1 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
2 use rustc::hir::def::Def;
3 use rustc::hir::{Expr, Expr_, Stmt, StmtSemi, BlockCheckMode, UnsafeSource, BiAnd, BiOr};
4 use utils::{in_macro, span_lint, snippet_opt, span_lint_and_then};
7 /// **What it does:** Checks for statements which have no effect.
9 /// **Why is this bad?** Similar to dead code, these statements are actually
10 /// executed. However, as they have no effect, all they do is make the code less
13 /// **Known problems:** None.
22 "statements with no effect"
25 /// **What it does:** Checks for expression statements that can be reduced to a
28 /// **Why is this bad?** Expressions by themselves often have no side-effects.
29 /// Having such expressions reduces readability.
31 /// **Known problems:** None.
35 /// compute_array()[0];
38 pub UNNECESSARY_OPERATION,
40 "outer expressions with no effect"
43 fn has_no_effect(cx: &LateContext, expr: &Expr) -> bool {
44 if in_macro(cx, expr.span) {
49 Expr_::ExprClosure(..) |
50 Expr_::ExprPath(..) => true,
51 Expr_::ExprIndex(ref a, ref b) |
52 Expr_::ExprBinary(_, ref a, ref b) => has_no_effect(cx, a) && has_no_effect(cx, b),
53 Expr_::ExprArray(ref v) |
54 Expr_::ExprTup(ref v) => v.iter().all(|val| has_no_effect(cx, val)),
55 Expr_::ExprRepeat(ref inner, _) |
56 Expr_::ExprCast(ref inner, _) |
57 Expr_::ExprType(ref inner, _) |
58 Expr_::ExprUnary(_, ref inner) |
59 Expr_::ExprField(ref inner, _) |
60 Expr_::ExprTupField(ref inner, _) |
61 Expr_::ExprAddrOf(_, ref inner) |
62 Expr_::ExprBox(ref inner) => has_no_effect(cx, inner),
63 Expr_::ExprStruct(_, ref fields, ref base) => {
64 fields.iter().all(|field| has_no_effect(cx, &field.expr)) &&
66 Some(ref base) => has_no_effect(cx, base),
70 Expr_::ExprCall(ref callee, ref args) => {
71 if let Expr_::ExprPath(ref qpath) = callee.node {
72 let def = cx.tcx.tables().qpath_def(qpath, callee.id);
77 Def::VariantCtor(..) => args.iter().all(|arg| has_no_effect(cx, arg)),
84 Expr_::ExprBlock(ref block) => {
85 block.stmts.is_empty() &&
86 if let Some(ref expr) = block.expr {
87 has_no_effect(cx, expr)
96 #[derive(Copy, Clone)]
99 impl LintPass for Pass {
100 fn get_lints(&self) -> LintArray {
101 lint_array!(NO_EFFECT, UNNECESSARY_OPERATION)
105 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Pass {
106 fn check_stmt(&mut self, cx: &LateContext<'a, 'tcx>, stmt: &'tcx Stmt) {
107 if let StmtSemi(ref expr, _) = stmt.node {
108 if has_no_effect(cx, expr) {
109 span_lint(cx, NO_EFFECT, stmt.span, "statement with no effect");
110 } else if let Some(reduced) = reduce_expression(cx, expr) {
111 let mut snippet = String::new();
113 if in_macro(cx, e.span) {
116 if let Some(snip) = snippet_opt(cx, e.span) {
117 snippet.push_str(&snip);
123 span_lint_and_then(cx, UNNECESSARY_OPERATION, stmt.span, "statement can be reduced", |db| {
124 db.span_suggestion(stmt.span, "replace it with", snippet);
132 fn reduce_expression<'a>(cx: &LateContext, expr: &'a Expr) -> Option<Vec<&'a Expr>> {
133 if in_macro(cx, expr.span) {
137 Expr_::ExprIndex(ref a, ref b) => Some(vec![&**a, &**b]),
138 Expr_::ExprBinary(ref binop, ref a, ref b) if binop.node != BiAnd && binop.node != BiOr => {
139 Some(vec![&**a, &**b])
141 Expr_::ExprArray(ref v) |
142 Expr_::ExprTup(ref v) => Some(v.iter().collect()),
143 Expr_::ExprRepeat(ref inner, _) |
144 Expr_::ExprCast(ref inner, _) |
145 Expr_::ExprType(ref inner, _) |
146 Expr_::ExprUnary(_, ref inner) |
147 Expr_::ExprField(ref inner, _) |
148 Expr_::ExprTupField(ref inner, _) |
149 Expr_::ExprAddrOf(_, ref inner) |
150 Expr_::ExprBox(ref inner) => reduce_expression(cx, inner).or_else(|| Some(vec![inner])),
151 Expr_::ExprStruct(_, ref fields, ref base) => {
152 Some(fields.iter().map(|f| &f.expr).chain(base).map(Deref::deref).collect())
154 Expr_::ExprCall(ref callee, ref args) => {
155 if let Expr_::ExprPath(ref qpath) = callee.node {
156 let def = cx.tcx.tables().qpath_def(qpath, callee.id);
160 Def::StructCtor(..) |
161 Def::VariantCtor(..) => Some(args.iter().collect()),
168 Expr_::ExprBlock(ref block) => {
169 if block.stmts.is_empty() {
170 block.expr.as_ref().and_then(|e| {
172 BlockCheckMode::UnsafeBlock(UnsafeSource::UserProvided) => None,
173 BlockCheckMode::DefaultBlock => Some(vec![&**e]),
174 // in case of compiler-inserted signaling blocks
175 _ => reduce_expression(cx, e),