use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass}; use rustc::middle::def::Def; use rustc_front::hir::{Expr, Expr_, Stmt, StmtSemi}; use utils::{in_macro, span_lint}; /// **What it does:** This lint checks for statements which have no effect. /// /// **Why is this bad?** Similar to dead code, these statements are actually executed. However, as they have no effect, all they do is make the code less readable. /// /// **Known problems:** None. /// /// **Example:** `0;` declare_lint! { pub NO_EFFECT, Warn, "statements with no effect" } fn has_no_effect(cx: &LateContext, expr: &Expr) -> bool { if in_macro(cx, expr.span) { return false; } match expr.node { Expr_::ExprLit(..) | Expr_::ExprClosure(..) | Expr_::ExprPath(..) => true, Expr_::ExprIndex(ref a, ref b) | Expr_::ExprBinary(_, ref a, ref b) => has_no_effect(cx, a) && has_no_effect(cx, b), Expr_::ExprVec(ref v) | Expr_::ExprTup(ref v) => v.iter().all(|val| has_no_effect(cx, val)), Expr_::ExprRepeat(ref inner, _) | Expr_::ExprCast(ref inner, _) | Expr_::ExprType(ref inner, _) | Expr_::ExprUnary(_, ref inner) | Expr_::ExprField(ref inner, _) | Expr_::ExprTupField(ref inner, _) | Expr_::ExprAddrOf(_, ref inner) | Expr_::ExprBox(ref inner) => has_no_effect(cx, inner), Expr_::ExprStruct(_, ref fields, ref base) => { fields.iter().all(|field| has_no_effect(cx, &field.expr)) && match *base { Some(ref base) => has_no_effect(cx, base), None => true, } } Expr_::ExprCall(ref callee, ref args) => { let def = cx.tcx.def_map.borrow().get(&callee.id).map(|d| d.full_def()); match def { Some(Def::Struct(..)) | Some(Def::Variant(..)) => args.iter().all(|arg| has_no_effect(cx, arg)), _ => false, } } Expr_::ExprBlock(ref block) => { block.stmts.is_empty() && if let Some(ref expr) = block.expr { has_no_effect(cx, expr) } else { false } } _ => false, } } #[derive(Copy, Clone)] pub struct NoEffectPass; impl LintPass for NoEffectPass { fn get_lints(&self) -> LintArray { lint_array!(NO_EFFECT) } } impl LateLintPass for NoEffectPass { fn check_stmt(&mut self, cx: &LateContext, stmt: &Stmt) { if let StmtSemi(ref expr, _) = stmt.node { if has_no_effect(cx, expr) { span_lint(cx, NO_EFFECT, stmt.span, "statement with no effect"); } } } }