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::{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(.., _) | Expr_::ExprPath(..) => true,
  49         Expr_::ExprIndex(ref a, ref b) | Expr_::ExprBinary(_, ref a, ref b) => {
  50             has_no_effect(cx, a) && has_no_effect(cx, b)
  51         },
  52         Expr_::ExprArray(ref v) | Expr_::ExprTup(ref v) => v.iter().all(|val| has_no_effect(cx, val)),
  53         Expr_::ExprRepeat(ref inner, _) |
  54         Expr_::ExprCast(ref inner, _) |
  55         Expr_::ExprType(ref inner, _) |
  56         Expr_::ExprUnary(_, ref inner) |
  57         Expr_::ExprField(ref inner, _) |
  58         Expr_::ExprTupField(ref inner, _) |
  59         Expr_::ExprAddrOf(_, ref inner) |
  60         Expr_::ExprBox(ref inner) => has_no_effect(cx, inner),
  61         Expr_::ExprStruct(_, ref fields, ref base) => {
  62             fields.iter().all(|field| has_no_effect(cx, &field.expr)) && match *base {
  63                 Some(ref base) => has_no_effect(cx, base),
  64                 None => true,
  65             }
  66         },
  67         Expr_::ExprCall(ref callee, ref args) => if let Expr_::ExprPath(ref qpath) = callee.node {
  68             let def = cx.tables.qpath_def(qpath, callee.hir_id);
  69             match def {
  70                 Def::Struct(..) | Def::Variant(..) | Def::StructCtor(..) | Def::VariantCtor(..) => {
  71                     args.iter().all(|arg| has_no_effect(cx, arg))
  72                 },
  73                 _ => false,
  74             }
  75         } else {
  76             false
  77         },
  78         Expr_::ExprBlock(ref block) => {
  79             block.stmts.is_empty() && if let Some(ref expr) = block.expr {
  80                 has_no_effect(cx, expr)
  81             } else {
  82                 false
  83             }
  84         },
  85         _ => false,
  86     }
  87 }
  88
  89 #[derive(Copy, Clone)]
  90 pub struct Pass;
  91
  92 impl LintPass for Pass {
  93     fn get_lints(&self) -> LintArray {
  94         lint_array!(NO_EFFECT, UNNECESSARY_OPERATION)
  95     }
  96 }
  97
  98 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Pass {
  99     fn check_stmt(&mut self, cx: &LateContext<'a, 'tcx>, stmt: &'tcx Stmt) {
 100         if let StmtSemi(ref expr, _) = stmt.node {
 101             if has_no_effect(cx, expr) {
 102                 span_lint(cx, NO_EFFECT, stmt.span, "statement with no effect");
 103             } else if let Some(reduced) = reduce_expression(cx, expr) {
 104                 let mut snippet = String::new();
 105                 for e in reduced {
 106                     if in_macro(e.span) {
 107                         return;
 108                     }
 109                     if let Some(snip) = snippet_opt(cx, e.span) {
 110                         snippet.push_str(&snip);
 111                         snippet.push(';');
 112                     } else {
 113                         return;
 114                     }
 115                 }
 116                 span_lint_and_sugg(
 117                     cx,
 118                     UNNECESSARY_OPERATION,
 119                     stmt.span,
 120                     "statement can be reduced",
 121                     "replace it with",
 122                     snippet,
 123                 );
 124             }
 125         }
 126     }
 127 }
 128
 129
 130 fn reduce_expression<'a>(cx: &LateContext, expr: &'a Expr) -> Option<Vec<&'a Expr>> {
 131     if in_macro(expr.span) {
 132         return None;
 133     }
 134     match expr.node {
 135         Expr_::ExprIndex(ref a, ref b) => Some(vec![&**a, &**b]),
 136         Expr_::ExprBinary(ref binop, ref a, ref b) if binop.node != BiAnd && binop.node != BiOr => {
 137             Some(vec![&**a, &**b])
 138         },
 139         Expr_::ExprArray(ref v) | Expr_::ExprTup(ref v) => Some(v.iter().collect()),
 140         Expr_::ExprRepeat(ref inner, _) |
 141         Expr_::ExprCast(ref inner, _) |
 142         Expr_::ExprType(ref inner, _) |
 143         Expr_::ExprUnary(_, ref inner) |
 144         Expr_::ExprField(ref inner, _) |
 145         Expr_::ExprTupField(ref inner, _) |
 146         Expr_::ExprAddrOf(_, ref inner) |
 147         Expr_::ExprBox(ref inner) => reduce_expression(cx, inner).or_else(|| Some(vec![inner])),
 148         Expr_::ExprStruct(_, ref fields, ref base) => Some(
 149             fields
 150                 .iter()
 151                 .map(|f| &f.expr)
 152                 .chain(base)
 153                 .map(Deref::deref)
 154                 .collect(),
 155         ),
 156         Expr_::ExprCall(ref callee, ref args) => if let Expr_::ExprPath(ref qpath) = callee.node {
 157             let def = cx.tables.qpath_def(qpath, callee.hir_id);
 158             match def {
 159                 Def::Struct(..) | Def::Variant(..) | Def::StructCtor(..) | Def::VariantCtor(..) => {
 160                     Some(args.iter().collect())
 161                 },
 162                 _ => None,
 163             }
 164         } else {
 165             None
 166         },
 167         Expr_::ExprBlock(ref block) => {
 168             if block.stmts.is_empty() {
 169                 block.expr.as_ref().and_then(|e| {
 170                     match block.rules {
 171                         BlockCheckMode::UnsafeBlock(UnsafeSource::UserProvided) => None,
 172                         BlockCheckMode::DefaultBlock => Some(vec![&**e]),
 173                         // in case of compiler-inserted signaling blocks
 174                         _ => reduce_expression(cx, e),
 175                     }
 176                 })
 177             } else {
 178                 None
 179             }
 180         },
 181         _ => None,
 182     }
 183 }