251 lines
8.4 KiB
Rust
251 lines
8.4 KiB
Rust
use rustc::hir::def_id::DefId;
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use rustc::hir::intravisit::{Visitor, walk_expr};
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use rustc::hir::*;
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use rustc::lint::*;
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use utils::{get_parent_expr, span_note_and_lint};
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/// **What it does:** Checks for a read and a write to the same variable where
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/// whether the read occurs before or after the write depends on the evaluation
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/// order of sub-expressions.
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///
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/// **Why is this bad?** It is often confusing to read. In addition, the
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/// sub-expression evaluation order for Rust is not well documented.
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///
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/// **Known problems:** Code which intentionally depends on the evaluation
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/// order, or which is correct for any evaluation order.
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///
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/// **Example:**
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/// ```rust
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/// let mut x = 0;
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/// let a = {x = 1; 1} + x;
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/// // Unclear whether a is 1 or 2.
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/// ```
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declare_lint! {
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pub EVAL_ORDER_DEPENDENCE,
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Warn,
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"whether a variable read occurs before a write depends on sub-expression evaluation order"
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}
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#[derive(Copy,Clone)]
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pub struct EvalOrderDependence;
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impl LintPass for EvalOrderDependence {
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fn get_lints(&self) -> LintArray {
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lint_array!(EVAL_ORDER_DEPENDENCE)
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}
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}
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impl LateLintPass for EvalOrderDependence {
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fn check_expr(&mut self, cx: &LateContext, expr: &Expr) {
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// Find a write to a local variable.
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match expr.node {
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ExprAssign(ref lhs, _) | ExprAssignOp(_, ref lhs, _) => {
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if let ExprPath(None, ref path) = lhs.node {
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if path.segments.len() == 1 {
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let var = cx.tcx.expect_def(lhs.id).def_id();
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let mut visitor = ReadVisitor {
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cx: cx,
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var: var,
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write_expr: expr,
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last_expr: expr,
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};
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check_for_unsequenced_reads(&mut visitor);
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}
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}
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}
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_ => {}
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}
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}
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}
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/// Walks up the AST from the the given write expression (`vis.write_expr`)
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/// looking for reads to the same variable that are unsequenced relative to the
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/// write.
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///
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/// This means reads for which there is a common ancestor between the read and
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/// the write such that
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///
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/// * evaluating the ancestor necessarily evaluates both the read and the write
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/// (for example, `&x` and `|| x = 1` don't necessarily evaluate `x`), and
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///
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/// * which one is evaluated first depends on the order of sub-expression
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/// evaluation. Blocks, `if`s, loops, `match`es, and the short-circuiting
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/// logical operators are considered to have a defined evaluation order.
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///
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/// When such a read is found, the lint is triggered.
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fn check_for_unsequenced_reads(vis: &mut ReadVisitor) {
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let map = &vis.cx.tcx.map;
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let mut cur_id = vis.write_expr.id;
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loop {
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let parent_id = map.get_parent_node(cur_id);
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if parent_id == cur_id {
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break;
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}
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let parent_node = match map.find(parent_id) {
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Some(parent) => parent,
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None => break,
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};
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let stop_early = match parent_node {
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map::Node::NodeExpr(expr) => check_expr(vis, expr),
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map::Node::NodeStmt(stmt) => check_stmt(vis, stmt),
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map::Node::NodeItem(_) => {
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// We reached the top of the function, stop.
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break;
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},
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_ => { StopEarly::KeepGoing }
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};
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match stop_early {
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StopEarly::Stop => break,
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StopEarly::KeepGoing => {},
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}
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cur_id = parent_id;
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}
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}
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/// Whether to stop early for the loop in `check_for_unsequenced_reads`. (If
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/// `check_expr` weren't an independent function, this would be unnecessary and
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/// we could just use `break`).
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enum StopEarly {
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KeepGoing,
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Stop,
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}
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fn check_expr<'v, 't>(vis: & mut ReadVisitor<'v, 't>, expr: &'v Expr) -> StopEarly {
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if expr.id == vis.last_expr.id {
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return StopEarly::KeepGoing;
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}
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match expr.node {
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ExprVec(_) |
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ExprTup(_) |
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ExprMethodCall(_, _, _) |
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ExprCall(_, _) |
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ExprAssign(_, _) |
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ExprIndex(_, _) |
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ExprRepeat(_, _) |
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ExprStruct(_, _, _) => {
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walk_expr(vis, expr);
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}
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ExprBinary(op, _, _) |
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ExprAssignOp(op, _, _) => {
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if op.node == BiAnd || op.node == BiOr {
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// x && y and x || y always evaluate x first, so these are
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// strictly sequenced.
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} else {
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walk_expr(vis, expr);
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}
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}
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ExprClosure(_, _, _, _) => {
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// Either
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//
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// * `var` is defined in the closure body, in which case we've
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// reached the top of the enclosing function and can stop, or
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//
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// * `var` is captured by the closure, in which case, because
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// evaluating a closure does not evaluate its body, we don't
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// necessarily have a write, so we need to stop to avoid
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// generating false positives.
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//
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// This is also the only place we need to stop early (grrr).
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return StopEarly::Stop;
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}
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// All other expressions either have only one child or strictly
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// sequence the evaluation order of their sub-expressions.
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_ => {}
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}
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vis.last_expr = expr;
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StopEarly::KeepGoing
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}
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fn check_stmt<'v, 't>(vis: &mut ReadVisitor<'v, 't>, stmt: &'v Stmt) -> StopEarly {
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match stmt.node {
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StmtExpr(ref expr, _) |
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StmtSemi(ref expr, _) => check_expr(vis, expr),
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StmtDecl(ref decl, _) => {
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// If the declaration is of a local variable, check its initializer
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// expression if it has one. Otherwise, keep going.
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let local = match decl.node {
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DeclLocal(ref local) => Some(local),
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_ => None,
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};
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local.and_then(|local| local.init.as_ref())
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.map_or(StopEarly::KeepGoing, |expr| check_expr(vis, expr))
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}
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}
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}
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/// A visitor that looks for reads from a variable.
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struct ReadVisitor<'v, 't: 'v> {
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cx: &'v LateContext<'v, 't>,
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/// The id of the variable we're looking for.
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var: DefId,
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/// The expressions where the write to the variable occurred (for reporting
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/// in the lint).
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write_expr: &'v Expr,
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/// The last (highest in the AST) expression we've checked, so we know not
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/// to recheck it.
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last_expr: &'v Expr,
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}
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impl<'v, 't> Visitor<'v> for ReadVisitor<'v, 't> {
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fn visit_expr(&mut self, expr: &'v Expr) {
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if expr.id == self.last_expr.id {
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return;
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}
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match expr.node {
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ExprPath(None, ref path) => {
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if path.segments.len() == 1 && self.cx.tcx.expect_def(expr.id).def_id() == self.var {
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if is_in_assignment_position(self.cx, expr) {
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// This is a write, not a read.
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} else {
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span_note_and_lint(
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self.cx,
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EVAL_ORDER_DEPENDENCE,
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expr.span,
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"unsequenced read of a variable",
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self.write_expr.span,
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"whether read occurs before this write depends on evaluation order"
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);
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}
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}
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}
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// We're about to descend a closure. Since we don't know when (or
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// if) the closure will be evaluated, any reads in it might not
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// occur here (or ever). Like above, bail to avoid false positives.
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ExprClosure(_, _, _, _) |
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// We want to avoid a false positive when a variable name occurs
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// only to have its address taken, so we stop here. Technically,
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// this misses some weird cases, eg.
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//
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// ```rust
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// let mut x = 0;
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// let a = foo(&{x = 1; x}, x);
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// ```
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//
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// TODO: fix this
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ExprAddrOf(_, _) => {
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return;
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}
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_ => {}
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}
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walk_expr(self, expr);
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}
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}
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/// Returns true if `expr` is the LHS of an assignment, like `expr = ...`.
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fn is_in_assignment_position(cx: &LateContext, expr: &Expr) -> bool {
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if let Some(parent) = get_parent_expr(cx, expr) {
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if let ExprAssign(ref lhs, _) = parent.node {
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return lhs.id == expr.id;
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}
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}
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false
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}
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