `if`s,
176 /// loops, `match`es, and the short-circuiting logical operators are considered to have a defined
177 /// evaluation order.
179 /// When such a read is found, the lint is triggered.
180 fn check_for_unsequenced_reads(vis: &mut ReadVisitor<'_, '_>) {
181 let map = &vis.cx.tcx.hir();
182 let mut cur_id = vis.write_expr.hir_id;
184 let parent_id = map.get_parent_node(cur_id);
185 if parent_id == cur_id {
188 let parent_node = match map.find(parent_id) {
189 Some(parent) => parent,
193 let stop_early = match parent_node {
194 Node::Expr(expr) => check_expr(vis, expr),
195 Node::Stmt(stmt) => check_stmt(vis, stmt),
197 // We reached the top of the function, stop.
200 _ => StopEarly::KeepGoing,
203 StopEarly::Stop => break,
204 StopEarly::KeepGoing => {},
211 /// Whether to stop early for the loop in `check_for_unsequenced_reads`. (If
212 /// `check_expr` weren't an independent function, this would be unnecessary and
213 /// we could just use `break`).
219 fn check_expr<'a, 'tcx>(vis: &mut ReadVisitor<'a, 'tcx>, expr: &'tcx Expr<'_>) -> StopEarly {
220 if expr.hir_id == vis.last_expr.hir_id {
221 return StopEarly::KeepGoing;
227 | ExprKind::MethodCall(..)
228 | ExprKind::Call(_, _)
229 | ExprKind::Assign(..)
230 | ExprKind::Index(_, _)
231 | ExprKind::Repeat(_, _)
232 | ExprKind::Struct(_, _, _) => {
233 walk_expr(vis, expr);
235 ExprKind::Binary(op, _, _) | ExprKind::AssignOp(op, _, _) => {
236 if op.node == BinOpKind::And || op.node == BinOpKind::Or {
237 // x && y and x || y always evaluate x first, so these are
238 // strictly sequenced.
240 walk_expr(vis, expr);
243 ExprKind::Closure(_, _, _, _, _) => {
246 // * `var` is defined in the closure body, in which case we've reached the top of the enclosing
247 // function and can stop, or
249 // * `var` is captured by the closure, in which case, because evaluating a closure does not evaluate
250 // its body, we don't necessarily have a write, so we need to stop to avoid generating false
253 // This is also the only place we need to stop early (grrr).
254 return StopEarly::Stop;
256 // All other expressions either have only one child or strictly
257 // sequence the evaluation order of their sub-expressions.
261 vis.last_expr = expr;
266 fn check_stmt<'a, 'tcx>(vis: &mut ReadVisitor<'a, 'tcx>, stmt: &'tcx Stmt<'_>) -> StopEarly {
268 StmtKind::Expr(ref expr) | StmtKind::Semi(ref expr) => check_expr(vis, expr),
269 // If the declaration is of a local variable, check its initializer
270 // expression if it has one. Otherwise, keep going.
271 StmtKind::Local(ref local) => local
274 .map_or(StopEarly::KeepGoing, |expr| check_expr(vis, expr)),
275 _ => StopEarly::KeepGoing,
279 /// A visitor that looks for reads from a variable.
280 struct ReadVisitor<'a, 'tcx> {
281 cx: &'a LateContext<'tcx>,
282 /// The ID of the variable we're looking for.
284 /// The expressions where the write to the variable occurred (for reporting
286 write_expr: &'tcx Expr<'tcx>,
287 /// The last (highest in the AST) expression we've checked, so we know not
289 last_expr: &'tcx Expr<'tcx>,
292 impl<'a, 'tcx> Visitor<'tcx> for ReadVisitor<'a, 'tcx> {
293 type Map = Map<'tcx>;
295 fn visit_expr(&mut self, expr: &'tcx Expr<'_>) {
296 if expr.hir_id == self.last_expr.hir_id {
300 if path_to_local_id(expr, self.var) {
301 // Check that this is a read, not a write.
302 if !is_in_assignment_position(self.cx, expr) {
305 EVAL_ORDER_DEPENDENCE,
307 "unsequenced read of a variable",
308 Some(self.write_expr.span),
309 "whether read occurs before this write depends on evaluation order",
314 // We're about to descend a closure. Since we don't know when (or
315 // if) the closure will be evaluated, any reads in it might not
316 // occur here (or ever). Like above, bail to avoid false positives.
317 ExprKind::Closure(_, _, _, _, _) |
319 // We want to avoid a false positive when a variable name occurs
320 // only to have its address taken, so we stop here. Technically,
321 // this misses some weird cases, eg.
325 // let a = foo(&{x = 1; x}, x);
329 ExprKind::AddrOf(_, _, _) => {
335 walk_expr(self, expr);
337 fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
338 NestedVisitorMap::None
342 /// Returns `true` if `expr` is the LHS of an assignment, like `expr = ...`.
343 fn is_in_assignment_position(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
344 if let Some(parent) = get_parent_expr(cx, expr) {
345 if let ExprKind::Assign(ref lhs, ..) = parent.kind {
346 return lhs.hir_id == expr.hir_id;