clippy_lints/src/no_effect.rs

   1 use crate::utils::{has_drop, in_macro, snippet_opt, span_lint, span_lint_and_sugg};
   2 use rustc::hir::def::Def;
   3 use rustc::hir::{BinOpKind, BlockCheckMode, Expr, ExprKind, Stmt, StmtKind, UnsafeSource};
   4 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
   5 use rustc::{declare_tool_lint, lint_array};
   6 use rustc_errors::Applicability;
   7 use std::ops::Deref;
   8
   9 /// **What it does:** Checks for statements which have no effect.
  10 ///
  11 /// **Why is this bad?** Similar to dead code, these statements are actually
  12 /// executed. However, as they have no effect, all they do is make the code less
  13 /// readable.
  14 ///
  15 /// **Known problems:** None.
  16 ///
  17 /// **Example:**
  18 /// ```rust
  19 /// 0;
  20 /// ```
  21 declare_clippy_lint! {
  22     pub NO_EFFECT,
  23     complexity,
  24     "statements with no effect"
  25 }
  26
  27 /// **What it does:** Checks for expression statements that can be reduced to a
  28 /// sub-expression.
  29 ///
  30 /// **Why is this bad?** Expressions by themselves often have no side-effects.
  31 /// Having such expressions reduces readability.
  32 ///
  33 /// **Known problems:** None.
  34 ///
  35 /// **Example:**
  36 /// ```rust
  37 /// compute_array()[0];
  38 /// ```
  39 declare_clippy_lint! {
  40     pub UNNECESSARY_OPERATION,
  41     complexity,
  42     "outer expressions with no effect"
  43 }
  44
  45 fn has_no_effect(cx: &LateContext<'_, '_>, expr: &Expr) -> bool {
  46     if in_macro(expr.span) {
  47         return false;
  48     }
  49     match expr.node {
  50         ExprKind::Lit(..) | ExprKind::Closure(.., _) => true,
  51         ExprKind::Path(..) => !has_drop(cx, cx.tables.expr_ty(expr)),
  52         ExprKind::Index(ref a, ref b) | ExprKind::Binary(_, ref a, ref b) => {
  53             has_no_effect(cx, a) && has_no_effect(cx, b)
  54         },
  55         ExprKind::Array(ref v) | ExprKind::Tup(ref v) => v.iter().all(|val| has_no_effect(cx, val)),
  56         ExprKind::Repeat(ref inner, _)
  57         | ExprKind::Cast(ref inner, _)
  58         | ExprKind::Type(ref inner, _)
  59         | ExprKind::Unary(_, ref inner)
  60         | ExprKind::Field(ref inner, _)
  61         | ExprKind::AddrOf(_, ref inner)
  62         | ExprKind::Box(ref inner) => has_no_effect(cx, inner),
  63         ExprKind::Struct(_, ref fields, ref base) => {
  64             !has_drop(cx, cx.tables.expr_ty(expr))
  65                 && fields.iter().all(|field| has_no_effect(cx, &field.expr))
  66                 && match *base {
  67                     Some(ref base) => has_no_effect(cx, base),
  68                     None => true,
  69                 }
  70         },
  71         ExprKind::Call(ref callee, ref args) => {
  72             if let ExprKind::Path(ref qpath) = callee.node {
  73                 let def = cx.tables.qpath_def(qpath, callee.hir_id);
  74                 match def {
  75                     Def::Struct(..) | Def::Variant(..) | Def::StructCtor(..) | Def::VariantCtor(..) => {
  76                         !has_drop(cx, cx.tables.expr_ty(expr)) && args.iter().all(|arg| has_no_effect(cx, arg))
  77                     },
  78                     _ => false,
  79                 }
  80             } else {
  81                 false
  82             }
  83         },
  84         ExprKind::Block(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 StmtKind::Semi(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                     Applicability::MachineApplicable,
 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         ExprKind::Index(ref a, ref b) => Some(vec![&**a, &**b]),
 143         ExprKind::Binary(ref binop, ref a, ref b) if binop.node != BinOpKind::And && binop.node != BinOpKind::Or => {
 144             Some(vec![&**a, &**b])
 145         },
 146         ExprKind::Array(ref v) | ExprKind::Tup(ref v) => Some(v.iter().collect()),
 147         ExprKind::Repeat(ref inner, _)
 148         | ExprKind::Cast(ref inner, _)
 149         | ExprKind::Type(ref inner, _)
 150         | ExprKind::Unary(_, ref inner)
 151         | ExprKind::Field(ref inner, _)
 152         | ExprKind::AddrOf(_, ref inner)
 153         | ExprKind::Box(ref inner) => reduce_expression(cx, inner).or_else(|| Some(vec![inner])),
 154         ExprKind::Struct(_, ref fields, ref base) => {
 155             if has_drop(cx, cx.tables.expr_ty(expr)) {
 156                 None
 157             } else {
 158                 Some(fields.iter().map(|f| &f.expr).chain(base).map(Deref::deref).collect())
 159             }
 160         },
 161         ExprKind::Call(ref callee, ref args) => {
 162             if let ExprKind::Path(ref qpath) = callee.node {
 163                 let def = cx.tables.qpath_def(qpath, callee.hir_id);
 164                 match def {
 165                     Def::Struct(..) | Def::Variant(..) | Def::StructCtor(..) | Def::VariantCtor(..)
 166                         if !has_drop(cx, cx.tables.expr_ty(expr)) =>
 167                     {
 168                         Some(args.iter().collect())
 169                     },
 170                     _ => None,
 171                 }
 172             } else {
 173                 None
 174             }
 175         },
 176         ExprKind::Block(ref block, _) => {
 177             if block.stmts.is_empty() {
 178                 block.expr.as_ref().and_then(|e| {
 179                     match block.rules {
 180                         BlockCheckMode::UnsafeBlock(UnsafeSource::UserProvided) => None,
 181                         BlockCheckMode::DefaultBlock => Some(vec![&**e]),
 182                         // in case of compiler-inserted signaling blocks
 183                         _ => reduce_expression(cx, e),
 184                     }
 185                 })
 186             } else {
 187                 None
 188             }
 189         },
 190         _ => None,
 191     }
 192 }