1 use clippy_utils::diagnostics::{span_lint, span_lint_and_note};
2 use clippy_utils::{get_parent_expr, path_to_local, path_to_local_id};
3 use if_chain::if_chain;
4 use rustc_hir::intravisit::{walk_expr, NestedVisitorMap, Visitor};
5 use rustc_hir::{BinOpKind, Block, Expr, ExprKind, Guard, HirId, Local, Node, Stmt, StmtKind};
6 use rustc_lint::{LateContext, LateLintPass};
7 use rustc_middle::hir::map::Map;
9 use rustc_session::{declare_lint_pass, declare_tool_lint};
11 declare_clippy_lint! {
12 /// **What it does:** Checks for a read and a write to the same variable where
13 /// whether the read occurs before or after the write depends on the evaluation
14 /// order of sub-expressions.
16 /// **Why is this bad?** It is often confusing to read. In addition, the
17 /// sub-expression evaluation order for Rust is not well documented.
19 /// **Known problems:** Code which intentionally depends on the evaluation
20 /// order, or which is correct for any evaluation order.
31 /// // Unclear whether a is 1 or 2.
40 pub EVAL_ORDER_DEPENDENCE,
42 "whether a variable read occurs before a write depends on sub-expression evaluation order"
45 declare_clippy_lint! {
46 /// **What it does:** Checks for diverging calls that are not match arms or
49 /// **Why is this bad?** It is often confusing to read. In addition, the
50 /// sub-expression evaluation order for Rust is not well documented.
52 /// **Known problems:** Someone might want to use `some_bool || panic!()` as a
57 /// # fn b() -> bool { true }
58 /// # fn c() -> bool { true }
59 /// let a = b() || panic!() || c();
60 /// // `c()` is dead, `panic!()` is only called if `b()` returns `false`
61 /// let x = (a, b, c, panic!());
62 /// // can simply be replaced by `panic!()`
64 pub DIVERGING_SUB_EXPRESSION,
66 "whether an expression contains a diverging sub expression"
69 declare_lint_pass!(EvalOrderDependence => [EVAL_ORDER_DEPENDENCE, DIVERGING_SUB_EXPRESSION]);
71 impl<'tcx> LateLintPass<'tcx> for EvalOrderDependence {
72 fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
73 // Find a write to a local variable.
75 if let ExprKind::Assign(lhs, ..) | ExprKind::AssignOp(_, lhs, _) = expr.kind;
76 if let Some(var) = path_to_local(lhs);
77 if expr.span.desugaring_kind().is_none();
78 then { var } else { return; }
80 let mut visitor = ReadVisitor {
86 check_for_unsequenced_reads(&mut visitor);
88 fn check_stmt(&mut self, cx: &LateContext<'tcx>, stmt: &'tcx Stmt<'_>) {
90 StmtKind::Local(local) => {
91 if let Local { init: Some(e), .. } = local {
92 DivergenceVisitor { cx }.visit_expr(e);
95 StmtKind::Expr(e) | StmtKind::Semi(e) => DivergenceVisitor { cx }.maybe_walk_expr(e),
96 StmtKind::Item(..) => {},
101 struct DivergenceVisitor<'a, 'tcx> {
102 cx: &'a LateContext<'tcx>,
105 impl<'a, 'tcx> DivergenceVisitor<'a, 'tcx> {
106 fn maybe_walk_expr(&mut self, e: &'tcx Expr<'_>) {
108 ExprKind::Closure(..) => {},
109 ExprKind::Match(e, arms, _) => {
112 if let Some(Guard::If(if_expr)) = arm.guard {
113 self.visit_expr(if_expr);
115 // make sure top level arm expressions aren't linted
116 self.maybe_walk_expr(&*arm.body);
119 _ => walk_expr(self, e),
122 fn report_diverging_sub_expr(&mut self, e: &Expr<'_>) {
123 span_lint(self.cx, DIVERGING_SUB_EXPRESSION, e.span, "sub-expression diverges");
127 impl<'a, 'tcx> Visitor<'tcx> for DivergenceVisitor<'a, 'tcx> {
128 type Map = Map<'tcx>;
130 fn visit_expr(&mut self, e: &'tcx Expr<'_>) {
132 ExprKind::Continue(_) | ExprKind::Break(_, _) | ExprKind::Ret(_) => self.report_diverging_sub_expr(e),
133 ExprKind::Call(func, _) => {
134 let typ = self.cx.typeck_results().expr_ty(func);
136 ty::FnDef(..) | ty::FnPtr(_) => {
137 let sig = typ.fn_sig(self.cx.tcx);
138 if let ty::Never = self.cx.tcx.erase_late_bound_regions(sig).output().kind() {
139 self.report_diverging_sub_expr(e);
145 ExprKind::MethodCall(..) => {
146 let borrowed_table = self.cx.typeck_results();
147 if borrowed_table.expr_ty(e).is_never() {
148 self.report_diverging_sub_expr(e);
152 // do not lint expressions referencing objects of type `!`, as that required a
153 // diverging expression
157 self.maybe_walk_expr(e);
159 fn visit_block(&mut self, _: &'tcx Block<'_>) {
160 // don't continue over blocks, LateLintPass already does that
162 fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
163 NestedVisitorMap::None
167 /// Walks up the AST from the given write expression (`vis.write_expr`) looking
168 /// for reads to the same variable that are unsequenced relative to the write.
170 /// This means reads for which there is a common ancestor between the read and
171 /// the write such that
173 /// * evaluating the ancestor necessarily evaluates both the read and the write (for example, `&x`
174 /// and `|| x = 1` don't necessarily evaluate `x`), and
176 /// * which one is evaluated first depends on the order of sub-expression evaluation. Blocks, `if`s,
177 /// loops, `match`es, and the short-circuiting logical operators are considered to have a defined
178 /// evaluation order.
180 /// When such a read is found, the lint is triggered.
181 fn check_for_unsequenced_reads(vis: &mut ReadVisitor<'_, '_>) {
182 let map = &vis.cx.tcx.hir();
183 let mut cur_id = vis.write_expr.hir_id;
185 let parent_id = map.get_parent_node(cur_id);
186 if parent_id == cur_id {
189 let parent_node = match map.find(parent_id) {
190 Some(parent) => parent,
194 let stop_early = match parent_node {
195 Node::Expr(expr) => check_expr(vis, expr),
196 Node::Stmt(stmt) => check_stmt(vis, stmt),
198 // We reached the top of the function, stop.
201 _ => StopEarly::KeepGoing,
204 StopEarly::Stop => break,
205 StopEarly::KeepGoing => {},
212 /// Whether to stop early for the loop in `check_for_unsequenced_reads`. (If
213 /// `check_expr` weren't an independent function, this would be unnecessary and
214 /// we could just use `break`).
220 fn check_expr<'a, 'tcx>(vis: &mut ReadVisitor<'a, 'tcx>, expr: &'tcx Expr<'_>) -> StopEarly {
221 if expr.hir_id == vis.last_expr.hir_id {
222 return StopEarly::KeepGoing;
228 | ExprKind::MethodCall(..)
229 | ExprKind::Call(_, _)
230 | ExprKind::Assign(..)
231 | ExprKind::Index(_, _)
232 | ExprKind::Repeat(_, _)
233 | ExprKind::Struct(_, _, _) => {
234 walk_expr(vis, expr);
236 ExprKind::Binary(op, _, _) | ExprKind::AssignOp(op, _, _) => {
237 if op.node == BinOpKind::And || op.node == BinOpKind::Or {
238 // x && y and x || y always evaluate x first, so these are
239 // strictly sequenced.
241 walk_expr(vis, expr);
244 ExprKind::Closure(_, _, _, _, _) => {
247 // * `var` is defined in the closure body, in which case we've reached the top of the enclosing
248 // function and can stop, or
250 // * `var` is captured by the closure, in which case, because evaluating a closure does not evaluate
251 // its body, we don't necessarily have a write, so we need to stop to avoid generating false
254 // This is also the only place we need to stop early (grrr).
255 return StopEarly::Stop;
257 // All other expressions either have only one child or strictly
258 // sequence the evaluation order of their sub-expressions.
262 vis.last_expr = expr;
267 fn check_stmt<'a, 'tcx>(vis: &mut ReadVisitor<'a, 'tcx>, stmt: &'tcx Stmt<'_>) -> StopEarly {
269 StmtKind::Expr(expr) | StmtKind::Semi(expr) => check_expr(vis, expr),
270 // If the declaration is of a local variable, check its initializer
271 // expression if it has one. Otherwise, keep going.
272 StmtKind::Local(local) => local
275 .map_or(StopEarly::KeepGoing, |expr| check_expr(vis, expr)),
276 StmtKind::Item(..) => StopEarly::KeepGoing,
280 /// A visitor that looks for reads from a variable.
281 struct ReadVisitor<'a, 'tcx> {
282 cx: &'a LateContext<'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<'tcx>,
288 /// The last (highest in the AST) expression we've checked, so we know not
290 last_expr: &'tcx Expr<'tcx>,
293 impl<'a, 'tcx> Visitor<'tcx> for ReadVisitor<'a, 'tcx> {
294 type Map = Map<'tcx>;
296 fn visit_expr(&mut self, expr: &'tcx Expr<'_>) {
297 if expr.hir_id == self.last_expr.hir_id {
301 if path_to_local_id(expr, self.var) {
302 // Check that this is a read, not a write.
303 if !is_in_assignment_position(self.cx, expr) {
306 EVAL_ORDER_DEPENDENCE,
308 &format!("unsequenced read of `{}`", self.cx.tcx.hir().name(self.var)),
309 Some(self.write_expr.span),
310 "whether read occurs before this write depends on evaluation order",
315 // We're about to descend a closure. Since we don't know when (or
316 // if) the closure will be evaluated, any reads in it might not
317 // occur here (or ever). Like above, bail to avoid false positives.
318 ExprKind::Closure(_, _, _, _, _) |
320 // We want to avoid a false positive when a variable name occurs
321 // only to have its address taken, so we stop here. Technically,
322 // this misses some weird cases, eg.
326 // let a = foo(&{x = 1; x}, x);
330 ExprKind::AddrOf(_, _, _) => {
336 walk_expr(self, expr);
338 fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
339 NestedVisitorMap::None
343 /// Returns `true` if `expr` is the LHS of an assignment, like `expr = ...`.
344 fn is_in_assignment_position(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
345 if let Some(parent) = get_parent_expr(cx, expr) {
346 if let ExprKind::Assign(lhs, ..) = parent.kind {
347 return lhs.hir_id == expr.hir_id;