clippy_lints/src/no_effect.rs

   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_sugg};
   5 use std::ops::Deref;
   6
   7 /// **What it does:** Checks for statements which have no effect.
   8 ///
   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
  11 /// readable.
  12 ///
  13 /// **Known problems:** None.
  14 ///
  15 /// **Example:**
  16 /// ```rust
  17 /// 0;
  18 /// ```
  19 declare_lint! {
  20     pub NO_EFFECT,
  21     Warn,
  22     "statements with no effect"
  23 }
  24
  25 /// **What it does:** Checks for expression statements that can be reduced to a
  26 /// sub-expression.
  27 ///
  28 /// **Why is this bad?** Expressions by themselves often have no side-effects.
  29 /// Having such expressions reduces readability.
  30 ///
  31 /// **Known problems:** None.
  32 ///
  33 /// **Example:**
  34 /// ```rust
  35 /// compute_array()[0];
  36 /// ```
  37 declare_lint! {
  38     pub UNNECESSARY_OPERATION,
  39     Warn,
  40     "outer expressions with no effect"
  41 }
  42
  43 fn has_no_effect(cx: &LateContext, expr: &Expr) -> bool {
  44     if in_macro(expr.span) {
  45         return false;
  46     }
  47     match expr.node {
  48         Expr_::ExprLit(..) |
  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)) &&
  65                 match *base {
  66                     Some(ref base) => has_no_effect(cx, base),
  67                     None => true,
  68                 }
  69         },
  70         Expr_::ExprCall(ref callee, ref args) => {
  71             if let Expr_::ExprPath(ref qpath) = callee.node {
  72                 let def = cx.tables.qpath_def(qpath, callee.hir_id);
  73                 match def {
  74                     Def::Struct(..) |
  75                     Def::Variant(..) |
  76                     Def::StructCtor(..) |
  77                     Def::VariantCtor(..) => args.iter().all(|arg| has_no_effect(cx, arg)),
  78                     _ => false,
  79                 }
  80             } else {
  81                 false
  82             }
  83         },
  84         Expr_::ExprBlock(ref block) => {
  85             block.stmts.is_empty() &&
  86                 if let Some(ref expr) = block.expr {
  87                     has_no_effect(cx, expr)
  88                 } else {
  89                     false
  90                 }
  91         },
  92         _ => false,
  93     }
  94 }
  95
  96 #[derive(Copy, Clone)]
  97 pub struct Pass;
  98
  99 impl LintPass for Pass {
 100     fn get_lints(&self) -> LintArray {
 101         lint_array!(NO_EFFECT, UNNECESSARY_OPERATION)
 102     }
 103 }
 104
 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();
 112                 for e in reduced {
 113                     if in_macro(e.span) {
 114                         return;
 115                     }
 116                     if let Some(snip) = snippet_opt(cx, e.span) {
 117                         snippet.push_str(&snip);
 118                         snippet.push(';');
 119                     } else {
 120                         return;
 121                     }
 122                 }
 123                 span_lint_and_sugg(
 124                     cx,
 125                     UNNECESSARY_OPERATION,
 126                     stmt.span,
 127                     "statement can be reduced",
 128                     "replace it with",
 129                     snippet,
 130                 );
 131             }
 132         }
 133     }
 134 }
 135
 136
 137 fn reduce_expression<'a>(cx: &LateContext, expr: &'a Expr) -> Option<Vec<&'a Expr>> {
 138     if in_macro(expr.span) {
 139         return None;
 140     }
 141     match expr.node {
 142         Expr_::ExprIndex(ref a, ref b) => Some(vec![&**a, &**b]),
 143         Expr_::ExprBinary(ref binop, ref a, ref b) if binop.node != BiAnd && binop.node != BiOr => {
 144             Some(vec![&**a, &**b])
 145         },
 146         Expr_::ExprArray(ref v) |
 147         Expr_::ExprTup(ref v) => Some(v.iter().collect()),
 148         Expr_::ExprRepeat(ref inner, _) |
 149         Expr_::ExprCast(ref inner, _) |
 150         Expr_::ExprType(ref inner, _) |
 151         Expr_::ExprUnary(_, ref inner) |
 152         Expr_::ExprField(ref inner, _) |
 153         Expr_::ExprTupField(ref inner, _) |
 154         Expr_::ExprAddrOf(_, ref inner) |
 155         Expr_::ExprBox(ref inner) => reduce_expression(cx, inner).or_else(|| Some(vec![inner])),
 156         Expr_::ExprStruct(_, ref fields, ref base) => {
 157             Some(
 158                 fields
 159                     .iter()
 160                     .map(|f| &f.expr)
 161                     .chain(base)
 162                     .map(Deref::deref)
 163                     .collect(),
 164             )
 165         },
 166         Expr_::ExprCall(ref callee, ref args) => {
 167             if let Expr_::ExprPath(ref qpath) = callee.node {
 168                 let def = cx.tables.qpath_def(qpath, callee.hir_id);
 169                 match def {
 170                     Def::Struct(..) |
 171                     Def::Variant(..) |
 172                     Def::StructCtor(..) |
 173                     Def::VariantCtor(..) => Some(args.iter().collect()),
 174                     _ => None,
 175                 }
 176             } else {
 177                 None
 178             }
 179         },
 180         Expr_::ExprBlock(ref block) => {
 181             if block.stmts.is_empty() {
 182                 block.expr.as_ref().and_then(|e| {
 183                     match block.rules {
 184                         BlockCheckMode::UnsafeBlock(UnsafeSource::UserProvided) => None,
 185                         BlockCheckMode::DefaultBlock => Some(vec![&**e]),
 186                         // in case of compiler-inserted signaling blocks
 187                         _ => reduce_expression(cx, e),
 188                     }
 189                 })
 190             } else {
 191                 None
 192             }
 193         },
 194         _ => None,
 195     }
 196 }