src/tools/clippy/clippy_lints/src/no_effect.rs

   1 use clippy_utils::diagnostics::{span_lint, span_lint_and_sugg};
   2 use clippy_utils::source::snippet_opt;
   3 use clippy_utils::ty::has_drop;
   4 use rustc_errors::Applicability;
   5 use rustc_hir::def::{DefKind, Res};
   6 use rustc_hir::{BinOpKind, BlockCheckMode, Expr, ExprKind, Stmt, StmtKind, UnsafeSource};
   7 use rustc_lint::{LateContext, LateLintPass};
   8 use rustc_session::{declare_lint_pass, declare_tool_lint};
   9 use std::ops::Deref;
  10
  11 declare_clippy_lint! {
  12     /// **What it does:** Checks for statements which have no effect.
  13     ///
  14     /// **Why is this bad?** Similar to dead code, these statements are actually
  15     /// executed. However, as they have no effect, all they do is make the code less
  16     /// readable.
  17     ///
  18     /// **Known problems:** None.
  19     ///
  20     /// **Example:**
  21     /// ```rust
  22     /// 0;
  23     /// ```
  24     pub NO_EFFECT,
  25     complexity,
  26     "statements with no effect"
  27 }
  28
  29 declare_clippy_lint! {
  30     /// **What it does:** Checks for expression statements that can be reduced to a
  31     /// sub-expression.
  32     ///
  33     /// **Why is this bad?** Expressions by themselves often have no side-effects.
  34     /// Having such expressions reduces readability.
  35     ///
  36     /// **Known problems:** None.
  37     ///
  38     /// **Example:**
  39     /// ```rust,ignore
  40     /// compute_array()[0];
  41     /// ```
  42     pub UNNECESSARY_OPERATION,
  43     complexity,
  44     "outer expressions with no effect"
  45 }
  46
  47 fn has_no_effect(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
  48     if expr.span.from_expansion() {
  49         return false;
  50     }
  51     match expr.kind {
  52         ExprKind::Lit(..) | ExprKind::Closure(..) => true,
  53         ExprKind::Path(..) => !has_drop(cx, cx.typeck_results().expr_ty(expr)),
  54         ExprKind::Index(ref a, ref b) | ExprKind::Binary(_, ref a, ref b) => {
  55             has_no_effect(cx, a) && has_no_effect(cx, b)
  56         },
  57         ExprKind::Array(ref v) | ExprKind::Tup(ref v) => v.iter().all(|val| has_no_effect(cx, val)),
  58         ExprKind::Repeat(ref inner, _)
  59         | ExprKind::Cast(ref inner, _)
  60         | ExprKind::Type(ref inner, _)
  61         | ExprKind::Unary(_, ref inner)
  62         | ExprKind::Field(ref inner, _)
  63         | ExprKind::AddrOf(_, _, ref inner)
  64         | ExprKind::Box(ref inner) => has_no_effect(cx, inner),
  65         ExprKind::Struct(_, ref fields, ref base) => {
  66             !has_drop(cx, cx.typeck_results().expr_ty(expr))
  67                 && fields.iter().all(|field| has_no_effect(cx, &field.expr))
  68                 && base.as_ref().map_or(true, |base| has_no_effect(cx, base))
  69         },
  70         ExprKind::Call(ref callee, ref args) => {
  71             if let ExprKind::Path(ref qpath) = callee.kind {
  72                 let res = cx.qpath_res(qpath, callee.hir_id);
  73                 match res {
  74                     Res::Def(DefKind::Struct | DefKind::Variant | DefKind::Ctor(..), ..) => {
  75                         !has_drop(cx, cx.typeck_results().expr_ty(expr))
  76                             && 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() && block.expr.as_ref().map_or(false, |expr| has_no_effect(cx, expr))
  86         },
  87         _ => false,
  88     }
  89 }
  90
  91 declare_lint_pass!(NoEffect => [NO_EFFECT, UNNECESSARY_OPERATION]);
  92
  93 impl<'tcx> LateLintPass<'tcx> for NoEffect {
  94     fn check_stmt(&mut self, cx: &LateContext<'tcx>, stmt: &'tcx Stmt<'_>) {
  95         if let StmtKind::Semi(ref expr) = stmt.kind {
  96             if has_no_effect(cx, expr) {
  97                 span_lint(cx, NO_EFFECT, stmt.span, "statement with no effect");
  98             } else if let Some(reduced) = reduce_expression(cx, expr) {
  99                 let mut snippet = String::new();
 100                 for e in reduced {
 101                     if e.span.from_expansion() {
 102                         return;
 103                     }
 104                     if let Some(snip) = snippet_opt(cx, e.span) {
 105                         snippet.push_str(&snip);
 106                         snippet.push(';');
 107                     } else {
 108                         return;
 109                     }
 110                 }
 111                 span_lint_and_sugg(
 112                     cx,
 113                     UNNECESSARY_OPERATION,
 114                     stmt.span,
 115                     "statement can be reduced",
 116                     "replace it with",
 117                     snippet,
 118                     Applicability::MachineApplicable,
 119                 );
 120             }
 121         }
 122     }
 123 }
 124
 125 fn reduce_expression<'a>(cx: &LateContext<'_>, expr: &'a Expr<'a>) -> Option<Vec<&'a Expr<'a>>> {
 126     if expr.span.from_expansion() {
 127         return None;
 128     }
 129     match expr.kind {
 130         ExprKind::Index(ref a, ref b) => Some(vec![&**a, &**b]),
 131         ExprKind::Binary(ref binop, ref a, ref b) if binop.node != BinOpKind::And && binop.node != BinOpKind::Or => {
 132             Some(vec![&**a, &**b])
 133         },
 134         ExprKind::Array(ref v) | ExprKind::Tup(ref v) => Some(v.iter().collect()),
 135         ExprKind::Repeat(ref inner, _)
 136         | ExprKind::Cast(ref inner, _)
 137         | ExprKind::Type(ref inner, _)
 138         | ExprKind::Unary(_, ref inner)
 139         | ExprKind::Field(ref inner, _)
 140         | ExprKind::AddrOf(_, _, ref inner)
 141         | ExprKind::Box(ref inner) => reduce_expression(cx, inner).or_else(|| Some(vec![inner])),
 142         ExprKind::Struct(_, ref fields, ref base) => {
 143             if has_drop(cx, cx.typeck_results().expr_ty(expr)) {
 144                 None
 145             } else {
 146                 Some(fields.iter().map(|f| &f.expr).chain(base).map(Deref::deref).collect())
 147             }
 148         },
 149         ExprKind::Call(ref callee, ref args) => {
 150             if let ExprKind::Path(ref qpath) = callee.kind {
 151                 let res = cx.qpath_res(qpath, callee.hir_id);
 152                 match res {
 153                     Res::Def(DefKind::Struct | DefKind::Variant | DefKind::Ctor(..), ..)
 154                         if !has_drop(cx, cx.typeck_results().expr_ty(expr)) =>
 155                     {
 156                         Some(args.iter().collect())
 157                     },
 158                     _ => None,
 159                 }
 160             } else {
 161                 None
 162             }
 163         },
 164         ExprKind::Block(ref block, _) => {
 165             if block.stmts.is_empty() {
 166                 block.expr.as_ref().and_then(|e| {
 167                     match block.rules {
 168                         BlockCheckMode::UnsafeBlock(UnsafeSource::UserProvided) => None,
 169                         BlockCheckMode::DefaultBlock => Some(vec![&**e]),
 170                         // in case of compiler-inserted signaling blocks
 171                         _ => reduce_expression(cx, e),
 172                     }
 173                 })
 174             } else {
 175                 None
 176             }
 177         },
 178         _ => None,
 179     }
 180 }