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_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.
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 /// # fn b() -> bool { true }
47 /// # fn c() -> bool { true }
48 /// let a = b() || panic!() || c();
49 /// // `c()` is dead, `panic!()` is only called if `b()` returns `false`
50 /// let x = (a, b, c, panic!());
51 /// // can simply be replaced by `panic!()`
53 pub DIVERGING_SUB_EXPRESSION,
55 "whether an expression contains a diverging sub expression"
58 declare_lint_pass!(EvalOrderDependence => [EVAL_ORDER_DEPENDENCE, DIVERGING_SUB_EXPRESSION]);
60 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for EvalOrderDependence {
61 fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
62 // Find a write to a local variable.
64 ExprKind::Assign(ref lhs, _) | ExprKind::AssignOp(_, ref lhs, _) => {
65 if let ExprKind::Path(ref qpath) = lhs.kind {
66 if let QPath::Resolved(_, ref path) = *qpath {
67 if path.segments.len() == 1 {
68 if let def::Res::Local(var) = cx.tables.qpath_res(qpath, lhs.hir_id) {
69 let mut visitor = ReadVisitor {
75 check_for_unsequenced_reads(&mut visitor);
84 fn check_stmt(&mut self, cx: &LateContext<'a, 'tcx>, stmt: &'tcx Stmt) {
86 StmtKind::Local(ref local) => {
87 if let Local { init: Some(ref e), .. } = **local {
88 DivergenceVisitor { cx }.visit_expr(e);
91 StmtKind::Expr(ref e) | StmtKind::Semi(ref e) => DivergenceVisitor { cx }.maybe_walk_expr(e),
92 StmtKind::Item(..) => {},
97 struct DivergenceVisitor<'a, 'tcx> {
98 cx: &'a LateContext<'a, 'tcx>,
101 impl<'a, 'tcx> DivergenceVisitor<'a, 'tcx> {
102 fn maybe_walk_expr(&mut self, e: &'tcx Expr) {
104 ExprKind::Closure(..) => {},
105 ExprKind::Match(ref e, ref arms, _) => {
108 if let Some(ref guard) = arm.guard {
110 Guard::If(if_expr) => self.visit_expr(if_expr),
113 // make sure top level arm expressions aren't linted
114 self.maybe_walk_expr(&*arm.body);
117 _ => walk_expr(self, e),
120 fn report_diverging_sub_expr(&mut self, e: &Expr) {
121 span_lint(self.cx, DIVERGING_SUB_EXPRESSION, e.span, "sub-expression diverges");
125 impl<'a, 'tcx> Visitor<'tcx> for DivergenceVisitor<'a, 'tcx> {
126 fn visit_expr(&mut self, e: &'tcx Expr) {
128 ExprKind::Continue(_) | ExprKind::Break(_, _) | ExprKind::Ret(_) => self.report_diverging_sub_expr(e),
129 ExprKind::Call(ref func, _) => {
130 let typ = self.cx.tables.expr_ty(func);
132 ty::FnDef(..) | ty::FnPtr(_) => {
133 let sig = typ.fn_sig(self.cx.tcx);
134 if let ty::Never = self.cx.tcx.erase_late_bound_regions(&sig).output().kind {
135 self.report_diverging_sub_expr(e);
141 ExprKind::MethodCall(..) => {
142 let borrowed_table = self.cx.tables;
143 if borrowed_table.expr_ty(e).is_never() {
144 self.report_diverging_sub_expr(e);
148 // do not lint expressions referencing objects of type `!`, as that required a
149 // diverging expression
153 self.maybe_walk_expr(e);
155 fn visit_block(&mut self, _: &'tcx Block) {
156 // don't continue over blocks, LateLintPass already does that
158 fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
159 NestedVisitorMap::None
163 /// Walks up the AST from the given write expression (`vis.write_expr`) looking
164 /// for reads to the same variable that are unsequenced relative to the write.
166 /// This means reads for which there is a common ancestor between the read and
167 /// the write such that
169 /// * evaluating the ancestor necessarily evaluates both the read and the write (for example, `&x`
170 /// and `|| x = 1` don't necessarily evaluate `x`), and
172 /// * which one is evaluated first depends on the order of sub-expression evaluation. Blocks, `if`s,
173 /// loops, `match`es, and the short-circuiting logical operators are considered to have a defined
174 /// evaluation order.
176 /// When such a read is found, the lint is triggered.
177 fn check_for_unsequenced_reads(vis: &mut ReadVisitor<'_, '_>) {
178 let map = &vis.cx.tcx.hir();
179 let mut cur_id = vis.write_expr.hir_id;
181 let parent_id = map.get_parent_node(cur_id);
182 if parent_id == cur_id {
185 let parent_node = match map.find(parent_id) {
186 Some(parent) => parent,
190 let stop_early = match parent_node {
191 Node::Expr(expr) => check_expr(vis, expr),
192 Node::Stmt(stmt) => check_stmt(vis, stmt),
194 // We reached the top of the function, stop.
197 _ => StopEarly::KeepGoing,
200 StopEarly::Stop => break,
201 StopEarly::KeepGoing => {},
208 /// Whether to stop early for the loop in `check_for_unsequenced_reads`. (If
209 /// `check_expr` weren't an independent function, this would be unnecessary and
210 /// we could just use `break`).
216 fn check_expr<'a, 'tcx>(vis: &mut ReadVisitor<'a, 'tcx>, expr: &'tcx Expr) -> StopEarly {
217 if expr.hir_id == vis.last_expr.hir_id {
218 return StopEarly::KeepGoing;
224 | ExprKind::MethodCall(..)
225 | ExprKind::Call(_, _)
226 | ExprKind::Assign(_, _)
227 | ExprKind::Index(_, _)
228 | ExprKind::Repeat(_, _)
229 | ExprKind::Struct(_, _, _) => {
230 walk_expr(vis, expr);
232 ExprKind::Binary(op, _, _) | ExprKind::AssignOp(op, _, _) => {
233 if op.node == BinOpKind::And || op.node == BinOpKind::Or {
234 // x && y and x || y always evaluate x first, so these are
235 // strictly sequenced.
237 walk_expr(vis, expr);
240 ExprKind::Closure(_, _, _, _, _) => {
243 // * `var` is defined in the closure body, in which case we've reached the top of the enclosing
244 // function and can stop, or
246 // * `var` is captured by the closure, in which case, because evaluating a closure does not evaluate
247 // its body, we don't necessarily have a write, so we need to stop to avoid generating false
250 // This is also the only place we need to stop early (grrr).
251 return StopEarly::Stop;
253 // All other expressions either have only one child or strictly
254 // sequence the evaluation order of their sub-expressions.
258 vis.last_expr = expr;
263 fn check_stmt<'a, 'tcx>(vis: &mut ReadVisitor<'a, 'tcx>, stmt: &'tcx Stmt) -> StopEarly {
265 StmtKind::Expr(ref expr) | StmtKind::Semi(ref expr) => check_expr(vis, expr),
266 // If the declaration is of a local variable, check its initializer
267 // expression if it has one. Otherwise, keep going.
268 StmtKind::Local(ref local) => local
271 .map_or(StopEarly::KeepGoing, |expr| check_expr(vis, expr)),
272 _ => StopEarly::KeepGoing,
276 /// A visitor that looks for reads from a variable.
277 struct ReadVisitor<'a, 'tcx> {
278 cx: &'a LateContext<'a, 'tcx>,
279 /// The ID of the variable we're looking for.
281 /// The expressions where the write to the variable occurred (for reporting
283 write_expr: &'tcx Expr,
284 /// The last (highest in the AST) expression we've checked, so we know not
286 last_expr: &'tcx Expr,
289 impl<'a, 'tcx> Visitor<'tcx> for ReadVisitor<'a, 'tcx> {
290 fn visit_expr(&mut self, expr: &'tcx Expr) {
291 if expr.hir_id == self.last_expr.hir_id {
296 ExprKind::Path(ref qpath) => {
298 if let QPath::Resolved(None, ref path) = *qpath;
299 if path.segments.len() == 1;
300 if let def::Res::Local(local_id) = self.cx.tables.qpath_res(qpath, expr.hir_id);
301 if local_id == self.var;
302 // Check that this is a read, not a write.
303 if !is_in_assignment_position(self.cx, expr);
307 EVAL_ORDER_DEPENDENCE,
309 "unsequenced read of a variable",
310 self.write_expr.span,
311 "whether read occurs before this write depends on evaluation order"
316 // We're about to descend a closure. Since we don't know when (or
317 // if) the closure will be evaluated, any reads in it might not
318 // occur here (or ever). Like above, bail to avoid false positives.
319 ExprKind::Closure(_, _, _, _, _) |
321 // We want to avoid a false positive when a variable name occurs
322 // only to have its address taken, so we stop here. Technically,
323 // this misses some weird cases, eg.
327 // let a = foo(&{x = 1; x}, x);
331 ExprKind::AddrOf(_, _) => {
337 walk_expr(self, expr);
339 fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
340 NestedVisitorMap::None
344 /// Returns `true` if `expr` is the LHS of an assignment, like `expr = ...`.
345 fn is_in_assignment_position(cx: &LateContext<'_, '_>, expr: &Expr) -> bool {
346 if let Some(parent) = get_parent_expr(cx, expr) {
347 if let ExprKind::Assign(ref lhs, _) = parent.kind {
348 return lhs.hir_id == expr.hir_id;