1 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
2 use rustc::hir::def::{Def, PathResolution};
3 use rustc::hir::{Expr, Expr_, Stmt, StmtSemi, BlockCheckMode, UnsafeSource};
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_::ExprVec(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 let def = cx.tcx.def_map.borrow().get(&callee.id).map(|d| d.full_def());
73 Some(Def::Struct(..)) |
74 Some(Def::Variant(..)) => args.iter().all(|arg| has_no_effect(cx, arg)),
78 Expr_::ExprBlock(ref block) => {
79 block.stmts.is_empty() &&
80 if let Some(ref expr) = block.expr {
81 has_no_effect(cx, expr)
90 #[derive(Copy, Clone)]
93 impl LintPass for Pass {
94 fn get_lints(&self) -> LintArray {
95 lint_array!(NO_EFFECT, UNNECESSARY_OPERATION)
99 impl LateLintPass for Pass {
100 fn check_stmt(&mut self, cx: &LateContext, stmt: &Stmt) {
101 if let StmtSemi(ref expr, _) = stmt.node {
102 if has_no_effect(cx, expr) {
103 span_lint(cx, NO_EFFECT, stmt.span, "statement with no effect");
104 } else if let Some(reduced) = reduce_expression(cx, expr) {
105 let mut snippet = String::new();
107 if in_macro(cx, e.span) {
110 if let Some(snip) = snippet_opt(cx, e.span) {
111 snippet.push_str(&snip);
117 span_lint_and_then(cx, UNNECESSARY_OPERATION, stmt.span, "statement can be reduced", |db| {
118 db.span_suggestion(stmt.span, "replace it with", snippet);
126 fn reduce_expression<'a>(cx: &LateContext, expr: &'a Expr) -> Option<Vec<&'a Expr>> {
127 if in_macro(cx, expr.span) {
131 Expr_::ExprIndex(ref a, ref b) |
132 Expr_::ExprBinary(_, ref a, ref b) => Some(vec![&**a, &**b]),
133 Expr_::ExprVec(ref v) |
134 Expr_::ExprTup(ref v) => Some(v.iter().map(Deref::deref).collect()),
135 Expr_::ExprRepeat(ref inner, _) |
136 Expr_::ExprCast(ref inner, _) |
137 Expr_::ExprType(ref inner, _) |
138 Expr_::ExprUnary(_, ref inner) |
139 Expr_::ExprField(ref inner, _) |
140 Expr_::ExprTupField(ref inner, _) |
141 Expr_::ExprAddrOf(_, ref inner) |
142 Expr_::ExprBox(ref inner) => reduce_expression(cx, inner).or_else(|| Some(vec![inner])),
143 Expr_::ExprStruct(_, ref fields, ref base) => {
144 Some(fields.iter().map(|f| &f.expr).chain(base).map(Deref::deref).collect())
146 Expr_::ExprCall(ref callee, ref args) => {
147 match cx.tcx.def_map.borrow().get(&callee.id).map(PathResolution::full_def) {
148 Some(Def::Struct(..)) |
149 Some(Def::Variant(..)) => Some(args.iter().map(Deref::deref).collect()),
153 Expr_::ExprBlock(ref block) => {
154 if block.stmts.is_empty() {
155 block.expr.as_ref().and_then(|e| {
157 BlockCheckMode::UnsafeBlock(UnsafeSource::UserProvided) => None,
158 BlockCheckMode::DefaultBlock => Some(vec![&**e]),
159 // in case of compiler-inserted signaling blocks
160 _ => reduce_expression(cx, e),