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 /// 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 declare_lint_pass!(EvalOrderDependence => [EVAL_ORDER_DEPENDENCE, DIVERGING_SUB_EXPRESSION]);
58 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for EvalOrderDependence {
59 fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
60 // Find a write to a local variable.
62 ExprKind::Assign(ref lhs, _) | ExprKind::AssignOp(_, ref lhs, _) => {
63 if let ExprKind::Path(ref qpath) = lhs.node {
64 if let QPath::Resolved(_, ref path) = *qpath {
65 if path.segments.len() == 1 {
66 if let def::Res::Local(var) = cx.tables.qpath_res(qpath, lhs.hir_id) {
67 let mut visitor = ReadVisitor {
73 check_for_unsequenced_reads(&mut visitor);
82 fn check_stmt(&mut self, cx: &LateContext<'a, 'tcx>, stmt: &'tcx Stmt) {
84 StmtKind::Local(ref local) => {
85 if let Local { init: Some(ref e), .. } = **local {
86 DivergenceVisitor { cx }.visit_expr(e);
89 StmtKind::Expr(ref e) | StmtKind::Semi(ref e) => DivergenceVisitor { cx }.maybe_walk_expr(e),
90 StmtKind::Item(..) => {},
95 struct DivergenceVisitor<'a, 'tcx> {
96 cx: &'a LateContext<'a, 'tcx>,
99 impl<'a, 'tcx> DivergenceVisitor<'a, 'tcx> {
100 fn maybe_walk_expr(&mut self, e: &'tcx Expr) {
102 ExprKind::Closure(.., _) => {},
103 ExprKind::Match(ref e, ref arms, _) => {
106 if let Some(ref guard) = arm.guard {
108 Guard::If(if_expr) => self.visit_expr(if_expr),
111 // make sure top level arm expressions aren't linted
112 self.maybe_walk_expr(&*arm.body);
115 _ => walk_expr(self, e),
118 fn report_diverging_sub_expr(&mut self, e: &Expr) {
119 span_lint(self.cx, DIVERGING_SUB_EXPRESSION, e.span, "sub-expression diverges");
123 impl<'a, 'tcx> Visitor<'tcx> for DivergenceVisitor<'a, 'tcx> {
124 fn visit_expr(&mut self, e: &'tcx Expr) {
126 ExprKind::Continue(_) | ExprKind::Break(_, _) | ExprKind::Ret(_) => self.report_diverging_sub_expr(e),
127 ExprKind::Call(ref func, _) => {
128 let typ = self.cx.tables.expr_ty(func);
130 ty::FnDef(..) | ty::FnPtr(_) => {
131 let sig = typ.fn_sig(self.cx.tcx);
132 if let ty::Never = self.cx.tcx.erase_late_bound_regions(&sig).output().sty {
133 self.report_diverging_sub_expr(e);
139 ExprKind::MethodCall(..) => {
140 let borrowed_table = self.cx.tables;
141 if borrowed_table.expr_ty(e).is_never() {
142 self.report_diverging_sub_expr(e);
146 // do not lint expressions referencing objects of type `!`, as that required a
147 // diverging expression
151 self.maybe_walk_expr(e);
153 fn visit_block(&mut self, _: &'tcx Block) {
154 // don't continue over blocks, LateLintPass already does that
156 fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
157 NestedVisitorMap::None
161 /// Walks up the AST from the given write expression (`vis.write_expr`) looking
162 /// for reads to the same variable that are unsequenced relative to the write.
164 /// This means reads for which there is a common ancestor between the read and
165 /// the write such that
167 /// * evaluating the ancestor necessarily evaluates both the read and the write (for example, `&x`
168 /// and `|| x = 1` don't necessarily evaluate `x`), and
170 /// * which one is evaluated first depends on the order of sub-expression evaluation. Blocks, `if`s,
171 /// loops, `match`es, and the short-circuiting logical operators are considered to have a defined
172 /// evaluation order.
174 /// When such a read is found, the lint is triggered.
175 fn check_for_unsequenced_reads(vis: &mut ReadVisitor<'_, '_>) {
176 let map = &vis.cx.tcx.hir();
177 let mut cur_id = vis.write_expr.hir_id;
179 let parent_id = map.get_parent_node(cur_id);
180 if parent_id == cur_id {
183 let parent_node = match map.find(parent_id) {
184 Some(parent) => parent,
188 let stop_early = match parent_node {
189 Node::Expr(expr) => check_expr(vis, expr),
190 Node::Stmt(stmt) => check_stmt(vis, stmt),
192 // We reached the top of the function, stop.
195 _ => StopEarly::KeepGoing,
198 StopEarly::Stop => break,
199 StopEarly::KeepGoing => {},
206 /// Whether to stop early for the loop in `check_for_unsequenced_reads`. (If
207 /// `check_expr` weren't an independent function, this would be unnecessary and
208 /// we could just use `break`).
214 fn check_expr<'a, 'tcx>(vis: &mut ReadVisitor<'a, 'tcx>, expr: &'tcx Expr) -> StopEarly {
215 if expr.hir_id == vis.last_expr.hir_id {
216 return StopEarly::KeepGoing;
222 | ExprKind::MethodCall(..)
223 | ExprKind::Call(_, _)
224 | ExprKind::Assign(_, _)
225 | ExprKind::Index(_, _)
226 | ExprKind::Repeat(_, _)
227 | ExprKind::Struct(_, _, _) => {
228 walk_expr(vis, expr);
230 ExprKind::Binary(op, _, _) | ExprKind::AssignOp(op, _, _) => {
231 if op.node == BinOpKind::And || op.node == BinOpKind::Or {
232 // x && y and x || y always evaluate x first, so these are
233 // strictly sequenced.
235 walk_expr(vis, expr);
238 ExprKind::Closure(_, _, _, _, _) => {
241 // * `var` is defined in the closure body, in which case we've reached the top of the enclosing
242 // function and can stop, or
244 // * `var` is captured by the closure, in which case, because evaluating a closure does not evaluate
245 // its body, we don't necessarily have a write, so we need to stop to avoid generating false
248 // This is also the only place we need to stop early (grrr).
249 return StopEarly::Stop;
251 // All other expressions either have only one child or strictly
252 // sequence the evaluation order of their sub-expressions.
256 vis.last_expr = expr;
261 fn check_stmt<'a, 'tcx>(vis: &mut ReadVisitor<'a, 'tcx>, stmt: &'tcx Stmt) -> StopEarly {
263 StmtKind::Expr(ref expr) | StmtKind::Semi(ref expr) => check_expr(vis, expr),
264 // If the declaration is of a local variable, check its initializer
265 // expression if it has one. Otherwise, keep going.
266 StmtKind::Local(ref local) => local
269 .map_or(StopEarly::KeepGoing, |expr| check_expr(vis, expr)),
270 _ => StopEarly::KeepGoing,
274 /// A visitor that looks for reads from a variable.
275 struct ReadVisitor<'a, 'tcx> {
276 cx: &'a LateContext<'a, 'tcx>,
277 /// The ID of the variable we're looking for.
279 /// The expressions where the write to the variable occurred (for reporting
281 write_expr: &'tcx Expr,
282 /// The last (highest in the AST) expression we've checked, so we know not
284 last_expr: &'tcx Expr,
287 impl<'a, 'tcx> Visitor<'tcx> for ReadVisitor<'a, 'tcx> {
288 fn visit_expr(&mut self, expr: &'tcx Expr) {
289 if expr.hir_id == self.last_expr.hir_id {
294 ExprKind::Path(ref qpath) => {
296 if let QPath::Resolved(None, ref path) = *qpath;
297 if path.segments.len() == 1;
298 if let def::Res::Local(local_id) = self.cx.tables.qpath_res(qpath, expr.hir_id);
299 if local_id == self.var;
300 // Check that this is a read, not a write.
301 if !is_in_assignment_position(self.cx, expr);
305 EVAL_ORDER_DEPENDENCE,
307 "unsequenced read of a variable",
308 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<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
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.node {
346 return lhs.hir_id == expr.hir_id;