clippy_lints/src/no_effect.rs

   1 use crate::utils::{snippet_opt, span_lint, span_lint_and_sugg};
   2 use clippy_utils::ty::has_drop;
   3 use rustc_errors::Applicability;
   4 use rustc_hir::def::{DefKind, Res};
   5 use rustc_hir::{BinOpKind, BlockCheckMode, Expr, ExprKind, Stmt, StmtKind, UnsafeSource};
   6 use rustc_lint::{LateContext, LateLintPass};
   7 use rustc_session::{declare_lint_pass, declare_tool_lint};
   8 use std::ops::Deref;
   9
  10 declare_clippy_lint! {
  11     /// **What it does:** Checks for statements which have no effect.
  12     ///
  13     /// **Why is this bad?** Similar to dead code, these statements are actually
  14     /// executed. However, as they have no effect, all they do is make the code less
  15     /// readable.
  16     ///
  17     /// **Known problems:** None.
  18     ///
  19     /// **Example:**
  20     /// ```rust
  21     /// 0;
  22     /// ```
  23     pub NO_EFFECT,
  24     complexity,
  25     "statements with no effect"
  26 }
  27
  28 declare_clippy_lint! {
  29     /// **What it does:** Checks for expression statements that can be reduced to a
  30     /// sub-expression.
  31     ///
  32     /// **Why is this bad?** Expressions by themselves often have no side-effects.
  33     /// Having such expressions reduces readability.
  34     ///
  35     /// **Known problems:** None.
  36     ///
  37     /// **Example:**
  38     /// ```rust,ignore
  39     /// compute_array()[0];
  40     /// ```
  41     pub UNNECESSARY_OPERATION,
  42     complexity,
  43     "outer expressions with no effect"
  44 }
  45
  46 fn has_no_effect(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
  47     if expr.span.from_expansion() {
  48         return false;
  49     }
  50     match expr.kind {
  51         ExprKind::Lit(..) | ExprKind::Closure(..) => true,
  52         ExprKind::Path(..) => !has_drop(cx, cx.typeck_results().expr_ty(expr)),
  53         ExprKind::Index(ref a, ref b) | ExprKind::Binary(_, ref a, ref b) => {
  54             has_no_effect(cx, a) && has_no_effect(cx, b)
  55         },
  56         ExprKind::Array(ref v) | ExprKind::Tup(ref v) => v.iter().all(|val| has_no_effect(cx, val)),
  57         ExprKind::Repeat(ref inner, _)
  58         | ExprKind::Cast(ref inner, _)
  59         | ExprKind::Type(ref inner, _)
  60         | ExprKind::Unary(_, ref inner)
  61         | ExprKind::Field(ref inner, _)
  62         | ExprKind::AddrOf(_, _, ref inner)
  63         | ExprKind::Box(ref inner) => has_no_effect(cx, inner),
  64         ExprKind::Struct(_, ref fields, ref base) => {
  65             !has_drop(cx, cx.typeck_results().expr_ty(expr))
  66                 && fields.iter().all(|field| has_no_effect(cx, &field.expr))
  67                 && base.as_ref().map_or(true, |base| has_no_effect(cx, base))
  68         },
  69         ExprKind::Call(ref callee, ref args) => {
  70             if let ExprKind::Path(ref qpath) = callee.kind {
  71                 let res = cx.qpath_res(qpath, callee.hir_id);
  72                 match res {
  73                     Res::Def(DefKind::Struct | DefKind::Variant | DefKind::Ctor(..), ..) => {
  74                         !has_drop(cx, cx.typeck_results().expr_ty(expr))
  75                             && args.iter().all(|arg| has_no_effect(cx, arg))
  76                     },
  77                     _ => false,
  78                 }
  79             } else {
  80                 false
  81             }
  82         },
  83         ExprKind::Block(ref block, _) => {
  84             block.stmts.is_empty() && block.expr.as_ref().map_or(false, |expr| has_no_effect(cx, expr))
  85         },
  86         _ => false,
  87     }
  88 }
  89
  90 declare_lint_pass!(NoEffect => [NO_EFFECT, UNNECESSARY_OPERATION]);
  91
  92 impl<'tcx> LateLintPass<'tcx> for NoEffect {
  93     fn check_stmt(&mut self, cx: &LateContext<'tcx>, stmt: &'tcx Stmt<'_>) {
  94         if let StmtKind::Semi(ref expr) = stmt.kind {
  95             if has_no_effect(cx, expr) {
  96                 span_lint(cx, NO_EFFECT, stmt.span, "statement with no effect");
  97             } else if let Some(reduced) = reduce_expression(cx, expr) {
  98                 let mut snippet = String::new();
  99                 for e in reduced {
 100                     if e.span.from_expansion() {
 101                         return;
 102                     }
 103                     if let Some(snip) = snippet_opt(cx, e.span) {
 104                         snippet.push_str(&snip);
 105                         snippet.push(';');
 106                     } else {
 107                         return;
 108                     }
 109                 }
 110                 span_lint_and_sugg(
 111                     cx,
 112                     UNNECESSARY_OPERATION,
 113                     stmt.span,
 114                     "statement can be reduced",
 115                     "replace it with",
 116                     snippet,
 117                     Applicability::MachineApplicable,
 118                 );
 119             }
 120         }
 121     }
 122 }
 123
 124 fn reduce_expression<'a>(cx: &LateContext<'_>, expr: &'a Expr<'a>) -> Option<Vec<&'a Expr<'a>>> {
 125     if expr.span.from_expansion() {
 126         return None;
 127     }
 128     match expr.kind {
 129         ExprKind::Index(ref a, ref b) => Some(vec![&**a, &**b]),
 130         ExprKind::Binary(ref binop, ref a, ref b) if binop.node != BinOpKind::And && binop.node != BinOpKind::Or => {
 131             Some(vec![&**a, &**b])
 132         },
 133         ExprKind::Array(ref v) | ExprKind::Tup(ref v) => Some(v.iter().collect()),
 134         ExprKind::Repeat(ref inner, _)
 135         | ExprKind::Cast(ref inner, _)
 136         | ExprKind::Type(ref inner, _)
 137         | ExprKind::Unary(_, ref inner)
 138         | ExprKind::Field(ref inner, _)
 139         | ExprKind::AddrOf(_, _, ref inner)
 140         | ExprKind::Box(ref inner) => reduce_expression(cx, inner).or_else(|| Some(vec![inner])),
 141         ExprKind::Struct(_, ref fields, ref base) => {
 142             if has_drop(cx, cx.typeck_results().expr_ty(expr)) {
 143                 None
 144             } else {
 145                 Some(fields.iter().map(|f| &f.expr).chain(base).map(Deref::deref).collect())
 146             }
 147         },
 148         ExprKind::Call(ref callee, ref args) => {
 149             if let ExprKind::Path(ref qpath) = callee.kind {
 150                 let res = cx.qpath_res(qpath, callee.hir_id);
 151                 match res {
 152                     Res::Def(DefKind::Struct | DefKind::Variant | DefKind::Ctor(..), ..)
 153                         if !has_drop(cx, cx.typeck_results().expr_ty(expr)) =>
 154                     {
 155                         Some(args.iter().collect())
 156                     },
 157                     _ => None,
 158                 }
 159             } else {
 160                 None
 161             }
 162         },
 163         ExprKind::Block(ref block, _) => {
 164             if block.stmts.is_empty() {
 165                 block.expr.as_ref().and_then(|e| {
 166                     match block.rules {
 167                         BlockCheckMode::UnsafeBlock(UnsafeSource::UserProvided) => None,
 168                         BlockCheckMode::DefaultBlock => Some(vec![&**e]),
 169                         // in case of compiler-inserted signaling blocks
 170                         _ => reduce_expression(cx, e),
 171                     }
 172                 })
 173             } else {
 174                 None
 175             }
 176         },
 177         _ => None,
 178     }
 179 }