1 use clippy_utils::diagnostics::{span_lint_hir, span_lint_hir_and_then};
2 use clippy_utils::source::snippet_opt;
3 use clippy_utils::ty::has_drop;
4 use rustc_errors::Applicability;
5 use rustc_hir::def::{DefKind, Res};
6 use rustc_hir::{BinOpKind, BlockCheckMode, Expr, ExprKind, Stmt, StmtKind, UnsafeSource};
7 use rustc_lint::{LateContext, LateLintPass};
8 use rustc_session::{declare_lint_pass, declare_tool_lint};
11 declare_clippy_lint! {
12 /// **What it does:** Checks for statements which have no effect.
14 /// **Why is this bad?** Similar to dead code, these statements are actually
15 /// executed. However, as they have no effect, all they do is make the code less
18 /// **Known problems:** None.
26 "statements with no effect"
29 declare_clippy_lint! {
30 /// **What it does:** Checks for expression statements that can be reduced to a
33 /// **Why is this bad?** Expressions by themselves often have no side-effects.
34 /// Having such expressions reduces readability.
36 /// **Known problems:** None.
40 /// compute_array()[0];
42 pub UNNECESSARY_OPERATION,
44 "outer expressions with no effect"
47 fn has_no_effect(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
48 if expr.span.from_expansion() {
52 ExprKind::Lit(..) | ExprKind::Closure(..) => true,
53 ExprKind::Path(..) => !has_drop(cx, cx.typeck_results().expr_ty(expr)),
54 ExprKind::Index(a, b) | ExprKind::Binary(_, a, b) => has_no_effect(cx, a) && has_no_effect(cx, b),
55 ExprKind::Array(v) | ExprKind::Tup(v) => v.iter().all(|val| has_no_effect(cx, val)),
56 ExprKind::Repeat(inner, _)
57 | ExprKind::Cast(inner, _)
58 | ExprKind::Type(inner, _)
59 | ExprKind::Unary(_, inner)
60 | ExprKind::Field(inner, _)
61 | ExprKind::AddrOf(_, _, inner)
62 | ExprKind::Box(inner) => has_no_effect(cx, inner),
63 ExprKind::Struct(_, fields, ref base) => {
64 !has_drop(cx, cx.typeck_results().expr_ty(expr))
65 && fields.iter().all(|field| has_no_effect(cx, field.expr))
66 && base.as_ref().map_or(true, |base| has_no_effect(cx, base))
68 ExprKind::Call(callee, args) => {
69 if let ExprKind::Path(ref qpath) = callee.kind {
70 let res = cx.qpath_res(qpath, callee.hir_id);
72 Res::Def(DefKind::Struct | DefKind::Variant | DefKind::Ctor(..), ..) => {
73 !has_drop(cx, cx.typeck_results().expr_ty(expr))
74 && args.iter().all(|arg| has_no_effect(cx, arg))
82 ExprKind::Block(block, _) => {
83 block.stmts.is_empty() && block.expr.as_ref().map_or(false, |expr| has_no_effect(cx, expr))
89 declare_lint_pass!(NoEffect => [NO_EFFECT, UNNECESSARY_OPERATION]);
91 impl<'tcx> LateLintPass<'tcx> for NoEffect {
92 fn check_stmt(&mut self, cx: &LateContext<'tcx>, stmt: &'tcx Stmt<'_>) {
93 if let StmtKind::Semi(expr) = stmt.kind {
94 if has_no_effect(cx, expr) {
95 span_lint_hir(cx, NO_EFFECT, expr.hir_id, stmt.span, "statement with no effect");
96 } else if let Some(reduced) = reduce_expression(cx, expr) {
97 let mut snippet = String::new();
99 if e.span.from_expansion() {
102 if let Some(snip) = snippet_opt(cx, e.span) {
103 snippet.push_str(&snip);
109 span_lint_hir_and_then(
111 UNNECESSARY_OPERATION,
114 "statement can be reduced",
116 diag.span_suggestion(stmt.span, "replace it with", snippet, Applicability::MachineApplicable);
124 fn reduce_expression<'a>(cx: &LateContext<'_>, expr: &'a Expr<'a>) -> Option<Vec<&'a Expr<'a>>> {
125 if expr.span.from_expansion() {
129 ExprKind::Index(a, b) => Some(vec![a, b]),
130 ExprKind::Binary(ref binop, a, b) if binop.node != BinOpKind::And && binop.node != BinOpKind::Or => {
133 ExprKind::Array(v) | ExprKind::Tup(v) => Some(v.iter().collect()),
134 ExprKind::Repeat(inner, _)
135 | ExprKind::Cast(inner, _)
136 | ExprKind::Type(inner, _)
137 | ExprKind::Unary(_, inner)
138 | ExprKind::Field(inner, _)
139 | ExprKind::AddrOf(_, _, inner)
140 | ExprKind::Box(inner) => reduce_expression(cx, inner).or_else(|| Some(vec![inner])),
141 ExprKind::Struct(_, fields, ref base) => {
142 if has_drop(cx, cx.typeck_results().expr_ty(expr)) {
145 Some(fields.iter().map(|f| &f.expr).chain(base).map(Deref::deref).collect())
148 ExprKind::Call(callee, args) => {
149 if let ExprKind::Path(ref qpath) = callee.kind {
150 let res = cx.qpath_res(qpath, callee.hir_id);
152 Res::Def(DefKind::Struct | DefKind::Variant | DefKind::Ctor(..), ..)
153 if !has_drop(cx, cx.typeck_results().expr_ty(expr)) =>
155 Some(args.iter().collect())
163 ExprKind::Block(block, _) => {
164 if block.stmts.is_empty() {
165 block.expr.as_ref().and_then(|e| {
167 BlockCheckMode::UnsafeBlock(UnsafeSource::UserProvided) => None,
168 BlockCheckMode::DefaultBlock => Some(vec![&**e]),
169 // in case of compiler-inserted signaling blocks
170 BlockCheckMode::UnsafeBlock(_) => reduce_expression(cx, e),