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::{DefKind, Res};
   3 use rustc::hir::{BinOpKind, BlockCheckMode, Expr, ExprKind, Stmt, StmtKind, UnsafeSource};
   4 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
   5 use rustc::{declare_lint_pass, declare_tool_lint};
   6 use rustc_errors::Applicability;
   7 use std::ops::Deref;
   8
   9 declare_clippy_lint! {
  10     /// **What it does:** Checks for statements which have no effect.
  11     ///
  12     /// **Why is this bad?** Similar to dead code, these statements are actually
  13     /// executed. However, as they have no effect, all they do is make the code less
  14     /// readable.
  15     ///
  16     /// **Known problems:** None.
  17     ///
  18     /// **Example:**
  19     /// ```rust
  20     /// 0;
  21     /// ```
  22     pub NO_EFFECT,
  23     complexity,
  24     "statements with no effect"
  25 }
  26
  27 declare_clippy_lint! {
  28     /// **What it does:** Checks for expression statements that can be reduced to a
  29     /// sub-expression.
  30     ///
  31     /// **Why is this bad?** Expressions by themselves often have no side-effects.
  32     /// Having such expressions reduces readability.
  33     ///
  34     /// **Known problems:** None.
  35     ///
  36     /// **Example:**
  37     /// ```rust
  38     /// compute_array()[0];
  39     /// ```
  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 res = cx.tables.qpath_res(qpath, callee.hir_id);
  74                 match res {
  75                     Res::Def(DefKind::Struct, ..) | Res::Def(DefKind::Variant, ..) | Res::Def(DefKind::Ctor(..), _) => {
  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 declare_lint_pass!(NoEffect => [NO_EFFECT, UNNECESSARY_OPERATION]);
  97
  98 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NoEffect {
  99     fn check_stmt(&mut self, cx: &LateContext<'a, 'tcx>, stmt: &'tcx Stmt) {
 100         if let StmtKind::Semi(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                     Applicability::MachineApplicable,
 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 != BinOpKind::And && binop.node != BinOpKind::Or => {
 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) => {
 148             if has_drop(cx, cx.tables.expr_ty(expr)) {
 149                 None
 150             } else {
 151                 Some(fields.iter().map(|f| &f.expr).chain(base).map(Deref::deref).collect())
 152             }
 153         },
 154         ExprKind::Call(ref callee, ref args) => {
 155             if let ExprKind::Path(ref qpath) = callee.node {
 156                 let res = cx.tables.qpath_res(qpath, callee.hir_id);
 157                 match res {
 158                     Res::Def(DefKind::Struct, ..) | Res::Def(DefKind::Variant, ..) | Res::Def(DefKind::Ctor(..), _)
 159                         if !has_drop(cx, cx.tables.expr_ty(expr)) =>
 160                     {
 161                         Some(args.iter().collect())
 162                     },
 163                     _ => None,
 164                 }
 165             } else {
 166                 None
 167             }
 168         },
 169         ExprKind::Block(ref block, _) => {
 170             if block.stmts.is_empty() {
 171                 block.expr.as_ref().and_then(|e| {
 172                     match block.rules {
 173                         BlockCheckMode::UnsafeBlock(UnsafeSource::UserProvided) => None,
 174                         BlockCheckMode::DefaultBlock => Some(vec![&**e]),
 175                         // in case of compiler-inserted signaling blocks
 176                         _ => reduce_expression(cx, e),
 177                     }
 178                 })
 179             } else {
 180                 None
 181             }
 182         },
 183         _ => None,
 184     }
 185 }