use clippy_utils::diagnostics::{span_lint_hir, span_lint_hir_and_then}; use clippy_utils::source::snippet_opt; use clippy_utils::ty::has_drop; use rustc_errors::Applicability; use rustc_hir::def::{DefKind, Res}; use rustc_hir::{is_range_literal, BinOpKind, BlockCheckMode, Expr, ExprKind, Stmt, StmtKind, UnsafeSource}; use rustc_lint::{LateContext, LateLintPass}; use rustc_session::{declare_lint_pass, 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. /// /// ### 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. /// /// ### 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.typeck_results().expr_ty(expr)), ExprKind::Index(a, b) | ExprKind::Binary(_, a, b) => has_no_effect(cx, a) && has_no_effect(cx, b), ExprKind::Array(v) | ExprKind::Tup(v) => v.iter().all(|val| has_no_effect(cx, val)), ExprKind::Repeat(inner, _) | ExprKind::Cast(inner, _) | ExprKind::Type(inner, _) | ExprKind::Unary(_, inner) | ExprKind::Field(inner, _) | ExprKind::AddrOf(_, _, inner) | ExprKind::Box(inner) => has_no_effect(cx, inner), ExprKind::Struct(_, fields, ref base) => { !has_drop(cx, cx.typeck_results().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(callee, args) => { if let ExprKind::Path(ref qpath) = callee.kind { let res = cx.qpath_res(qpath, callee.hir_id); let def_matched = matches!( res, Res::Def(DefKind::Struct | DefKind::Variant | DefKind::Ctor(..), ..) ); if def_matched || is_range_literal(expr) { !has_drop(cx, cx.typeck_results().expr_ty(expr)) && args.iter().all(|arg| has_no_effect(cx, arg)) } else { false } } else { false } }, ExprKind::Block(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<'tcx> LateLintPass<'tcx> for NoEffect { fn check_stmt(&mut self, cx: &LateContext<'tcx>, stmt: &'tcx Stmt<'_>) { if let StmtKind::Semi(expr) = stmt.kind { if has_no_effect(cx, expr) { span_lint_hir(cx, NO_EFFECT, expr.hir_id, stmt.span, "statement with no effect"); } else if let Some(reduced) = reduce_expression(cx, expr) { for e in &reduced { if e.span.from_expansion() { return; } } if let ExprKind::Index(..) = &expr.kind { let snippet; if_chain! { if let Some(arr) = snippet_opt(cx, reduced[0].span); if let Some(func) = snippet_opt(cx, reduced[1].span); then { snippet = format!("assert!({}.len() > {});", &arr, &func); } else { return; } } span_lint_hir_and_then( cx, UNNECESSARY_OPERATION, expr.hir_id, stmt.span, "unnecessary operation", |diag| { diag.span_suggestion( stmt.span, "statement can be written as", snippet, Applicability::MaybeIncorrect, ); }, ); } else { let mut snippet = String::new(); for e in reduced { if let Some(snip) = snippet_opt(cx, e.span) { snippet.push_str(&snip); snippet.push(';'); } else { return; } } span_lint_hir_and_then( cx, UNNECESSARY_OPERATION, expr.hir_id, stmt.span, "unnecessary operation", |diag| { diag.span_suggestion( stmt.span, "statement can be reduced to", snippet, Applicability::MachineApplicable, ); }, ); } } } } } fn reduce_expression<'a>(cx: &LateContext<'_>, expr: &'a Expr<'a>) -> Option>> { if expr.span.from_expansion() { return None; } match expr.kind { ExprKind::Index(a, b) => Some(vec![a, b]), ExprKind::Binary(ref binop, a, b) if binop.node != BinOpKind::And && binop.node != BinOpKind::Or => { Some(vec![a, b]) }, ExprKind::Array(v) | ExprKind::Tup(v) => Some(v.iter().collect()), ExprKind::Repeat(inner, _) | ExprKind::Cast(inner, _) | ExprKind::Type(inner, _) | ExprKind::Unary(_, inner) | ExprKind::Field(inner, _) | ExprKind::AddrOf(_, _, inner) | ExprKind::Box(inner) => reduce_expression(cx, inner).or_else(|| Some(vec![inner])), ExprKind::Struct(_, fields, ref base) => { if has_drop(cx, cx.typeck_results().expr_ty(expr)) { None } else { Some(fields.iter().map(|f| &f.expr).chain(base).map(Deref::deref).collect()) } }, ExprKind::Call(callee, args) => { if let ExprKind::Path(ref qpath) = callee.kind { let res = cx.qpath_res(qpath, callee.hir_id); match res { Res::Def(DefKind::Struct | DefKind::Variant | DefKind::Ctor(..), ..) if !has_drop(cx, cx.typeck_results().expr_ty(expr)) => { Some(args.iter().collect()) }, _ => None, } } else { None } }, ExprKind::Block(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 BlockCheckMode::UnsafeBlock(_) => reduce_expression(cx, e), } }) } else { None } }, _ => None, } }