use crate::utils::{has_drop, qpath_res, snippet_opt, span_lint, span_lint_and_sugg}; use rustc::declare_lint_pass; use rustc::hir::def::{DefKind, Res}; use rustc::hir::{BinOpKind, BlockCheckMode, Expr, ExprKind, Stmt, StmtKind, UnsafeSource}; use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass}; use rustc_errors::Applicability; use rustc_session::declare_tool_lint; use std::ops::Deref; declare_clippy_lint! { /// **What it does:** 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:** /// ```rust /// 0; /// ``` pub NO_EFFECT, complexity, "statements with no effect" } declare_clippy_lint! { /// **What it does:** Checks for expression statements that can be reduced to a /// sub-expression. /// /// **Why is this bad?** Expressions by themselves often have no side-effects. /// Having such expressions reduces readability. /// /// **Known problems:** None. /// /// **Example:** /// ```rust,ignore /// compute_array()[0]; /// ``` pub UNNECESSARY_OPERATION, complexity, "outer expressions with no effect" } fn has_no_effect(cx: &LateContext<'_, '_>, expr: &Expr) -> bool { if expr.span.from_expansion() { return false; } match expr.kind { ExprKind::Lit(..) | ExprKind::Closure(..) => true, ExprKind::Path(..) => !has_drop(cx, cx.tables.expr_ty(expr)), ExprKind::Index(ref a, ref b) | ExprKind::Binary(_, ref a, ref b) => { has_no_effect(cx, a) && has_no_effect(cx, b) }, ExprKind::Array(ref v) | ExprKind::Tup(ref v) => v.iter().all(|val| has_no_effect(cx, val)), ExprKind::Repeat(ref inner, _) | ExprKind::Cast(ref inner, _) | ExprKind::Type(ref inner, _) | ExprKind::Unary(_, ref inner) | ExprKind::Field(ref inner, _) | ExprKind::AddrOf(_, _, ref inner) | ExprKind::Box(ref inner) => has_no_effect(cx, inner), ExprKind::Struct(_, ref fields, ref base) => { !has_drop(cx, cx.tables.expr_ty(expr)) && fields.iter().all(|field| has_no_effect(cx, &field.expr)) && base.as_ref().map_or(true, |base| has_no_effect(cx, base)) }, ExprKind::Call(ref callee, ref args) => { if let ExprKind::Path(ref qpath) = callee.kind { let res = qpath_res(cx, qpath, callee.hir_id); match res { Res::Def(DefKind::Struct, ..) | Res::Def(DefKind::Variant, ..) | Res::Def(DefKind::Ctor(..), _) => { !has_drop(cx, cx.tables.expr_ty(expr)) && args.iter().all(|arg| has_no_effect(cx, arg)) }, _ => false, } } else { false } }, ExprKind::Block(ref block, _) => { block.stmts.is_empty() && block.expr.as_ref().map_or(false, |expr| has_no_effect(cx, expr)) }, _ => false, } } declare_lint_pass!(NoEffect => [NO_EFFECT, UNNECESSARY_OPERATION]); impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NoEffect { fn check_stmt(&mut self, cx: &LateContext<'a, 'tcx>, stmt: &'tcx Stmt) { if let StmtKind::Semi(ref expr) = stmt.kind { if has_no_effect(cx, expr) { span_lint(cx, NO_EFFECT, stmt.span, "statement with no effect"); } else if let Some(reduced) = reduce_expression(cx, expr) { let mut snippet = String::new(); for e in reduced { if e.span.from_expansion() { return; } if let Some(snip) = snippet_opt(cx, e.span) { snippet.push_str(&snip); snippet.push(';'); } else { return; } } span_lint_and_sugg( cx, UNNECESSARY_OPERATION, stmt.span, "statement can be reduced", "replace it with", snippet, Applicability::MachineApplicable, ); } } } } fn reduce_expression<'a>(cx: &LateContext<'_, '_>, expr: &'a Expr) -> Option> { if expr.span.from_expansion() { return None; } match expr.kind { ExprKind::Index(ref a, ref b) => Some(vec![&**a, &**b]), ExprKind::Binary(ref binop, ref a, ref b) if binop.node != BinOpKind::And && binop.node != BinOpKind::Or => { Some(vec![&**a, &**b]) }, ExprKind::Array(ref v) | ExprKind::Tup(ref v) => Some(v.iter().collect()), ExprKind::Repeat(ref inner, _) | ExprKind::Cast(ref inner, _) | ExprKind::Type(ref inner, _) | ExprKind::Unary(_, ref inner) | ExprKind::Field(ref inner, _) | ExprKind::AddrOf(_, _, ref inner) | ExprKind::Box(ref inner) => reduce_expression(cx, inner).or_else(|| Some(vec![inner])), ExprKind::Struct(_, ref fields, ref base) => { if has_drop(cx, cx.tables.expr_ty(expr)) { None } else { Some(fields.iter().map(|f| &f.expr).chain(base).map(Deref::deref).collect()) } }, ExprKind::Call(ref callee, ref args) => { if let ExprKind::Path(ref qpath) = callee.kind { let res = qpath_res(cx, qpath, callee.hir_id); match res { Res::Def(DefKind::Struct, ..) | Res::Def(DefKind::Variant, ..) | Res::Def(DefKind::Ctor(..), _) if !has_drop(cx, cx.tables.expr_ty(expr)) => { Some(args.iter().collect()) }, _ => None, } } else { None } }, ExprKind::Block(ref block, _) => { if block.stmts.is_empty() { block.expr.as_ref().and_then(|e| { match block.rules { BlockCheckMode::UnsafeBlock(UnsafeSource::UserProvided) => None, BlockCheckMode::DefaultBlock => Some(vec![&**e]), // in case of compiler-inserted signaling blocks _ => reduce_expression(cx, e), } }) } else { None } }, _ => None, } }