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