clippy_lints/src/no_effect.rs

   1 use crate::utils::{has_drop, qpath_res, snippet_opt, span_lint, span_lint_and_sugg};
   2 use rustc_errors::Applicability;
   3 use rustc_hir::def::{DefKind, Res};
   4 use rustc_hir::{BinOpKind, BlockCheckMode, Expr, ExprKind, Stmt, StmtKind, UnsafeSource};
   5 use rustc_lint::{LateContext, LateLintPass};
   6 use rustc_session::{declare_lint_pass, declare_tool_lint};
   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,ignore
  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 expr.span.from_expansion() {
  47         return false;
  48     }
  49     match expr.kind {
  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                 && base.as_ref().map_or(true, |base| has_no_effect(cx, base))
  67         },
  68         ExprKind::Call(ref callee, ref args) => {
  69             if let ExprKind::Path(ref qpath) = callee.kind {
  70                 let res = qpath_res(cx, qpath, callee.hir_id);
  71                 match res {
  72                     Res::Def(DefKind::Struct | DefKind::Variant | DefKind::Ctor(..), ..) => {
  73                         !has_drop(cx, cx.tables.expr_ty(expr)) && args.iter().all(|arg| has_no_effect(cx, arg))
  74                     },
  75                     _ => false,
  76                 }
  77             } else {
  78                 false
  79             }
  80         },
  81         ExprKind::Block(ref block, _) => {
  82             block.stmts.is_empty() && block.expr.as_ref().map_or(false, |expr| has_no_effect(cx, expr))
  83         },
  84         _ => false,
  85     }
  86 }
  87
  88 declare_lint_pass!(NoEffect => [NO_EFFECT, UNNECESSARY_OPERATION]);
  89
  90 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NoEffect {
  91     fn check_stmt(&mut self, cx: &LateContext<'a, 'tcx>, stmt: &'tcx Stmt<'_>) {
  92         if let StmtKind::Semi(ref expr) = stmt.kind {
  93             if has_no_effect(cx, expr) {
  94                 span_lint(cx, NO_EFFECT, stmt.span, "statement with no effect");
  95             } else if let Some(reduced) = reduce_expression(cx, expr) {
  96                 let mut snippet = String::new();
  97                 for e in reduced {
  98                     if e.span.from_expansion() {
  99                         return;
 100                     }
 101                     if let Some(snip) = snippet_opt(cx, e.span) {
 102                         snippet.push_str(&snip);
 103                         snippet.push(';');
 104                     } else {
 105                         return;
 106                     }
 107                 }
 108                 span_lint_and_sugg(
 109                     cx,
 110                     UNNECESSARY_OPERATION,
 111                     stmt.span,
 112                     "statement can be reduced",
 113                     "replace it with",
 114                     snippet,
 115                     Applicability::MachineApplicable,
 116                 );
 117             }
 118         }
 119     }
 120 }
 121
 122 fn reduce_expression<'a>(cx: &LateContext<'_, '_>, expr: &'a Expr<'a>) -> Option<Vec<&'a Expr<'a>>> {
 123     if expr.span.from_expansion() {
 124         return None;
 125     }
 126     match expr.kind {
 127         ExprKind::Index(ref a, ref b) => Some(vec![&**a, &**b]),
 128         ExprKind::Binary(ref binop, ref a, ref b) if binop.node != BinOpKind::And && binop.node != BinOpKind::Or => {
 129             Some(vec![&**a, &**b])
 130         },
 131         ExprKind::Array(ref v) | ExprKind::Tup(ref v) => Some(v.iter().collect()),
 132         ExprKind::Repeat(ref inner, _)
 133         | ExprKind::Cast(ref inner, _)
 134         | ExprKind::Type(ref inner, _)
 135         | ExprKind::Unary(_, ref inner)
 136         | ExprKind::Field(ref inner, _)
 137         | ExprKind::AddrOf(_, _, ref inner)
 138         | ExprKind::Box(ref inner) => reduce_expression(cx, inner).or_else(|| Some(vec![inner])),
 139         ExprKind::Struct(_, ref fields, ref base) => {
 140             if has_drop(cx, cx.tables.expr_ty(expr)) {
 141                 None
 142             } else {
 143                 Some(fields.iter().map(|f| &f.expr).chain(base).map(Deref::deref).collect())
 144             }
 145         },
 146         ExprKind::Call(ref callee, ref args) => {
 147             if let ExprKind::Path(ref qpath) = callee.kind {
 148                 let res = qpath_res(cx, qpath, callee.hir_id);
 149                 match res {
 150                     Res::Def(DefKind::Struct | DefKind::Variant | DefKind::Ctor(..), ..)
 151                         if !has_drop(cx, cx.tables.expr_ty(expr)) =>
 152                     {
 153                         Some(args.iter().collect())
 154                     },
 155                     _ => None,
 156                 }
 157             } else {
 158                 None
 159             }
 160         },
 161         ExprKind::Block(ref block, _) => {
 162             if block.stmts.is_empty() {
 163                 block.expr.as_ref().and_then(|e| {
 164                     match block.rules {
 165                         BlockCheckMode::UnsafeBlock(UnsafeSource::UserProvided) => None,
 166                         BlockCheckMode::DefaultBlock => Some(vec![&**e]),
 167                         // in case of compiler-inserted signaling blocks
 168                         _ => reduce_expression(cx, e),
 169                     }
 170                 })
 171             } else {
 172                 None
 173             }
 174         },
 175         _ => None,
 176     }
 177 }