1 use rustc::hir::def_id::DefId;
2 use rustc::hir::intravisit::{Visitor, walk_expr, NestedVisitorMap};
6 use utils::{get_parent_expr, span_note_and_lint, span_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.
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 statements.
32 /// **Why is this bad?** It is often confusing to read. In addition, the
33 /// sub-expression evaluation order for Rust is not well documented.
35 /// **Known problems:** Someone might want to use `some_bool || panic!()` as a shorthand.
39 /// let a = b() || panic!() || c();
40 /// // `c()` is dead, `panic!()` is only called if `b()` returns `false`
41 /// let x = (a, b, c, panic!());
42 /// // can simply be replaced by `panic!()`
45 pub DIVERGING_SUB_EXPRESSION,
47 "whether an expression contains a diverging sub expression"
51 pub struct EvalOrderDependence;
53 impl LintPass for EvalOrderDependence {
54 fn get_lints(&self) -> LintArray {
55 lint_array!(EVAL_ORDER_DEPENDENCE, DIVERGING_SUB_EXPRESSION)
59 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for EvalOrderDependence {
60 fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
61 // Find a write to a local variable.
63 ExprAssign(ref lhs, _) | ExprAssignOp(_, ref lhs, _) => {
64 if let ExprPath(ref qpath) = lhs.node {
65 if let QPath::Resolved(_, ref path) = *qpath {
66 if path.segments.len() == 1 {
67 let var = cx.tcx.tables().qpath_def(qpath, lhs.id).def_id();
68 let mut visitor = ReadVisitor {
74 check_for_unsequenced_reads(&mut visitor);
82 fn check_stmt(&mut self, cx: &LateContext<'a, 'tcx>, stmt: &'tcx Stmt) {
84 StmtExpr(ref e, _) | StmtSemi(ref e, _) => DivergenceVisitor { cx: cx }.maybe_walk_expr(e),
85 StmtDecl(ref d, _) => {
86 if let DeclLocal(ref local) = d.node {
87 if let Local { init: Some(ref e), .. } = **local {
88 DivergenceVisitor { cx: cx }.visit_expr(e);
96 struct DivergenceVisitor<'a, 'tcx: 'a> {
97 cx: &'a LateContext<'a, 'tcx>,
100 impl<'a, 'tcx> DivergenceVisitor<'a, 'tcx> {
101 fn maybe_walk_expr(&mut self, e: &'tcx Expr) {
103 ExprClosure(..) => {},
104 ExprMatch(ref e, ref arms, _) => {
107 if let Some(ref guard) = arm.guard {
108 self.visit_expr(guard);
110 // make sure top level arm expressions aren't linted
111 self.maybe_walk_expr(&*arm.body);
114 _ => walk_expr(self, e),
117 fn report_diverging_sub_expr(&mut self, e: &Expr) {
120 DIVERGING_SUB_EXPRESSION,
122 "sub-expression diverges",
127 impl<'a, 'tcx> Visitor<'tcx> for DivergenceVisitor<'a, 'tcx> {
128 fn visit_expr(&mut self, e: &'tcx Expr) {
132 ExprRet(_) => self.report_diverging_sub_expr(e),
133 ExprCall(ref func, _) => match self.cx.tcx.tables().expr_ty(func).sty {
134 ty::TyFnDef(_, _, fn_ty) |
135 ty::TyFnPtr(fn_ty) => if let ty::TyNever = self.cx.tcx.erase_late_bound_regions(&fn_ty.sig).output().sty {
136 self.report_diverging_sub_expr(e);
140 ExprMethodCall(..) => {
141 let method_call = ty::MethodCall::expr(e.id);
142 let borrowed_table = self.cx.tcx.tables.borrow();
143 let method_type = borrowed_table.method_map.get(&method_call).expect("This should never happen.");
144 let result_ty = method_type.ty.fn_ret();
145 if let ty::TyNever = self.cx.tcx.erase_late_bound_regions(&result_ty).sty {
146 self.report_diverging_sub_expr(e);
150 // do not lint expressions referencing objects of type `!`, as that required a diverging expression to begin with
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::All(&self.cx.tcx.map)
163 /// Walks up the AST from the the given write expression (`vis.write_expr`)
164 /// looking for reads to the same variable that are unsequenced relative to the
167 /// This means reads for which there is a common ancestor between the read and
168 /// the write such that
170 /// * evaluating the ancestor necessarily evaluates both the read and the write
171 /// (for example, `&x` and `|| x = 1` don't necessarily evaluate `x`), and
173 /// * which one is evaluated first depends on the order of sub-expression
174 /// evaluation. Blocks, `if`s, loops, `match`es, and the short-circuiting
175 /// logical operators are considered to have a defined evaluation order.
177 /// When such a read is found, the lint is triggered.
178 fn check_for_unsequenced_reads(vis: &mut ReadVisitor) {
179 let map = &vis.cx.tcx.map;
180 let mut cur_id = vis.write_expr.id;
182 let parent_id = map.get_parent_node(cur_id);
183 if parent_id == cur_id {
186 let parent_node = match map.find(parent_id) {
187 Some(parent) => parent,
191 let stop_early = match parent_node {
192 map::Node::NodeExpr(expr) => check_expr(vis, expr),
193 map::Node::NodeStmt(stmt) => check_stmt(vis, stmt),
194 map::Node::NodeItem(_) => {
195 // We reached the top of the function, stop.
198 _ => { StopEarly::KeepGoing }
201 StopEarly::Stop => break,
202 StopEarly::KeepGoing => {},
209 /// Whether to stop early for the loop in `check_for_unsequenced_reads`. (If
210 /// `check_expr` weren't an independent function, this would be unnecessary and
211 /// we could just use `break`).
217 fn check_expr<'a, 'tcx>(vis: & mut ReadVisitor<'a, 'tcx>, expr: &'tcx Expr) -> StopEarly {
218 if expr.id == vis.last_expr.id {
219 return StopEarly::KeepGoing;
225 ExprMethodCall(_, _, _) |
230 ExprStruct(_, _, _) => {
231 walk_expr(vis, expr);
233 ExprBinary(op, _, _) |
234 ExprAssignOp(op, _, _) => {
235 if op.node == BiAnd || op.node == BiOr {
236 // x && y and x || y always evaluate x first, so these are
237 // strictly sequenced.
239 walk_expr(vis, expr);
242 ExprClosure(_, _, _, _) => {
245 // * `var` is defined in the closure body, in which case we've
246 // reached the top of the enclosing function and can stop, or
248 // * `var` is captured by the closure, in which case, because
249 // evaluating a closure does not evaluate its body, we don't
250 // necessarily have a write, so we need to stop to avoid
251 // generating false positives.
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 StmtExpr(ref expr, _) |
269 StmtSemi(ref expr, _) => check_expr(vis, expr),
270 StmtDecl(ref decl, _) => {
271 // If the declaration is of a local variable, check its initializer
272 // expression if it has one. Otherwise, keep going.
273 let local = match decl.node {
274 DeclLocal(ref local) => Some(local),
277 local.and_then(|local| local.init.as_ref())
278 .map_or(StopEarly::KeepGoing, |expr| check_expr(vis, expr))
283 /// A visitor that looks for reads from a variable.
284 struct ReadVisitor<'a, 'tcx: 'a> {
285 cx: &'a LateContext<'a, 'tcx>,
286 /// The id of the variable we're looking for.
288 /// The expressions where the write to the variable occurred (for reporting
290 write_expr: &'tcx Expr,
291 /// The last (highest in the AST) expression we've checked, so we know not
293 last_expr: &'tcx Expr,
296 impl<'a, 'tcx> Visitor<'tcx> for ReadVisitor<'a, 'tcx> {
297 fn visit_expr(&mut self, expr: &'tcx Expr) {
298 if expr.id == self.last_expr.id {
303 ExprPath(ref qpath) => {
304 if let QPath::Resolved(None, ref path) = *qpath {
305 if path.segments.len() == 1 && self.cx.tcx.tables().qpath_def(qpath, expr.id).def_id() == self.var {
306 if is_in_assignment_position(self.cx, expr) {
307 // This is a write, not a read.
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"
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 ExprClosure(_, _, _, _) |
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 ExprAddrOf(_, _) => {
342 walk_expr(self, expr);
344 fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
345 NestedVisitorMap::All(&self.cx.tcx.map)
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 ExprAssign(ref lhs, _) = parent.node {
353 return lhs.id == expr.id;