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(a, b) | ExprKind::Binary(_, a, b) => has_no_effect(cx, a) && has_no_effect(cx, b),
  55         ExprKind::Array(v) | ExprKind::Tup(v) => v.iter().all(|val| has_no_effect(cx, val)),
  56         ExprKind::Repeat(inner, _)
  57         | ExprKind::Cast(inner, _)
  58         | ExprKind::Type(inner, _)
  59         | ExprKind::Unary(_, inner)
  60         | ExprKind::Field(inner, _)
  61         | ExprKind::AddrOf(_, _, inner)
  62         | ExprKind::Box(inner) => has_no_effect(cx, inner),
  63         ExprKind::Struct(_, fields, ref base) => {
  64             !has_drop(cx, cx.typeck_results().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(callee, args) => {
  69             if let ExprKind::Path(ref qpath) = callee.kind {
  70                 let res = cx.qpath_res(qpath, callee.hir_id);
  71                 match res {
  72                     Res::Def(DefKind::Struct | DefKind::Variant | DefKind::Ctor(..), ..) => {
  73                         !has_drop(cx, cx.typeck_results().expr_ty(expr))
  74                             && args.iter().all(|arg| has_no_effect(cx, arg))
  75                     },
  76                     _ => false,
  77                 }
  78             } else {
  79                 false
  80             }
  81         },
  82         ExprKind::Block(block, _) => {
  83             block.stmts.is_empty() && block.expr.as_ref().map_or(false, |expr| has_no_effect(cx, expr))
  84         },
  85         _ => false,
  86     }
  87 }
  88
  89 declare_lint_pass!(NoEffect => [NO_EFFECT, UNNECESSARY_OPERATION]);
  90
  91 impl<'tcx> LateLintPass<'tcx> for NoEffect {
  92     fn check_stmt(&mut self, cx: &LateContext<'tcx>, stmt: &'tcx Stmt<'_>) {
  93         if let StmtKind::Semi(expr) = stmt.kind {
  94             if has_no_effect(cx, expr) {
  95                 span_lint(cx, NO_EFFECT, stmt.span, "statement with no effect");
  96             } else if let Some(reduced) = reduce_expression(cx, expr) {
  97                 let mut snippet = String::new();
  98                 for e in reduced {
  99                     if e.span.from_expansion() {
 100                         return;
 101                     }
 102                     if let Some(snip) = snippet_opt(cx, e.span) {
 103                         snippet.push_str(&snip);
 104                         snippet.push(';');
 105                     } else {
 106                         return;
 107                     }
 108                 }
 109                 span_lint_and_sugg(
 110                     cx,
 111                     UNNECESSARY_OPERATION,
 112                     stmt.span,
 113                     "statement can be reduced",
 114                     "replace it with",
 115                     snippet,
 116                     Applicability::MachineApplicable,
 117                 );
 118             }
 119         }
 120     }
 121 }
 122
 123 fn reduce_expression<'a>(cx: &LateContext<'_>, expr: &'a Expr<'a>) -> Option<Vec<&'a Expr<'a>>> {
 124     if expr.span.from_expansion() {
 125         return None;
 126     }
 127     match expr.kind {
 128         ExprKind::Index(a, b) => Some(vec![a, b]),
 129         ExprKind::Binary(ref binop, a, b) if binop.node != BinOpKind::And && binop.node != BinOpKind::Or => {
 130             Some(vec![a, b])
 131         },
 132         ExprKind::Array(v) | ExprKind::Tup(v) => Some(v.iter().collect()),
 133         ExprKind::Repeat(inner, _)
 134         | ExprKind::Cast(inner, _)
 135         | ExprKind::Type(inner, _)
 136         | ExprKind::Unary(_, inner)
 137         | ExprKind::Field(inner, _)
 138         | ExprKind::AddrOf(_, _, inner)
 139         | ExprKind::Box(inner) => reduce_expression(cx, inner).or_else(|| Some(vec![inner])),
 140         ExprKind::Struct(_, fields, ref base) => {
 141             if has_drop(cx, cx.typeck_results().expr_ty(expr)) {
 142                 None
 143             } else {
 144                 Some(fields.iter().map(|f| &f.expr).chain(base).map(Deref::deref).collect())
 145             }
 146         },
 147         ExprKind::Call(callee, args) => {
 148             if let ExprKind::Path(ref qpath) = callee.kind {
 149                 let res = cx.qpath_res(qpath, callee.hir_id);
 150                 match res {
 151                     Res::Def(DefKind::Struct | DefKind::Variant | DefKind::Ctor(..), ..)
 152                         if !has_drop(cx, cx.typeck_results().expr_ty(expr)) =>
 153                     {
 154                         Some(args.iter().collect())
 155                     },
 156                     _ => None,
 157                 }
 158             } else {
 159                 None
 160             }
 161         },
 162         ExprKind::Block(block, _) => {
 163             if block.stmts.is_empty() {
 164                 block.expr.as_ref().and_then(|e| {
 165                     match block.rules {
 166                         BlockCheckMode::UnsafeBlock(UnsafeSource::UserProvided) => None,
 167                         BlockCheckMode::DefaultBlock => Some(vec![&**e]),
 168                         // in case of compiler-inserted signaling blocks
 169                         _ => reduce_expression(cx, e),
 170                     }
 171                 })
 172             } else {
 173                 None
 174             }
 175         },
 176         _ => None,
 177     }
 178 }