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

   1 use clippy_utils::diagnostics::{span_lint_hir, span_lint_hir_and_then};
   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::{is_range_literal, 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
  13     /// Checks for statements which have no effect.
  14     ///
  15     /// ### Why is this bad?
  16     /// Similar to dead code, these statements are actually
  17     /// executed. However, as they have no effect, all they do is make the code less
  18     /// readable.
  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
  31     /// Checks for expression statements that can be reduced to a
  32     /// sub-expression.
  33     ///
  34     /// ### Why is this bad?
  35     /// Expressions by themselves often have no side-effects.
  36     /// Having such expressions reduces readability.
  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                 let def_matched = matches!(
  72                     res,
  73                     Res::Def(DefKind::Struct | DefKind::Variant | DefKind::Ctor(..), ..)
  74                 );
  75                 if def_matched || is_range_literal(expr) {
  76                     !has_drop(cx, cx.typeck_results().expr_ty(expr)) && args.iter().all(|arg| has_no_effect(cx, arg))
  77                 } else {
  78                     false
  79                 }
  80             } else {
  81                 false
  82             }
  83         },
  84         ExprKind::Block(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(expr) = stmt.kind {
  96             if has_no_effect(cx, expr) {
  97                 span_lint_hir(cx, NO_EFFECT, expr.hir_id, 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_hir_and_then(
 112                     cx,
 113                     UNNECESSARY_OPERATION,
 114                     expr.hir_id,
 115                     stmt.span,
 116                     "statement can be reduced",
 117                     |diag| {
 118                         diag.span_suggestion(stmt.span, "replace it with", snippet, Applicability::MachineApplicable);
 119                     },
 120                 );
 121             }
 122         }
 123     }
 124 }
 125
 126 fn reduce_expression<'a>(cx: &LateContext<'_>, expr: &'a Expr<'a>) -> Option<Vec<&'a Expr<'a>>> {
 127     if expr.span.from_expansion() {
 128         return None;
 129     }
 130     match expr.kind {
 131         ExprKind::Index(a, b) => Some(vec![a, b]),
 132         ExprKind::Binary(ref binop, a, b) if binop.node != BinOpKind::And && binop.node != BinOpKind::Or => {
 133             Some(vec![a, b])
 134         },
 135         ExprKind::Array(v) | ExprKind::Tup(v) => Some(v.iter().collect()),
 136         ExprKind::Repeat(inner, _)
 137         | ExprKind::Cast(inner, _)
 138         | ExprKind::Type(inner, _)
 139         | ExprKind::Unary(_, inner)
 140         | ExprKind::Field(inner, _)
 141         | ExprKind::AddrOf(_, _, inner)
 142         | ExprKind::Box(inner) => reduce_expression(cx, inner).or_else(|| Some(vec![inner])),
 143         ExprKind::Struct(_, fields, ref base) => {
 144             if has_drop(cx, cx.typeck_results().expr_ty(expr)) {
 145                 None
 146             } else {
 147                 Some(fields.iter().map(|f| &f.expr).chain(base).map(Deref::deref).collect())
 148             }
 149         },
 150         ExprKind::Call(callee, args) => {
 151             if let ExprKind::Path(ref qpath) = callee.kind {
 152                 let res = cx.qpath_res(qpath, callee.hir_id);
 153                 match res {
 154                     Res::Def(DefKind::Struct | DefKind::Variant | DefKind::Ctor(..), ..)
 155                         if !has_drop(cx, cx.typeck_results().expr_ty(expr)) =>
 156                     {
 157                         Some(args.iter().collect())
 158                     },
 159                     _ => None,
 160                 }
 161             } else {
 162                 None
 163             }
 164         },
 165         ExprKind::Block(block, _) => {
 166             if block.stmts.is_empty() {
 167                 block.expr.as_ref().and_then(|e| {
 168                     match block.rules {
 169                         BlockCheckMode::UnsafeBlock(UnsafeSource::UserProvided) => None,
 170                         BlockCheckMode::DefaultBlock => Some(vec![&**e]),
 171                         // in case of compiler-inserted signaling blocks
 172                         BlockCheckMode::UnsafeBlock(_) => reduce_expression(cx, e),
 173                     }
 174                 })
 175             } else {
 176                 None
 177             }
 178         },
 179         _ => None,
 180     }
 181 }