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::lint::{LateContext, LateLintPass, LintArray, LintPass};
  12 use crate::rustc::{declare_tool_lint, lint_array};
  13 use crate::rustc::hir::def::Def;
  14 use crate::rustc::hir::{BinOpKind, BlockCheckMode, Expr, ExprKind, Stmt, StmtKind, UnsafeSource};
  15 use crate::utils::{has_drop, in_macro, snippet_opt, span_lint, span_lint_and_sugg};
  16 use std::ops::Deref;
  17
  18 /// **What it does:** Checks for statements which have no effect.
  19 ///
  20 /// **Why is this bad?** Similar to dead code, these statements are actually
  21 /// executed. However, as they have no effect, all they do is make the code less
  22 /// readable.
  23 ///
  24 /// **Known problems:** None.
  25 ///
  26 /// **Example:**
  27 /// ```rust
  28 /// 0;
  29 /// ```
  30 declare_clippy_lint! {
  31     pub NO_EFFECT,
  32     complexity,
  33     "statements with no effect"
  34 }
  35
  36 /// **What it does:** Checks for expression statements that can be reduced to a
  37 /// sub-expression.
  38 ///
  39 /// **Why is this bad?** Expressions by themselves often have no side-effects.
  40 /// Having such expressions reduces readability.
  41 ///
  42 /// **Known problems:** None.
  43 ///
  44 /// **Example:**
  45 /// ```rust
  46 /// compute_array()[0];
  47 /// ```
  48 declare_clippy_lint! {
  49     pub UNNECESSARY_OPERATION,
  50     complexity,
  51     "outer expressions with no effect"
  52 }
  53
  54 fn has_no_effect(cx: &LateContext<'_, '_>, expr: &Expr) -> bool {
  55     if in_macro(expr.span) {
  56         return false;
  57     }
  58     match expr.node {
  59         ExprKind::Lit(..) | ExprKind::Closure(.., _) => true,
  60         ExprKind::Path(..) => !has_drop(cx, expr),
  61         ExprKind::Index(ref a, ref b) | ExprKind::Binary(_, ref a, ref b) => {
  62             has_no_effect(cx, a) && has_no_effect(cx, b)
  63         },
  64         ExprKind::Array(ref v) | ExprKind::Tup(ref v) => v.iter().all(|val| has_no_effect(cx, val)),
  65         ExprKind::Repeat(ref inner, _) |
  66         ExprKind::Cast(ref inner, _) |
  67         ExprKind::Type(ref inner, _) |
  68         ExprKind::Unary(_, ref inner) |
  69         ExprKind::Field(ref inner, _) |
  70         ExprKind::AddrOf(_, ref inner) |
  71         ExprKind::Box(ref inner) => has_no_effect(cx, inner),
  72         ExprKind::Struct(_, ref fields, ref base) => {
  73             !has_drop(cx, expr) && fields.iter().all(|field| has_no_effect(cx, &field.expr)) && match *base {
  74                 Some(ref base) => has_no_effect(cx, base),
  75                 None => true,
  76             }
  77         },
  78         ExprKind::Call(ref callee, ref args) => if let ExprKind::Path(ref qpath) = callee.node {
  79             let def = cx.tables.qpath_def(qpath, callee.hir_id);
  80             match def {
  81                 Def::Struct(..) | Def::Variant(..) | Def::StructCtor(..) | Def::VariantCtor(..) => {
  82                     !has_drop(cx, expr) && args.iter().all(|arg| has_no_effect(cx, arg))
  83                 },
  84                 _ => false,
  85             }
  86         } else {
  87             false
  88         },
  89         ExprKind::Block(ref block, _) => {
  90             block.stmts.is_empty() && if let Some(ref expr) = block.expr {
  91                 has_no_effect(cx, expr)
  92             } else {
  93                 false
  94             }
  95         },
  96         _ => false,
  97     }
  98 }
  99
 100 #[derive(Copy, Clone)]
 101 pub struct Pass;
 102
 103 impl LintPass for Pass {
 104     fn get_lints(&self) -> LintArray {
 105         lint_array!(NO_EFFECT, UNNECESSARY_OPERATION)
 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                 );
 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) => if has_drop(cx, expr) {
 159             None
 160         } else {
 161             Some(
 162                 fields
 163                     .iter()
 164                     .map(|f| &f.expr)
 165                     .chain(base)
 166                     .map(Deref::deref)
 167                     .collect(),
 168             )
 169         },
 170         ExprKind::Call(ref callee, ref args) => if let ExprKind::Path(ref qpath) = callee.node {
 171             let def = cx.tables.qpath_def(qpath, callee.hir_id);
 172             match def {
 173                 Def::Struct(..) | Def::Variant(..) | Def::StructCtor(..) | Def::VariantCtor(..)
 174                     if !has_drop(cx, expr) =>
 175                 {
 176                     Some(args.iter().collect())
 177                 },
 178                 _ => None,
 179             }
 180         } else {
 181             None
 182         },
 183         ExprKind::Block(ref block, _) => {
 184             if block.stmts.is_empty() {
 185                 block.expr.as_ref().and_then(|e| {
 186                     match block.rules {
 187                         BlockCheckMode::UnsafeBlock(UnsafeSource::UserProvided) => None,
 188                         BlockCheckMode::DefaultBlock => Some(vec![&**e]),
 189                         // in case of compiler-inserted signaling blocks
 190                         _ => reduce_expression(cx, e),
 191                     }
 192                 })
 193             } else {
 194                 None
 195             }
 196         },
 197         _ => None,
 198     }
 199 }