clippy_lints/src/no_effect.rs

   1 // Copyright 2014-2018 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution.
   3 //
   4 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   5 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   6 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   7 // option. This file may not be copied, modified, or distributed
   8 // except according to those terms.
   9
  10
  11 use crate::rustc::hir::def::Def;
  12 use crate::rustc::hir::{BinOpKind, BlockCheckMode, Expr, ExprKind, Stmt, StmtKind, UnsafeSource};
  13 use crate::rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
  14 use crate::rustc::{declare_tool_lint, lint_array};
  15 use crate::rustc_errors::Applicability;
  16 use crate::utils::{has_drop, in_macro, snippet_opt, span_lint, span_lint_and_sugg};
  17 use std::ops::Deref;
  18
  19 /// **What it does:** Checks for statements which have no effect.
  20 ///
  21 /// **Why is this bad?** Similar to dead code, these statements are actually
  22 /// executed. However, as they have no effect, all they do is make the code less
  23 /// readable.
  24 ///
  25 /// **Known problems:** None.
  26 ///
  27 /// **Example:**
  28 /// ```rust
  29 /// 0;
  30 /// ```
  31 declare_clippy_lint! {
  32     pub NO_EFFECT,
  33     complexity,
  34     "statements with no effect"
  35 }
  36
  37 /// **What it does:** Checks for expression statements that can be reduced to a
  38 /// sub-expression.
  39 ///
  40 /// **Why is this bad?** Expressions by themselves often have no side-effects.
  41 /// Having such expressions reduces readability.
  42 ///
  43 /// **Known problems:** None.
  44 ///
  45 /// **Example:**
  46 /// ```rust
  47 /// compute_array()[0];
  48 /// ```
  49 declare_clippy_lint! {
  50     pub UNNECESSARY_OPERATION,
  51     complexity,
  52     "outer expressions with no effect"
  53 }
  54
  55 fn has_no_effect(cx: &LateContext<'_, '_>, expr: &Expr) -> bool {
  56     if in_macro(expr.span) {
  57         return false;
  58     }
  59     match expr.node {
  60         ExprKind::Lit(..) | ExprKind::Closure(.., _) => true,
  61         ExprKind::Path(..) => !has_drop(cx, expr),
  62         ExprKind::Index(ref a, ref b) | ExprKind::Binary(_, ref a, ref b) => {
  63             has_no_effect(cx, a) && has_no_effect(cx, b)
  64         },
  65         ExprKind::Array(ref v) | ExprKind::Tup(ref v) => v.iter().all(|val| has_no_effect(cx, val)),
  66         ExprKind::Repeat(ref inner, _) |
  67         ExprKind::Cast(ref inner, _) |
  68         ExprKind::Type(ref inner, _) |
  69         ExprKind::Unary(_, ref inner) |
  70         ExprKind::Field(ref inner, _) |
  71         ExprKind::AddrOf(_, ref inner) |
  72         ExprKind::Box(ref inner) => has_no_effect(cx, inner),
  73         ExprKind::Struct(_, ref fields, ref base) => {
  74             !has_drop(cx, expr) && fields.iter().all(|field| has_no_effect(cx, &field.expr)) && match *base {
  75                 Some(ref base) => has_no_effect(cx, base),
  76                 None => true,
  77             }
  78         },
  79         ExprKind::Call(ref callee, ref args) => if let ExprKind::Path(ref qpath) = callee.node {
  80             let def = cx.tables.qpath_def(qpath, callee.hir_id);
  81             match def {
  82                 Def::Struct(..) | Def::Variant(..) | Def::StructCtor(..) | Def::VariantCtor(..) => {
  83                     !has_drop(cx, expr) && args.iter().all(|arg| has_no_effect(cx, arg))
  84                 },
  85                 _ => false,
  86             }
  87         } else {
  88             false
  89         },
  90         ExprKind::Block(ref block, _) => {
  91             block.stmts.is_empty() && if let Some(ref expr) = block.expr {
  92                 has_no_effect(cx, expr)
  93             } else {
  94                 false
  95             }
  96         },
  97         _ => false,
  98     }
  99 }
 100
 101 #[derive(Copy, Clone)]
 102 pub struct Pass;
 103
 104 impl LintPass for Pass {
 105     fn get_lints(&self) -> LintArray {
 106         lint_array!(NO_EFFECT, UNNECESSARY_OPERATION)
 107     }
 108 }
 109
 110 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Pass {
 111     fn check_stmt(&mut self, cx: &LateContext<'a, 'tcx>, stmt: &'tcx Stmt) {
 112         if let StmtKind::Semi(ref expr, _) = stmt.node {
 113             if has_no_effect(cx, expr) {
 114                 span_lint(cx, NO_EFFECT, stmt.span, "statement with no effect");
 115             } else if let Some(reduced) = reduce_expression(cx, expr) {
 116                 let mut snippet = String::new();
 117                 for e in reduced {
 118                     if in_macro(e.span) {
 119                         return;
 120                     }
 121                     if let Some(snip) = snippet_opt(cx, e.span) {
 122                         snippet.push_str(&snip);
 123                         snippet.push(';');
 124                     } else {
 125                         return;
 126                     }
 127                 }
 128                 span_lint_and_sugg(
 129                     cx,
 130                     UNNECESSARY_OPERATION,
 131                     stmt.span,
 132                     "statement can be reduced",
 133                     "replace it with",
 134                     snippet,
 135                     Applicability::MachineApplicable,
 136                 );
 137             }
 138         }
 139     }
 140 }
 141
 142
 143 fn reduce_expression<'a>(cx: &LateContext<'_, '_>, expr: &'a Expr) -> Option<Vec<&'a Expr>> {
 144     if in_macro(expr.span) {
 145         return None;
 146     }
 147     match expr.node {
 148         ExprKind::Index(ref a, ref b) => Some(vec![&**a, &**b]),
 149         ExprKind::Binary(ref binop, ref a, ref b) if binop.node != BinOpKind::And && binop.node != BinOpKind::Or => {
 150             Some(vec![&**a, &**b])
 151         },
 152         ExprKind::Array(ref v) | ExprKind::Tup(ref v) => Some(v.iter().collect()),
 153         ExprKind::Repeat(ref inner, _) |
 154         ExprKind::Cast(ref inner, _) |
 155         ExprKind::Type(ref inner, _) |
 156         ExprKind::Unary(_, ref inner) |
 157         ExprKind::Field(ref inner, _) |
 158         ExprKind::AddrOf(_, ref inner) |
 159         ExprKind::Box(ref inner) => reduce_expression(cx, inner).or_else(|| Some(vec![inner])),
 160         ExprKind::Struct(_, ref fields, ref base) => if has_drop(cx, expr) {
 161             None
 162         } else {
 163             Some(
 164                 fields
 165                     .iter()
 166                     .map(|f| &f.expr)
 167                     .chain(base)
 168                     .map(Deref::deref)
 169                     .collect(),
 170             )
 171         },
 172         ExprKind::Call(ref callee, ref args) => if let ExprKind::Path(ref qpath) = callee.node {
 173             let def = cx.tables.qpath_def(qpath, callee.hir_id);
 174             match def {
 175                 Def::Struct(..) | Def::Variant(..) | Def::StructCtor(..) | Def::VariantCtor(..)
 176                     if !has_drop(cx, expr) =>
 177                 {
 178                     Some(args.iter().collect())
 179                 },
 180                 _ => None,
 181             }
 182         } else {
 183             None
 184         },
 185         ExprKind::Block(ref block, _) => {
 186             if block.stmts.is_empty() {
 187                 block.expr.as_ref().and_then(|e| {
 188                     match block.rules {
 189                         BlockCheckMode::UnsafeBlock(UnsafeSource::UserProvided) => None,
 190                         BlockCheckMode::DefaultBlock => Some(vec![&**e]),
 191                         // in case of compiler-inserted signaling blocks
 192                         _ => reduce_expression(cx, e),
 193                     }
 194                 })
 195             } else {
 196                 None
 197             }
 198         },
 199         _ => None,
 200     }
 201 }