clippy_lints/src/no_effect.rs

   1 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
   2 use rustc::hir::def::Def;
   3 use rustc::hir::{BiAnd, BiOr, BlockCheckMode, Expr, Expr_, Stmt, StmtSemi, UnsafeSource};
   4 use utils::{has_drop, in_macro, snippet_opt, span_lint, 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(..) | Expr_::ExprClosure(.., _) => true,
  49         Expr_::ExprPath(..) => !has_drop(cx, expr),
  50         Expr_::ExprIndex(ref a, ref b) | Expr_::ExprBinary(_, ref a, ref b) => {
  51             has_no_effect(cx, a) && has_no_effect(cx, b)
  52         },
  53         Expr_::ExprArray(ref v) | Expr_::ExprTup(ref v) => v.iter().all(|val| has_no_effect(cx, val)),
  54         Expr_::ExprRepeat(ref inner, _) |
  55         Expr_::ExprCast(ref inner, _) |
  56         Expr_::ExprType(ref inner, _) |
  57         Expr_::ExprUnary(_, ref inner) |
  58         Expr_::ExprField(ref inner, _) |
  59         Expr_::ExprTupField(ref inner, _) |
  60         Expr_::ExprAddrOf(_, ref inner) |
  61         Expr_::ExprBox(ref inner) => has_no_effect(cx, inner),
  62         Expr_::ExprStruct(_, ref fields, ref base) => {
  63             !has_drop(cx, expr) && fields.iter().all(|field| has_no_effect(cx, &field.expr)) && match *base {
  64                 Some(ref base) => has_no_effect(cx, base),
  65                 None => true,
  66             }
  67         },
  68         Expr_::ExprCall(ref callee, ref args) => if let Expr_::ExprPath(ref qpath) = callee.node {
  69             let def = cx.tables.qpath_def(qpath, callee.hir_id);
  70             match def {
  71                 Def::Struct(..) | Def::Variant(..) | Def::StructCtor(..) | Def::VariantCtor(..) => {
  72                     !has_drop(cx, expr) && args.iter().all(|arg| has_no_effect(cx, arg))
  73                 },
  74                 _ => false,
  75             }
  76         } else {
  77             false
  78         },
  79         Expr_::ExprBlock(ref block) => {
  80             block.stmts.is_empty() && if let Some(ref expr) = block.expr {
  81                 has_no_effect(cx, expr)
  82             } else {
  83                 false
  84             }
  85         },
  86         _ => false,
  87     }
  88 }
  89
  90 #[derive(Copy, Clone)]
  91 pub struct Pass;
  92
  93 impl LintPass for Pass {
  94     fn get_lints(&self) -> LintArray {
  95         lint_array!(NO_EFFECT, UNNECESSARY_OPERATION)
  96     }
  97 }
  98
  99 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Pass {
 100     fn check_stmt(&mut self, cx: &LateContext<'a, 'tcx>, stmt: &'tcx 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();
 106                 for e in reduced {
 107                     if in_macro(e.span) {
 108                         return;
 109                     }
 110                     if let Some(snip) = snippet_opt(cx, e.span) {
 111                         snippet.push_str(&snip);
 112                         snippet.push(';');
 113                     } else {
 114                         return;
 115                     }
 116                 }
 117                 span_lint_and_sugg(
 118                     cx,
 119                     UNNECESSARY_OPERATION,
 120                     stmt.span,
 121                     "statement can be reduced",
 122                     "replace it with",
 123                     snippet,
 124                 );
 125             }
 126         }
 127     }
 128 }
 129
 130
 131 fn reduce_expression<'a>(cx: &LateContext, expr: &'a Expr) -> Option<Vec<&'a Expr>> {
 132     if in_macro(expr.span) {
 133         return None;
 134     }
 135     match expr.node {
 136         Expr_::ExprIndex(ref a, ref b) => Some(vec![&**a, &**b]),
 137         Expr_::ExprBinary(ref binop, ref a, ref b) if binop.node != BiAnd && binop.node != BiOr => {
 138             Some(vec![&**a, &**b])
 139         },
 140         Expr_::ExprArray(ref v) | Expr_::ExprTup(ref v) => Some(v.iter().collect()),
 141         Expr_::ExprRepeat(ref inner, _) |
 142         Expr_::ExprCast(ref inner, _) |
 143         Expr_::ExprType(ref inner, _) |
 144         Expr_::ExprUnary(_, ref inner) |
 145         Expr_::ExprField(ref inner, _) |
 146         Expr_::ExprTupField(ref inner, _) |
 147         Expr_::ExprAddrOf(_, ref inner) |
 148         Expr_::ExprBox(ref inner) => reduce_expression(cx, inner).or_else(|| Some(vec![inner])),
 149         Expr_::ExprStruct(_, ref fields, ref base) => if has_drop(cx, expr) {
 150             None
 151         } else {
 152             Some(
 153                 fields
 154                     .iter()
 155                     .map(|f| &f.expr)
 156                     .chain(base)
 157                     .map(Deref::deref)
 158                     .collect(),
 159             )
 160         },
 161         Expr_::ExprCall(ref callee, ref args) => if let Expr_::ExprPath(ref qpath) = callee.node {
 162             let def = cx.tables.qpath_def(qpath, callee.hir_id);
 163             match def {
 164                 Def::Struct(..) | Def::Variant(..) | Def::StructCtor(..) | Def::VariantCtor(..)
 165                     if !has_drop(cx, expr) =>
 166                 {
 167                     Some(args.iter().collect())
 168                 },
 169                 _ => None,
 170             }
 171         } else {
 172             None
 173         },
 174         Expr_::ExprBlock(ref block) => {
 175             if block.stmts.is_empty() {
 176                 block.expr.as_ref().and_then(|e| {
 177                     match block.rules {
 178                         BlockCheckMode::UnsafeBlock(UnsafeSource::UserProvided) => None,
 179                         BlockCheckMode::DefaultBlock => Some(vec![&**e]),
 180                         // in case of compiler-inserted signaling blocks
 181                         _ => reduce_expression(cx, e),
 182                     }
 183                 })
 184             } else {
 185                 None
 186             }
 187         },
 188         _ => None,
 189     }
 190 }