1 use crate::utils::{get_parent_expr, span_lint, span_note_and_lint};
2 use if_chain::if_chain;
3 use rustc::hir::intravisit::{walk_expr, NestedVisitorMap, Visitor};
5 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
7 use rustc::{declare_tool_lint, lint_array};
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.
27 /// // Unclear whether a is 1 or 2.
29 pub EVAL_ORDER_DEPENDENCE,
31 "whether a variable read occurs before a write depends on sub-expression evaluation order"
34 declare_clippy_lint! {
35 /// **What it does:** Checks for diverging calls that are not match arms or
38 /// **Why is this bad?** It is often confusing to read. In addition, the
39 /// sub-expression evaluation order for Rust is not well documented.
41 /// **Known problems:** Someone might want to use `some_bool || panic!()` as a
46 /// let a = b() || panic!() || c();
47 /// // `c()` is dead, `panic!()` is only called if `b()` returns `false`
48 /// let x = (a, b, c, panic!());
49 /// // can simply be replaced by `panic!()`
51 pub DIVERGING_SUB_EXPRESSION,
53 "whether an expression contains a diverging sub expression"
56 #[derive(Copy, Clone)]
57 pub struct EvalOrderDependence;
59 impl LintPass for EvalOrderDependence {
60 fn get_lints(&self) -> LintArray {
61 lint_array!(EVAL_ORDER_DEPENDENCE, DIVERGING_SUB_EXPRESSION)
64 fn name(&self) -> &'static str {
69 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for EvalOrderDependence {
70 fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
71 // Find a write to a local variable.
73 ExprKind::Assign(ref lhs, _) | ExprKind::AssignOp(_, ref lhs, _) => {
74 if let ExprKind::Path(ref qpath) = lhs.node {
75 if let QPath::Resolved(_, ref path) = *qpath {
76 if path.segments.len() == 1 {
77 if let def::Def::Local(var) = cx.tables.qpath_def(qpath, lhs.hir_id) {
78 let mut visitor = ReadVisitor {
84 check_for_unsequenced_reads(&mut visitor);
93 fn check_stmt(&mut self, cx: &LateContext<'a, 'tcx>, stmt: &'tcx Stmt) {
95 StmtKind::Local(ref local) => {
96 if let Local { init: Some(ref e), .. } = **local {
97 DivergenceVisitor { cx }.visit_expr(e);
100 StmtKind::Expr(ref e) | StmtKind::Semi(ref e) => DivergenceVisitor { cx }.maybe_walk_expr(e),
101 StmtKind::Item(..) => {},
106 struct DivergenceVisitor<'a, 'tcx: 'a> {
107 cx: &'a LateContext<'a, 'tcx>,
110 impl<'a, 'tcx> DivergenceVisitor<'a, 'tcx> {
111 fn maybe_walk_expr(&mut self, e: &'tcx Expr) {
113 ExprKind::Closure(.., _) => {},
114 ExprKind::Match(ref e, ref arms, _) => {
117 if let Some(ref guard) = arm.guard {
119 Guard::If(if_expr) => self.visit_expr(if_expr),
122 // make sure top level arm expressions aren't linted
123 self.maybe_walk_expr(&*arm.body);
126 _ => walk_expr(self, e),
129 fn report_diverging_sub_expr(&mut self, e: &Expr) {
130 span_lint(self.cx, DIVERGING_SUB_EXPRESSION, e.span, "sub-expression diverges");
134 impl<'a, 'tcx> Visitor<'tcx> for DivergenceVisitor<'a, 'tcx> {
135 fn visit_expr(&mut self, e: &'tcx Expr) {
137 ExprKind::Continue(_) | ExprKind::Break(_, _) | ExprKind::Ret(_) => self.report_diverging_sub_expr(e),
138 ExprKind::Call(ref func, _) => {
139 let typ = self.cx.tables.expr_ty(func);
141 ty::FnDef(..) | ty::FnPtr(_) => {
142 let sig = typ.fn_sig(self.cx.tcx);
143 if let ty::Never = self.cx.tcx.erase_late_bound_regions(&sig).output().sty {
144 self.report_diverging_sub_expr(e);
150 ExprKind::MethodCall(..) => {
151 let borrowed_table = self.cx.tables;
152 if borrowed_table.expr_ty(e).is_never() {
153 self.report_diverging_sub_expr(e);
157 // do not lint expressions referencing objects of type `!`, as that required a
158 // diverging expression
162 self.maybe_walk_expr(e);
164 fn visit_block(&mut self, _: &'tcx Block) {
165 // don't continue over blocks, LateLintPass already does that
167 fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
168 NestedVisitorMap::None
172 /// Walks up the AST from the given write expression (`vis.write_expr`) looking
173 /// for reads to the same variable that are unsequenced relative to the write.
175 /// This means reads for which there is a common ancestor between the read and
176 /// the write such that
178 /// * evaluating the ancestor necessarily evaluates both the read and the write (for example, `&x`
179 /// and `|| x = 1` don't necessarily evaluate `x`), and
181 /// * which one is evaluated first depends on the order of sub-expression evaluation. Blocks, `if`s,
182 /// loops, `match`es, and the short-circuiting logical operators are considered to have a defined
183 /// evaluation order.
185 /// When such a read is found, the lint is triggered.
186 fn check_for_unsequenced_reads(vis: &mut ReadVisitor<'_, '_>) {
187 let map = &vis.cx.tcx.hir();
188 let mut cur_id = vis.write_expr.hir_id;
190 let parent_id = map.get_parent_node_by_hir_id(cur_id);
191 if parent_id == cur_id {
194 let parent_node = match map.find_by_hir_id(parent_id) {
195 Some(parent) => parent,
199 let stop_early = match parent_node {
200 Node::Expr(expr) => check_expr(vis, expr),
201 Node::Stmt(stmt) => check_stmt(vis, stmt),
203 // We reached the top of the function, stop.
206 _ => StopEarly::KeepGoing,
209 StopEarly::Stop => break,
210 StopEarly::KeepGoing => {},
217 /// Whether to stop early for the loop in `check_for_unsequenced_reads`. (If
218 /// `check_expr` weren't an independent function, this would be unnecessary and
219 /// we could just use `break`).
225 fn check_expr<'a, 'tcx>(vis: &mut ReadVisitor<'a, 'tcx>, expr: &'tcx Expr) -> StopEarly {
226 if expr.hir_id == vis.last_expr.hir_id {
227 return StopEarly::KeepGoing;
233 | ExprKind::MethodCall(..)
234 | ExprKind::Call(_, _)
235 | ExprKind::Assign(_, _)
236 | ExprKind::Index(_, _)
237 | ExprKind::Repeat(_, _)
238 | ExprKind::Struct(_, _, _) => {
239 walk_expr(vis, expr);
241 ExprKind::Binary(op, _, _) | ExprKind::AssignOp(op, _, _) => {
242 if op.node == BinOpKind::And || op.node == BinOpKind::Or {
243 // x && y and x || y always evaluate x first, so these are
244 // strictly sequenced.
246 walk_expr(vis, expr);
249 ExprKind::Closure(_, _, _, _, _) => {
252 // * `var` is defined in the closure body, in which case we've reached the top of the enclosing
253 // function and can stop, or
255 // * `var` is captured by the closure, in which case, because evaluating a closure does not evaluate
256 // its body, we don't necessarily have a write, so we need to stop to avoid generating false
259 // This is also the only place we need to stop early (grrr).
260 return StopEarly::Stop;
262 // All other expressions either have only one child or strictly
263 // sequence the evaluation order of their sub-expressions.
267 vis.last_expr = expr;
272 fn check_stmt<'a, 'tcx>(vis: &mut ReadVisitor<'a, 'tcx>, stmt: &'tcx Stmt) -> StopEarly {
274 StmtKind::Expr(ref expr) | StmtKind::Semi(ref expr) => check_expr(vis, expr),
275 // If the declaration is of a local variable, check its initializer
276 // expression if it has one. Otherwise, keep going.
277 StmtKind::Local(ref local) => local
280 .map_or(StopEarly::KeepGoing, |expr| check_expr(vis, expr)),
281 _ => StopEarly::KeepGoing,
285 /// A visitor that looks for reads from a variable.
286 struct ReadVisitor<'a, 'tcx: 'a> {
287 cx: &'a LateContext<'a, 'tcx>,
288 /// The ID of the variable we're looking for.
290 /// The expressions where the write to the variable occurred (for reporting
292 write_expr: &'tcx Expr,
293 /// The last (highest in the AST) expression we've checked, so we know not
295 last_expr: &'tcx Expr,
298 impl<'a, 'tcx> Visitor<'tcx> for ReadVisitor<'a, 'tcx> {
299 fn visit_expr(&mut self, expr: &'tcx Expr) {
300 if expr.hir_id == self.last_expr.hir_id {
305 ExprKind::Path(ref qpath) => {
307 if let QPath::Resolved(None, ref path) = *qpath;
308 if path.segments.len() == 1;
309 if let def::Def::Local(local_id) = self.cx.tables.qpath_def(qpath, expr.hir_id);
310 if local_id == self.var;
311 // Check that this is a read, not a write.
312 if !is_in_assignment_position(self.cx, expr);
316 EVAL_ORDER_DEPENDENCE,
318 "unsequenced read of a variable",
319 self.write_expr.span,
320 "whether read occurs before this write depends on evaluation order"
325 // We're about to descend a closure. Since we don't know when (or
326 // if) the closure will be evaluated, any reads in it might not
327 // occur here (or ever). Like above, bail to avoid false positives.
328 ExprKind::Closure(_, _, _, _, _) |
330 // We want to avoid a false positive when a variable name occurs
331 // only to have its address taken, so we stop here. Technically,
332 // this misses some weird cases, eg.
336 // let a = foo(&{x = 1; x}, x);
340 ExprKind::AddrOf(_, _) => {
346 walk_expr(self, expr);
348 fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
349 NestedVisitorMap::None
353 /// Returns `true` if `expr` is the LHS of an assignment, like `expr = ...`.
354 fn is_in_assignment_position(cx: &LateContext<'_, '_>, expr: &Expr) -> bool {
355 if let Some(parent) = get_parent_expr(cx, expr) {
356 if let ExprKind::Assign(ref lhs, _) = parent.node {
357 return lhs.hir_id == expr.hir_id;