clippy_lints/src/no_effect.rs

   1 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
   2 use rustc::hir::def::Def;
   3 use rustc::hir::{BinOpKind, BlockCheckMode, Expr, ExprKind, Stmt, StmtKind, UnsafeSource};
   4 use crate::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_clippy_lint! {
  20     pub NO_EFFECT,
  21     complexity,
  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_clippy_lint! {
  38     pub UNNECESSARY_OPERATION,
  39     complexity,
  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         ExprKind::Lit(..) | ExprKind::Closure(.., _) => true,
  49         ExprKind::Path(..) => !has_drop(cx, expr),
  50         ExprKind::Index(ref a, ref b) | ExprKind::Binary(_, ref a, ref b) => {
  51             has_no_effect(cx, a) && has_no_effect(cx, b)
  52         },
  53         ExprKind::Array(ref v) | ExprKind::Tup(ref v) => v.iter().all(|val| has_no_effect(cx, val)),
  54         ExprKind::Repeat(ref inner, _) |
  55         ExprKind::Cast(ref inner, _) |
  56         ExprKind::Type(ref inner, _) |
  57         ExprKind::Unary(_, ref inner) |
  58         ExprKind::Field(ref inner, _) |
  59         ExprKind::AddrOf(_, ref inner) |
  60         ExprKind::Box(ref inner) => has_no_effect(cx, inner),
  61         ExprKind::Struct(_, ref fields, ref base) => {
  62             !has_drop(cx, expr) && 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         ExprKind::Call(ref callee, ref args) => if let ExprKind::Path(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                     !has_drop(cx, expr) && args.iter().all(|arg| has_no_effect(cx, arg))
  72                 },
  73                 _ => false,
  74             }
  75         } else {
  76             false
  77         },
  78         ExprKind::Block(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         ExprKind::Index(ref a, ref b) => Some(vec![&**a, &**b]),
 136         ExprKind::Binary(ref binop, ref a, ref b) if binop.node != BiAnd && binop.node != BiOr => {
 137             Some(vec![&**a, &**b])
 138         },
 139         ExprKind::Array(ref v) | ExprKind::Tup(ref v) => Some(v.iter().collect()),
 140         ExprKind::Repeat(ref inner, _) |
 141         ExprKind::Cast(ref inner, _) |
 142         ExprKind::Type(ref inner, _) |
 143         ExprKind::Unary(_, ref inner) |
 144         ExprKind::Field(ref inner, _) |
 145         ExprKind::AddrOf(_, ref inner) |
 146         ExprKind::Box(ref inner) => reduce_expression(cx, inner).or_else(|| Some(vec![inner])),
 147         ExprKind::Struct(_, ref fields, ref base) => if has_drop(cx, expr) {
 148             None
 149         } else {
 150             Some(
 151                 fields
 152                     .iter()
 153                     .map(|f| &f.expr)
 154                     .chain(base)
 155                     .map(Deref::deref)
 156                     .collect(),
 157             )
 158         },
 159         ExprKind::Call(ref callee, ref args) => if let ExprKind::Path(ref qpath) = callee.node {
 160             let def = cx.tables.qpath_def(qpath, callee.hir_id);
 161             match def {
 162                 Def::Struct(..) | Def::Variant(..) | Def::StructCtor(..) | Def::VariantCtor(..)
 163                     if !has_drop(cx, expr) =>
 164                 {
 165                     Some(args.iter().collect())
 166                 },
 167                 _ => None,
 168             }
 169         } else {
 170             None
 171         },
 172         ExprKind::Block(ref block, _) => {
 173             if block.stmts.is_empty() {
 174                 block.expr.as_ref().and_then(|e| {
 175                     match block.rules {
 176                         BlockCheckMode::UnsafeBlock(UnsafeSource::UserProvided) => None,
 177                         BlockCheckMode::DefaultBlock => Some(vec![&**e]),
 178                         // in case of compiler-inserted signaling blocks
 179                         _ => reduce_expression(cx, e),
 180                     }
 181                 })
 182             } else {
 183                 None
 184             }
 185         },
 186         _ => None,
 187     }
 188 }