use rustc::hir::intravisit::{walk_expr, NestedVisitorMap, Visitor};
use rustc::hir::*;
use rustc::ty;
use rustc::lint::*;
use syntax::ast;
use utils::{get_parent_expr, span_lint, 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"
}
/// **What it does:** Checks for diverging calls that are not match arms or
/// statements.
///
/// **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:** Someone might want to use `some_bool || panic!()` as a
/// shorthand.
///
/// **Example:**
/// ```rust
/// let a = b() || panic!() || c();
/// // `c()` is dead, `panic!()` is only called if `b()` returns `false`
/// let x = (a, b, c, panic!());
/// // can simply be replaced by `panic!()`
/// ```
declare_lint! {
pub DIVERGING_SUB_EXPRESSION,
Warn,
"whether an expression contains a diverging sub expression"
}
#[derive(Copy, Clone)]
pub struct EvalOrderDependence;
impl LintPass for EvalOrderDependence {
fn get_lints(&self) -> LintArray {
lint_array!(EVAL_ORDER_DEPENDENCE, DIVERGING_SUB_EXPRESSION)
}
}
impl<'a, 'tcx> LateLintPass<'a, 'tcx> for EvalOrderDependence {
fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
// Find a write to a local variable.
match expr.node {
ExprAssign(ref lhs, _) | ExprAssignOp(_, ref lhs, _) => if let ExprPath(ref qpath) = lhs.node {
if let QPath::Resolved(_, ref path) = *qpath {
if path.segments.len() == 1 {
if let def::Def::Local(var) = cx.tables.qpath_def(qpath, lhs.hir_id) {
let mut visitor = ReadVisitor {
cx,
var,
write_expr: expr,
last_expr: expr,
};
check_for_unsequenced_reads(&mut visitor);
}
}
}
},
_ => {},
}
}
fn check_stmt(&mut self, cx: &LateContext<'a, 'tcx>, stmt: &'tcx Stmt) {
match stmt.node {
StmtExpr(ref e, _) | StmtSemi(ref e, _) => DivergenceVisitor { cx }.maybe_walk_expr(e),
StmtDecl(ref d, _) => if let DeclLocal(ref local) = d.node {
if let Local {
init: Some(ref e), ..
} = **local
{
DivergenceVisitor { cx }.visit_expr(e);
}
},
}
}
}
struct DivergenceVisitor<'a, 'tcx: 'a> {
cx: &'a LateContext<'a, 'tcx>,
}
impl<'a, 'tcx> DivergenceVisitor<'a, 'tcx> {
fn maybe_walk_expr(&mut self, e: &'tcx Expr) {
match e.node {
ExprClosure(.., _) => {},
ExprMatch(ref e, ref arms, _) => {
self.visit_expr(e);
for arm in arms {
if let Some(ref guard) = arm.guard {
self.visit_expr(guard);
}
// make sure top level arm expressions aren't linted
self.maybe_walk_expr(&*arm.body);
}
},
_ => walk_expr(self, e),
}
}
fn report_diverging_sub_expr(&mut self, e: &Expr) {
span_lint(self.cx, DIVERGING_SUB_EXPRESSION, e.span, "sub-expression diverges");
}
}
impl<'a, 'tcx> Visitor<'tcx> for DivergenceVisitor<'a, 'tcx> {
fn visit_expr(&mut self, e: &'tcx Expr) {
match e.node {
ExprAgain(_) | ExprBreak(_, _) | ExprRet(_) => self.report_diverging_sub_expr(e),
ExprCall(ref func, _) => {
let typ = self.cx.tables.expr_ty(func);
match typ.sty {
ty::TyFnDef(..) | ty::TyFnPtr(_) => {
let sig = typ.fn_sig(self.cx.tcx);
if let ty::TyNever = self.cx.tcx.erase_late_bound_regions(&sig).output().sty {
self.report_diverging_sub_expr(e);
}
},
_ => {},
}
},
ExprMethodCall(..) => {
let borrowed_table = self.cx.tables;
if borrowed_table.expr_ty(e).is_never() {
self.report_diverging_sub_expr(e);
}
},
_ => {
// do not lint expressions referencing objects of type `!`, as that required a
// diverging expression
// to begin with
},
}
self.maybe_walk_expr(e);
}
fn visit_block(&mut self, _: &'tcx Block) {
// don't continue over blocks, LateLintPass already does that
}
fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
NestedVisitorMap::None
}
}
/// Walks up the AST from 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.hir;
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<'a, 'tcx>(vis: &mut ReadVisitor<'a, 'tcx>, expr: &'tcx Expr) -> StopEarly {
if expr.id == vis.last_expr.id {
return StopEarly::KeepGoing;
}
match expr.node {
ExprArray(_) |
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<'a, 'tcx>(vis: &mut ReadVisitor<'a, 'tcx>, stmt: &'tcx 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<'a, 'tcx: 'a> {
cx: &'a LateContext<'a, 'tcx>,
/// The id of the variable we're looking for.
var: ast::NodeId,
/// The expressions where the write to the variable occurred (for reporting
/// in the lint).
write_expr: &'tcx Expr,
/// The last (highest in the AST) expression we've checked, so we know not
/// to recheck it.
last_expr: &'tcx Expr,
}
impl<'a, 'tcx> Visitor<'tcx> for ReadVisitor<'a, 'tcx> {
fn visit_expr(&mut self, expr: &'tcx Expr) {
if expr.id == self.last_expr.id {
return;
}
match expr.node {
ExprPath(ref qpath) => {
if_chain! {
if let QPath::Resolved(None, ref path) = *qpath;
if path.segments.len() == 1;
if let def::Def::Local(local_id) = self.cx.tables.qpath_def(qpath, expr.hir_id);
if local_id == self.var;
// Check that this is a read, not a write.
if !is_in_assignment_position(self.cx, expr);
then {
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);
}
fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
NestedVisitorMap::None
}
}
/// 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
}