1 use crate::utils::{get_parent_expr, span_lint, span_lint_and_note};
2 use if_chain::if_chain;
3 use rustc::hir::map::Map;
5 use rustc_hir::intravisit::{walk_expr, NestedVisitorMap, Visitor};
6 use rustc_hir::{def, BinOpKind, Block, Expr, ExprKind, Guard, HirId, Local, Node, QPath, Stmt, StmtKind};
7 use rustc_lint::{LateContext, LateLintPass};
8 use rustc_session::{declare_lint_pass, declare_tool_lint};
10 declare_clippy_lint! {
11 /// **What it does:** Checks for a read and a write to the same variable where
12 /// whether the read occurs before or after the write depends on the evaluation
13 /// order of sub-expressions.
15 /// **Why is this bad?** It is often confusing to read. In addition, the
16 /// sub-expression evaluation order for Rust is not well documented.
18 /// **Known problems:** Code which intentionally depends on the evaluation
19 /// order, or which is correct for any evaluation order.
28 /// // Unclear whether a is 1 or 2.
30 pub EVAL_ORDER_DEPENDENCE,
32 "whether a variable read occurs before a write depends on sub-expression evaluation order"
35 declare_clippy_lint! {
36 /// **What it does:** Checks for diverging calls that are not match arms or
39 /// **Why is this bad?** It is often confusing to read. In addition, the
40 /// sub-expression evaluation order for Rust is not well documented.
42 /// **Known problems:** Someone might want to use `some_bool || panic!()` as a
47 /// # fn b() -> bool { true }
48 /// # fn c() -> bool { true }
49 /// let a = b() || panic!() || c();
50 /// // `c()` is dead, `panic!()` is only called if `b()` returns `false`
51 /// let x = (a, b, c, panic!());
52 /// // can simply be replaced by `panic!()`
54 pub DIVERGING_SUB_EXPRESSION,
56 "whether an expression contains a diverging sub expression"
59 declare_lint_pass!(EvalOrderDependence => [EVAL_ORDER_DEPENDENCE, DIVERGING_SUB_EXPRESSION]);
61 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for EvalOrderDependence {
62 fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr<'_>) {
63 // Find a write to a local variable.
65 ExprKind::Assign(ref lhs, ..) | ExprKind::AssignOp(_, ref lhs, _) => {
66 if let ExprKind::Path(ref qpath) = lhs.kind {
67 if let QPath::Resolved(_, ref path) = *qpath {
68 if path.segments.len() == 1 {
69 if let def::Res::Local(var) = cx.tables.qpath_res(qpath, lhs.hir_id) {
70 let mut visitor = ReadVisitor {
76 check_for_unsequenced_reads(&mut visitor);
85 fn check_stmt(&mut self, cx: &LateContext<'a, 'tcx>, stmt: &'tcx Stmt<'_>) {
87 StmtKind::Local(ref local) => {
88 if let Local { init: Some(ref e), .. } = **local {
89 DivergenceVisitor { cx }.visit_expr(e);
92 StmtKind::Expr(ref e) | StmtKind::Semi(ref e) => DivergenceVisitor { cx }.maybe_walk_expr(e),
93 StmtKind::Item(..) => {},
98 struct DivergenceVisitor<'a, 'tcx> {
99 cx: &'a LateContext<'a, 'tcx>,
102 impl<'a, 'tcx> DivergenceVisitor<'a, 'tcx> {
103 fn maybe_walk_expr(&mut self, e: &'tcx Expr<'_>) {
105 ExprKind::Closure(..) => {},
106 ExprKind::Match(ref e, arms, _) => {
109 if let Some(ref guard) = arm.guard {
111 Guard::If(if_expr) => 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.tables.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.tables;
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<'a, '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 {
301 ExprKind::Path(ref qpath) => {
303 if let QPath::Resolved(None, ref path) = *qpath;
304 if path.segments.len() == 1;
305 if let def::Res::Local(local_id) = self.cx.tables.qpath_res(qpath, expr.hir_id);
306 if local_id == self.var;
307 // Check that this is a read, not a write.
308 if !is_in_assignment_position(self.cx, expr);
312 EVAL_ORDER_DEPENDENCE,
314 "unsequenced read of a variable",
315 self.write_expr.span,
316 "whether read occurs before this write depends on evaluation order"
321 // We're about to descend a closure. Since we don't know when (or
322 // if) the closure will be evaluated, any reads in it might not
323 // occur here (or ever). Like above, bail to avoid false positives.
324 ExprKind::Closure(_, _, _, _, _) |
326 // We want to avoid a false positive when a variable name occurs
327 // only to have its address taken, so we stop here. Technically,
328 // this misses some weird cases, eg.
332 // let a = foo(&{x = 1; x}, x);
336 ExprKind::AddrOf(_, _, _) => {
342 walk_expr(self, expr);
344 fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
345 NestedVisitorMap::None
349 /// Returns `true` if `expr` is the LHS of an assignment, like `expr = ...`.
350 fn is_in_assignment_position(cx: &LateContext<'_, '_>, expr: &Expr<'_>) -> bool {
351 if let Some(parent) = get_parent_expr(cx, expr) {
352 if let ExprKind::Assign(ref lhs, ..) = parent.kind {
353 return lhs.hir_id == expr.hir_id;