356 lines
13 KiB
Rust
356 lines
13 KiB
Rust
use crate::utils::{get_parent_expr, span_lint, span_lint_and_note};
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use if_chain::if_chain;
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use rustc_hir::intravisit::{walk_expr, NestedVisitorMap, Visitor};
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use rustc_hir::{def, BinOpKind, Block, Expr, ExprKind, Guard, HirId, Local, Node, QPath, Stmt, StmtKind};
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use rustc_lint::{LateContext, LateLintPass};
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use rustc_middle::hir::map::Map;
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use rustc_middle::ty;
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use rustc_session::{declare_lint_pass, declare_tool_lint};
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declare_clippy_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 = {
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/// x = 1;
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/// 1
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/// } + x;
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/// // Unclear whether a is 1 or 2.
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/// ```
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pub EVAL_ORDER_DEPENDENCE,
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complexity,
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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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declare_clippy_lint! {
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/// **What it does:** Checks for diverging calls that are not match arms or
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/// statements.
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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:** Someone might want to use `some_bool || panic!()` as a
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/// shorthand.
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///
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/// **Example:**
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/// ```rust,no_run
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/// # fn b() -> bool { true }
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/// # fn c() -> bool { true }
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/// let a = b() || panic!() || c();
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/// // `c()` is dead, `panic!()` is only called if `b()` returns `false`
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/// let x = (a, b, c, panic!());
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/// // can simply be replaced by `panic!()`
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/// ```
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pub DIVERGING_SUB_EXPRESSION,
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complexity,
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"whether an expression contains a diverging sub expression"
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}
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declare_lint_pass!(EvalOrderDependence => [EVAL_ORDER_DEPENDENCE, DIVERGING_SUB_EXPRESSION]);
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impl<'a, 'tcx> LateLintPass<'a, 'tcx> for EvalOrderDependence {
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fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr<'_>) {
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// Find a write to a local variable.
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match expr.kind {
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ExprKind::Assign(ref lhs, ..) | ExprKind::AssignOp(_, ref lhs, _) => {
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if let ExprKind::Path(ref qpath) = lhs.kind {
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if let QPath::Resolved(_, ref path) = *qpath {
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if path.segments.len() == 1 {
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if let def::Res::Local(var) = cx.tables.qpath_res(qpath, lhs.hir_id) {
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let mut visitor = ReadVisitor {
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cx,
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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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}
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fn check_stmt(&mut self, cx: &LateContext<'a, 'tcx>, stmt: &'tcx Stmt<'_>) {
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match stmt.kind {
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StmtKind::Local(ref local) => {
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if let Local { init: Some(ref e), .. } = **local {
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DivergenceVisitor { cx }.visit_expr(e);
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}
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},
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StmtKind::Expr(ref e) | StmtKind::Semi(ref e) => DivergenceVisitor { cx }.maybe_walk_expr(e),
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StmtKind::Item(..) => {},
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}
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}
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}
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struct DivergenceVisitor<'a, 'tcx> {
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cx: &'a LateContext<'a, 'tcx>,
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}
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impl<'a, 'tcx> DivergenceVisitor<'a, 'tcx> {
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fn maybe_walk_expr(&mut self, e: &'tcx Expr<'_>) {
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match e.kind {
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ExprKind::Closure(..) => {},
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ExprKind::Match(ref e, arms, _) => {
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self.visit_expr(e);
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for arm in arms {
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if let Some(Guard::If(if_expr)) = arm.guard {
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self.visit_expr(if_expr)
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}
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// make sure top level arm expressions aren't linted
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self.maybe_walk_expr(&*arm.body);
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}
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},
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_ => walk_expr(self, e),
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}
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}
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fn report_diverging_sub_expr(&mut self, e: &Expr<'_>) {
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span_lint(self.cx, DIVERGING_SUB_EXPRESSION, e.span, "sub-expression diverges");
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}
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}
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impl<'a, 'tcx> Visitor<'tcx> for DivergenceVisitor<'a, 'tcx> {
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type Map = Map<'tcx>;
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fn visit_expr(&mut self, e: &'tcx Expr<'_>) {
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match e.kind {
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ExprKind::Continue(_) | ExprKind::Break(_, _) | ExprKind::Ret(_) => self.report_diverging_sub_expr(e),
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ExprKind::Call(ref func, _) => {
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let typ = self.cx.tables.expr_ty(func);
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match typ.kind {
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ty::FnDef(..) | ty::FnPtr(_) => {
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let sig = typ.fn_sig(self.cx.tcx);
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if let ty::Never = self.cx.tcx.erase_late_bound_regions(&sig).output().kind {
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self.report_diverging_sub_expr(e);
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}
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},
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_ => {},
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}
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},
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ExprKind::MethodCall(..) => {
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let borrowed_table = self.cx.tables;
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if borrowed_table.expr_ty(e).is_never() {
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self.report_diverging_sub_expr(e);
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}
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},
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_ => {
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// do not lint expressions referencing objects of type `!`, as that required a
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// diverging expression
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// to begin with
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},
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}
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self.maybe_walk_expr(e);
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}
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fn visit_block(&mut self, _: &'tcx Block<'_>) {
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// don't continue over blocks, LateLintPass already does that
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}
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fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
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NestedVisitorMap::None
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}
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}
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/// Walks up the AST from the given write expression (`vis.write_expr`) looking
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/// for reads to the same variable that are unsequenced relative to the 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 (for example, `&x`
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/// 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 evaluation. Blocks, `if`s,
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/// loops, `match`es, and the short-circuiting logical operators are considered to have a defined
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/// 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.hir();
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let mut cur_id = vis.write_expr.hir_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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Node::Expr(expr) => check_expr(vis, expr),
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Node::Stmt(stmt) => check_stmt(vis, stmt),
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Node::Item(_) => {
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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<'a, 'tcx>(vis: &mut ReadVisitor<'a, 'tcx>, expr: &'tcx Expr<'_>) -> StopEarly {
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if expr.hir_id == vis.last_expr.hir_id {
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return StopEarly::KeepGoing;
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}
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match expr.kind {
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ExprKind::Array(_)
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| ExprKind::Tup(_)
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| ExprKind::MethodCall(..)
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| ExprKind::Call(_, _)
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| ExprKind::Assign(..)
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| ExprKind::Index(_, _)
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| ExprKind::Repeat(_, _)
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| ExprKind::Struct(_, _, _) => {
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walk_expr(vis, expr);
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},
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ExprKind::Binary(op, _, _) | ExprKind::AssignOp(op, _, _) => {
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if op.node == BinOpKind::And || op.node == BinOpKind::Or {
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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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ExprKind::Closure(_, _, _, _, _) => {
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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 reached the top of the enclosing
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// function and can stop, or
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//
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// * `var` is captured by the closure, in which case, because evaluating a closure does not evaluate
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// its body, we don't necessarily have a write, so we need to stop to avoid generating false
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// 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<'a, 'tcx>(vis: &mut ReadVisitor<'a, 'tcx>, stmt: &'tcx Stmt<'_>) -> StopEarly {
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match stmt.kind {
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StmtKind::Expr(ref expr) | StmtKind::Semi(ref expr) => check_expr(vis, expr),
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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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StmtKind::Local(ref local) => local
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.init
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.as_ref()
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.map_or(StopEarly::KeepGoing, |expr| check_expr(vis, expr)),
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_ => StopEarly::KeepGoing,
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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<'a, 'tcx> {
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cx: &'a LateContext<'a, 'tcx>,
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/// The ID of the variable we're looking for.
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var: HirId,
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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: &'tcx Expr<'tcx>,
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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: &'tcx Expr<'tcx>,
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}
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impl<'a, 'tcx> Visitor<'tcx> for ReadVisitor<'a, 'tcx> {
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type Map = Map<'tcx>;
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fn visit_expr(&mut self, expr: &'tcx Expr<'_>) {
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if expr.hir_id == self.last_expr.hir_id {
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return;
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}
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match expr.kind {
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ExprKind::Path(ref qpath) => {
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if_chain! {
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if let QPath::Resolved(None, ref path) = *qpath;
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if path.segments.len() == 1;
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if let def::Res::Local(local_id) = self.cx.tables.qpath_res(qpath, expr.hir_id);
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if local_id == self.var;
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// Check that this is a read, not a write.
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if !is_in_assignment_position(self.cx, expr);
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then {
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span_lint_and_note(
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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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Some(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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ExprKind::Closure(_, _, _, _, _) |
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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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ExprKind::AddrOf(_, _, _) => {
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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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fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
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NestedVisitorMap::None
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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 ExprKind::Assign(ref lhs, ..) = parent.kind {
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return lhs.hir_id == expr.hir_id;
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}
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}
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false
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}
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