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 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 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     fn name(&self) -> &'static str {
 105         "NoEffect"
 106     }
 107 }
 108
 109 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Pass {
 110     fn check_stmt(&mut self, cx: &LateContext<'a, 'tcx>, stmt: &'tcx Stmt) {
 111         if let StmtKind::Semi(ref expr) = stmt.node {
 112             if has_no_effect(cx, expr) {
 113                 span_lint(cx, NO_EFFECT, stmt.span, "statement with no effect");
 114             } else if let Some(reduced) = reduce_expression(cx, expr) {
 115                 let mut snippet = String::new();
 116                 for e in reduced {
 117                     if in_macro(e.span) {
 118                         return;
 119                     }
 120                     if let Some(snip) = snippet_opt(cx, e.span) {
 121                         snippet.push_str(&snip);
 122                         snippet.push(';');
 123                     } else {
 124                         return;
 125                     }
 126                 }
 127                 span_lint_and_sugg(
 128                     cx,
 129                     UNNECESSARY_OPERATION,
 130                     stmt.span,
 131                     "statement can be reduced",
 132                     "replace it with",
 133                     snippet,
 134                     Applicability::MachineApplicable,
 135                 );
 136             }
 137         }
 138     }
 139 }
 140
 141 fn reduce_expression<'a>(cx: &LateContext<'_, '_>, expr: &'a Expr) -> Option<Vec<&'a Expr>> {
 142     if in_macro(expr.span) {
 143         return None;
 144     }
 145     match expr.node {
 146         ExprKind::Index(ref a, ref b) => Some(vec![&**a, &**b]),
 147         ExprKind::Binary(ref binop, ref a, ref b) if binop.node != BinOpKind::And && binop.node != BinOpKind::Or => {
 148             Some(vec![&**a, &**b])
 149         },
 150         ExprKind::Array(ref v) | ExprKind::Tup(ref v) => Some(v.iter().collect()),
 151         ExprKind::Repeat(ref inner, _)
 152         | ExprKind::Cast(ref inner, _)
 153         | ExprKind::Type(ref inner, _)
 154         | ExprKind::Unary(_, ref inner)
 155         | ExprKind::Field(ref inner, _)
 156         | ExprKind::AddrOf(_, ref inner)
 157         | ExprKind::Box(ref inner) => reduce_expression(cx, inner).or_else(|| Some(vec![inner])),
 158         ExprKind::Struct(_, ref fields, ref base) => {
 159             if has_drop(cx, cx.tables.expr_ty(expr)) {
 160                 None
 161             } else {
 162                 Some(fields.iter().map(|f| &f.expr).chain(base).map(Deref::deref).collect())
 163             }
 164         },
 165         ExprKind::Call(ref callee, ref args) => {
 166             if let ExprKind::Path(ref qpath) = callee.node {
 167                 let def = cx.tables.qpath_def(qpath, callee.hir_id);
 168                 match def {
 169                     Def::Struct(..) | Def::Variant(..) | Def::StructCtor(..) | Def::VariantCtor(..)
 170                         if !has_drop(cx, cx.tables.expr_ty(expr)) =>
 171                     {
 172                         Some(args.iter().collect())
 173                     },
 174                     _ => None,
 175                 }
 176             } else {
 177                 None
 178             }
 179         },
 180         ExprKind::Block(ref block, _) => {
 181             if block.stmts.is_empty() {
 182                 block.expr.as_ref().and_then(|e| {
 183                     match block.rules {
 184                         BlockCheckMode::UnsafeBlock(UnsafeSource::UserProvided) => None,
 185                         BlockCheckMode::DefaultBlock => Some(vec![&**e]),
 186                         // in case of compiler-inserted signaling blocks
 187                         _ => reduce_expression(cx, e),
 188                     }
 189                 })
 190             } else {
 191                 None
 192             }
 193         },
 194         _ => None,
 195     }
 196 }