1 use rustc::hir::def_id::DefId;
2 use rustc::hir::intravisit::{Visitor, walk_expr};
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 LateLintPass for EvalOrderDependence {
60 fn check_expr(&mut self, cx: &LateContext, expr: &Expr) {
61 // Find a write to a local variable.
63 ExprAssign(ref lhs, _) | ExprAssignOp(_, ref lhs, _) => {
64 if let ExprPath(None, ref path) = lhs.node {
65 if path.segments.len() == 1 {
66 let var = cx.tcx.expect_def(lhs.id).def_id();
67 let mut visitor = ReadVisitor {
73 check_for_unsequenced_reads(&mut visitor);
80 fn check_stmt(&mut self, cx: &LateContext, stmt: &Stmt) {
82 StmtExpr(ref e, _) | StmtSemi(ref e, _) => DivergenceVisitor(cx).maybe_walk_expr(e),
83 StmtDecl(ref d, _) => {
84 if let DeclLocal(ref local) = d.node {
85 if let Local { init: Some(ref e), .. } = **local {
86 DivergenceVisitor(cx).visit_expr(e);
94 struct DivergenceVisitor<'a, 'tcx: 'a>(&'a LateContext<'a, 'tcx>);
96 impl<'a, 'tcx> DivergenceVisitor<'a, 'tcx> {
97 fn maybe_walk_expr(&mut self, e: &Expr) {
99 ExprClosure(..) => {},
100 ExprMatch(ref e, ref arms, _) => {
103 if let Some(ref guard) = arm.guard {
104 self.visit_expr(guard);
106 // make sure top level arm expressions aren't linted
107 self.maybe_walk_expr(&*arm.body);
110 _ => walk_expr(self, e),
113 fn report_diverging_sub_expr(&mut self, e: &Expr) {
116 DIVERGING_SUB_EXPRESSION,
118 "sub-expression diverges",
123 impl<'a, 'tcx, 'v> Visitor<'v> for DivergenceVisitor<'a, 'tcx> {
124 fn visit_expr(&mut self, e: &'v Expr) {
128 ExprRet(_) => self.report_diverging_sub_expr(e),
129 ExprCall(ref func, _) => match self.0.tcx.expr_ty(func).sty {
130 ty::TyFnDef(_, _, fn_ty) |
131 ty::TyFnPtr(fn_ty) => if let ty::TyNever = self.0.tcx.erase_late_bound_regions(&fn_ty.sig).output.sty {
132 self.report_diverging_sub_expr(e);
136 ExprMethodCall(..) => {
137 let method_call = ty::MethodCall::expr(e.id);
138 let borrowed_table = self.0.tcx.tables.borrow();
139 let method_type = borrowed_table.method_map.get(&method_call).expect("This should never happen.");
140 let result_ty = method_type.ty.fn_ret();
141 if let ty::TyNever = self.0.tcx.erase_late_bound_regions(&result_ty).sty {
142 self.report_diverging_sub_expr(e);
146 // do not lint expressions referencing objects of type `!`, as that required a diverging expression to begin with
149 self.maybe_walk_expr(e);
151 fn visit_block(&mut self, _: &'v Block) {
152 // don't continue over blocks, LateLintPass already does that
156 /// Walks up the AST from the the given write expression (`vis.write_expr`)
157 /// looking for reads to the same variable that are unsequenced relative to the
160 /// This means reads for which there is a common ancestor between the read and
161 /// the write such that
163 /// * evaluating the ancestor necessarily evaluates both the read and the write
164 /// (for example, `&x` and `|| x = 1` don't necessarily evaluate `x`), and
166 /// * which one is evaluated first depends on the order of sub-expression
167 /// evaluation. Blocks, `if`s, loops, `match`es, and the short-circuiting
168 /// logical operators are considered to have a defined evaluation order.
170 /// When such a read is found, the lint is triggered.
171 fn check_for_unsequenced_reads(vis: &mut ReadVisitor) {
172 let map = &vis.cx.tcx.map;
173 let mut cur_id = vis.write_expr.id;
175 let parent_id = map.get_parent_node(cur_id);
176 if parent_id == cur_id {
179 let parent_node = match map.find(parent_id) {
180 Some(parent) => parent,
184 let stop_early = match parent_node {
185 map::Node::NodeExpr(expr) => check_expr(vis, expr),
186 map::Node::NodeStmt(stmt) => check_stmt(vis, stmt),
187 map::Node::NodeItem(_) => {
188 // We reached the top of the function, stop.
191 _ => { StopEarly::KeepGoing }
194 StopEarly::Stop => break,
195 StopEarly::KeepGoing => {},
202 /// Whether to stop early for the loop in `check_for_unsequenced_reads`. (If
203 /// `check_expr` weren't an independent function, this would be unnecessary and
204 /// we could just use `break`).
210 fn check_expr<'v, 't>(vis: & mut ReadVisitor<'v, 't>, expr: &'v Expr) -> StopEarly {
211 if expr.id == vis.last_expr.id {
212 return StopEarly::KeepGoing;
218 ExprMethodCall(_, _, _) |
223 ExprStruct(_, _, _) => {
224 walk_expr(vis, expr);
226 ExprBinary(op, _, _) |
227 ExprAssignOp(op, _, _) => {
228 if op.node == BiAnd || op.node == BiOr {
229 // x && y and x || y always evaluate x first, so these are
230 // strictly sequenced.
232 walk_expr(vis, expr);
235 ExprClosure(_, _, _, _) => {
238 // * `var` is defined in the closure body, in which case we've
239 // reached the top of the enclosing function and can stop, or
241 // * `var` is captured by the closure, in which case, because
242 // evaluating a closure does not evaluate its body, we don't
243 // necessarily have a write, so we need to stop to avoid
244 // generating false positives.
246 // This is also the only place we need to stop early (grrr).
247 return StopEarly::Stop;
249 // All other expressions either have only one child or strictly
250 // sequence the evaluation order of their sub-expressions.
254 vis.last_expr = expr;
259 fn check_stmt<'v, 't>(vis: &mut ReadVisitor<'v, 't>, stmt: &'v Stmt) -> StopEarly {
261 StmtExpr(ref expr, _) |
262 StmtSemi(ref expr, _) => check_expr(vis, expr),
263 StmtDecl(ref decl, _) => {
264 // If the declaration is of a local variable, check its initializer
265 // expression if it has one. Otherwise, keep going.
266 let local = match decl.node {
267 DeclLocal(ref local) => Some(local),
270 local.and_then(|local| local.init.as_ref())
271 .map_or(StopEarly::KeepGoing, |expr| check_expr(vis, expr))
276 /// A visitor that looks for reads from a variable.
277 struct ReadVisitor<'v, 't: 'v> {
278 cx: &'v LateContext<'v, 't>,
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: &'v Expr,
284 /// The last (highest in the AST) expression we've checked, so we know not
289 impl<'v, 't> Visitor<'v> for ReadVisitor<'v, 't> {
290 fn visit_expr(&mut self, expr: &'v Expr) {
291 if expr.id == self.last_expr.id {
296 ExprPath(None, ref path) => {
297 if path.segments.len() == 1 && self.cx.tcx.expect_def(expr.id).def_id() == self.var {
298 if is_in_assignment_position(self.cx, expr) {
299 // This is a write, not a read.
303 EVAL_ORDER_DEPENDENCE,
305 "unsequenced read of a variable",
306 self.write_expr.span,
307 "whether read occurs before this write depends on evaluation order"
312 // We're about to descend a closure. Since we don't know when (or
313 // if) the closure will be evaluated, any reads in it might not
314 // occur here (or ever). Like above, bail to avoid false positives.
315 ExprClosure(_, _, _, _) |
317 // We want to avoid a false positive when a variable name occurs
318 // only to have its address taken, so we stop here. Technically,
319 // this misses some weird cases, eg.
323 // let a = foo(&{x = 1; x}, x);
327 ExprAddrOf(_, _) => {
333 walk_expr(self, expr);
337 /// Returns true if `expr` is the LHS of an assignment, like `expr = ...`.
338 fn is_in_assignment_position(cx: &LateContext, expr: &Expr) -> bool {
339 if let Some(parent) = get_parent_expr(cx, expr) {
340 if let ExprAssign(ref lhs, _) = parent.node {
341 return lhs.id == expr.id;