1 use rustc::hir::intravisit::{walk_expr, NestedVisitorMap, Visitor};
6 use utils::{get_parent_expr, span_lint, span_note_and_lint};
8 /// **What it does:** Checks for a read and a write to the same variable where
9 /// whether the read occurs before or after the write depends on the evaluation
10 /// order of sub-expressions.
12 /// **Why is this bad?** It is often confusing to read. In addition, the
13 /// sub-expression evaluation order for Rust is not well documented.
15 /// **Known problems:** Code which intentionally depends on the evaluation
16 /// order, or which is correct for any evaluation order.
21 /// let a = {x = 1; 1} + x;
22 /// // Unclear whether a is 1 or 2.
24 declare_clippy_lint! {
25 pub EVAL_ORDER_DEPENDENCE,
27 "whether a variable read occurs before a write depends on sub-expression evaluation order"
30 /// **What it does:** Checks for diverging calls that are not match arms or
33 /// **Why is this bad?** It is often confusing to read. In addition, the
34 /// sub-expression evaluation order for Rust is not well documented.
36 /// **Known problems:** Someone might want to use `some_bool || panic!()` as a
41 /// let a = b() || panic!() || c();
42 /// // `c()` is dead, `panic!()` is only called if `b()` returns `false`
43 /// let x = (a, b, c, panic!());
44 /// // can simply be replaced by `panic!()`
46 declare_clippy_lint! {
47 pub DIVERGING_SUB_EXPRESSION,
49 "whether an expression contains a diverging sub expression"
52 #[derive(Copy, Clone)]
53 pub struct EvalOrderDependence;
55 impl LintPass for EvalOrderDependence {
56 fn get_lints(&self) -> LintArray {
57 lint_array!(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 ExprAssign(ref lhs, _) | ExprAssignOp(_, ref lhs, _) => if let ExprPath(ref qpath) = lhs.node {
66 if let QPath::Resolved(_, ref path) = *qpath {
67 if path.segments.len() == 1 {
68 if let def::Def::Local(var) = cx.tables.qpath_def(qpath, lhs.hir_id) {
69 let mut visitor = ReadVisitor {
75 check_for_unsequenced_reads(&mut visitor);
83 fn check_stmt(&mut self, cx: &LateContext<'a, 'tcx>, stmt: &'tcx Stmt) {
85 StmtExpr(ref e, _) | StmtSemi(ref e, _) => DivergenceVisitor { cx }.maybe_walk_expr(e),
86 StmtDecl(ref d, _) => if let DeclLocal(ref local) = d.node {
91 DivergenceVisitor { cx }.visit_expr(e);
98 struct DivergenceVisitor<'a, 'tcx: 'a> {
99 cx: &'a LateContext<'a, 'tcx>,
102 impl<'a, 'tcx> DivergenceVisitor<'a, 'tcx> {
103 fn maybe_walk_expr(&mut self, e: &'tcx Expr) {
105 ExprClosure(.., _) => {},
106 ExprMatch(ref e, ref arms, _) => {
109 if let Some(ref guard) = arm.guard {
110 self.visit_expr(guard);
112 // make sure top level arm expressions aren't linted
113 self.maybe_walk_expr(&*arm.body);
116 _ => walk_expr(self, e),
119 fn report_diverging_sub_expr(&mut self, e: &Expr) {
120 span_lint(self.cx, DIVERGING_SUB_EXPRESSION, e.span, "sub-expression diverges");
124 impl<'a, 'tcx> Visitor<'tcx> for DivergenceVisitor<'a, 'tcx> {
125 fn visit_expr(&mut self, e: &'tcx Expr) {
127 ExprAgain(_) | ExprBreak(_, _) | ExprRet(_) => self.report_diverging_sub_expr(e),
128 ExprCall(ref func, _) => {
129 let typ = self.cx.tables.expr_ty(func);
131 ty::TyFnDef(..) | ty::TyFnPtr(_) => {
132 let sig = typ.fn_sig(self.cx.tcx);
133 if let ty::TyNever = self.cx.tcx.erase_late_bound_regions(&sig).output().sty {
134 self.report_diverging_sub_expr(e);
140 ExprMethodCall(..) => {
141 let borrowed_table = self.cx.tables;
142 if borrowed_table.expr_ty(e).is_never() {
143 self.report_diverging_sub_expr(e);
147 // do not lint expressions referencing objects of type `!`, as that required a
148 // diverging expression
152 self.maybe_walk_expr(e);
154 fn visit_block(&mut self, _: &'tcx Block) {
155 // don't continue over blocks, LateLintPass already does that
157 fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
158 NestedVisitorMap::None
162 /// Walks up the AST from the given write expression (`vis.write_expr`) looking
163 /// for reads to the same variable that are unsequenced relative to the write.
165 /// This means reads for which there is a common ancestor between the read and
166 /// the write such that
168 /// * evaluating the ancestor necessarily evaluates both the read and the write
169 /// (for example, `&x` and `|| x = 1` don't necessarily evaluate `x`), and
171 /// * which one is evaluated first depends on the order of sub-expression
172 /// evaluation. Blocks, `if`s, loops, `match`es, and the short-circuiting
173 /// logical operators are considered to have a defined evaluation order.
175 /// When such a read is found, the lint is triggered.
176 fn check_for_unsequenced_reads(vis: &mut ReadVisitor) {
177 let map = &vis.cx.tcx.hir;
178 let mut cur_id = vis.write_expr.id;
180 let parent_id = map.get_parent_node(cur_id);
181 if parent_id == cur_id {
184 let parent_node = match map.find(parent_id) {
185 Some(parent) => parent,
189 let stop_early = match parent_node {
190 map::Node::NodeExpr(expr) => check_expr(vis, expr),
191 map::Node::NodeStmt(stmt) => check_stmt(vis, stmt),
192 map::Node::NodeItem(_) => {
193 // We reached the top of the function, stop.
196 _ => StopEarly::KeepGoing,
199 StopEarly::Stop => break,
200 StopEarly::KeepGoing => {},
207 /// Whether to stop early for the loop in `check_for_unsequenced_reads`. (If
208 /// `check_expr` weren't an independent function, this would be unnecessary and
209 /// we could just use `break`).
215 fn check_expr<'a, 'tcx>(vis: &mut ReadVisitor<'a, 'tcx>, expr: &'tcx Expr) -> StopEarly {
216 if expr.id == vis.last_expr.id {
217 return StopEarly::KeepGoing;
228 ExprStruct(_, _, _) => {
229 walk_expr(vis, expr);
231 ExprBinary(op, _, _) | ExprAssignOp(op, _, _) => {
232 if op.node == BiAnd || op.node == BiOr {
233 // x && y and x || y always evaluate x first, so these are
234 // strictly sequenced.
236 walk_expr(vis, expr);
239 ExprClosure(_, _, _, _, _) => {
242 // * `var` is defined in the closure body, in which case we've
243 // reached the top of the enclosing function and can stop, or
245 // * `var` is captured by the closure, in which case, because
246 // evaluating a closure does not evaluate its body, we don't
247 // necessarily have a write, so we need to stop to avoid
248 // generating false positives.
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 StmtExpr(ref expr, _) | StmtSemi(ref expr, _) => check_expr(vis, expr),
266 StmtDecl(ref decl, _) => {
267 // If the declaration is of a local variable, check its initializer
268 // expression if it has one. Otherwise, keep going.
269 let local = match decl.node {
270 DeclLocal(ref local) => Some(local),
274 .and_then(|local| local.init.as_ref())
275 .map_or(StopEarly::KeepGoing, |expr| check_expr(vis, expr))
280 /// A visitor that looks for reads from a variable.
281 struct ReadVisitor<'a, 'tcx: 'a> {
282 cx: &'a LateContext<'a, 'tcx>,
283 /// The id of the variable we're looking for.
285 /// The expressions where the write to the variable occurred (for reporting
287 write_expr: &'tcx Expr,
288 /// The last (highest in the AST) expression we've checked, so we know not
290 last_expr: &'tcx Expr,
293 impl<'a, 'tcx> Visitor<'tcx> for ReadVisitor<'a, 'tcx> {
294 fn visit_expr(&mut self, expr: &'tcx Expr) {
295 if expr.id == self.last_expr.id {
300 ExprPath(ref qpath) => {
302 if let QPath::Resolved(None, ref path) = *qpath;
303 if path.segments.len() == 1;
304 if let def::Def::Local(local_id) = self.cx.tables.qpath_def(qpath, expr.hir_id);
305 if local_id == self.var;
306 // Check that this is a read, not a write.
307 if !is_in_assignment_position(self.cx, expr);
311 EVAL_ORDER_DEPENDENCE,
313 "unsequenced read of a variable",
314 self.write_expr.span,
315 "whether read occurs before this write depends on evaluation order"
320 // We're about to descend a closure. Since we don't know when (or
321 // if) the closure will be evaluated, any reads in it might not
322 // occur here (or ever). Like above, bail to avoid false positives.
323 ExprClosure(_, _, _, _, _) |
325 // We want to avoid a false positive when a variable name occurs
326 // only to have its address taken, so we stop here. Technically,
327 // this misses some weird cases, eg.
331 // let a = foo(&{x = 1; x}, x);
335 ExprAddrOf(_, _) => {
341 walk_expr(self, expr);
343 fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
344 NestedVisitorMap::None
348 /// Returns true if `expr` is the LHS of an assignment, like `expr = ...`.
349 fn is_in_assignment_position(cx: &LateContext, expr: &Expr) -> bool {
350 if let Some(parent) = get_parent_expr(cx, expr) {
351 if let ExprAssign(ref lhs, _) = parent.node {
352 return lhs.id == expr.id;