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::is_lint_allowed;
   3 use clippy_utils::source::snippet_opt;
   4 use clippy_utils::ty::has_drop;
   5 use rustc_errors::Applicability;
   6 use rustc_hir::def::{DefKind, Res};
   7 use rustc_hir::{is_range_literal, BinOpKind, BlockCheckMode, Expr, ExprKind, PatKind, Stmt, StmtKind, UnsafeSource};
   8 use rustc_lint::{LateContext, LateLintPass};
   9 use rustc_session::{declare_lint_pass, declare_tool_lint};
  10 use std::ops::Deref;
  11
  12 declare_clippy_lint! {
  13     /// ### What it does
  14     /// Checks for statements which have no effect.
  15     ///
  16     /// ### Why is this bad?
  17     /// Unlike dead code, these statements are actually
  18     /// executed. However, as they have no effect, all they do is make the code less
  19     /// readable.
  20     ///
  21     /// ### Example
  22     /// ```rust
  23     /// 0;
  24     /// ```
  25     #[clippy::version = "pre 1.29.0"]
  26     pub NO_EFFECT,
  27     complexity,
  28     "statements with no effect"
  29 }
  30
  31 declare_clippy_lint! {
  32     /// ### What it does
  33     /// Checks for binding to underscore prefixed variable without side-effects.
  34     ///
  35     /// ### Why is this bad?
  36     /// Unlike dead code, these bindings are actually
  37     /// executed. However, as they have no effect and shouldn't be used further on, all they
  38     /// do is make the code less readable.
  39     ///
  40     /// ### Known problems
  41     /// Further usage of this variable is not checked, which can lead to false positives if it is
  42     /// used later in the code.
  43     ///
  44     /// ### Example
  45     /// ```rust,ignore
  46     /// let _i_serve_no_purpose = 1;
  47     /// ```
  48     #[clippy::version = "1.58.0"]
  49     pub NO_EFFECT_UNDERSCORE_BINDING,
  50     pedantic,
  51     "binding to `_` prefixed variable with no side-effect"
  52 }
  53
  54 declare_clippy_lint! {
  55     /// ### What it does
  56     /// Checks for expression statements that can be reduced to a
  57     /// sub-expression.
  58     ///
  59     /// ### Why is this bad?
  60     /// Expressions by themselves often have no side-effects.
  61     /// Having such expressions reduces readability.
  62     ///
  63     /// ### Example
  64     /// ```rust,ignore
  65     /// compute_array()[0];
  66     /// ```
  67     #[clippy::version = "pre 1.29.0"]
  68     pub UNNECESSARY_OPERATION,
  69     complexity,
  70     "outer expressions with no effect"
  71 }
  72
  73 declare_lint_pass!(NoEffect => [NO_EFFECT, UNNECESSARY_OPERATION, NO_EFFECT_UNDERSCORE_BINDING]);
  74
  75 impl<'tcx> LateLintPass<'tcx> for NoEffect {
  76     fn check_stmt(&mut self, cx: &LateContext<'tcx>, stmt: &'tcx Stmt<'_>) {
  77         if check_no_effect(cx, stmt) {
  78             return;
  79         }
  80         check_unnecessary_operation(cx, stmt);
  81     }
  82 }
  83
  84 fn check_no_effect(cx: &LateContext<'tcx>, stmt: &'tcx Stmt<'_>) -> bool {
  85     if let StmtKind::Semi(expr) = stmt.kind {
  86         if has_no_effect(cx, expr) {
  87             span_lint_hir(cx, NO_EFFECT, expr.hir_id, stmt.span, "statement with no effect");
  88             return true;
  89         }
  90     } else if let StmtKind::Local(local) = stmt.kind {
  91         if_chain! {
  92             if !is_lint_allowed(cx, NO_EFFECT_UNDERSCORE_BINDING, local.hir_id);
  93             if let Some(init) = local.init;
  94             if !local.pat.span.from_expansion();
  95             if has_no_effect(cx, init);
  96             if let PatKind::Binding(_, _, ident, _) = local.pat.kind;
  97             if ident.name.to_ident_string().starts_with('_');
  98             then {
  99                 span_lint_hir(
 100                     cx,
 101                     NO_EFFECT_UNDERSCORE_BINDING,
 102                     init.hir_id,
 103                     stmt.span,
 104                     "binding to `_` prefixed variable with no side-effect"
 105                 );
 106                 return true;
 107             }
 108         }
 109     }
 110     false
 111 }
 112
 113 fn has_no_effect(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
 114     if expr.span.from_expansion() {
 115         return false;
 116     }
 117     match expr.kind {
 118         ExprKind::Lit(..) | ExprKind::Closure(..) => true,
 119         ExprKind::Path(..) => !has_drop(cx, cx.typeck_results().expr_ty(expr)),
 120         ExprKind::Index(a, b) | ExprKind::Binary(_, a, b) => has_no_effect(cx, a) && has_no_effect(cx, b),
 121         ExprKind::Array(v) | ExprKind::Tup(v) => v.iter().all(|val| has_no_effect(cx, val)),
 122         ExprKind::Repeat(inner, _)
 123         | ExprKind::Cast(inner, _)
 124         | ExprKind::Type(inner, _)
 125         | ExprKind::Unary(_, inner)
 126         | ExprKind::Field(inner, _)
 127         | ExprKind::AddrOf(_, _, inner)
 128         | ExprKind::Box(inner) => has_no_effect(cx, inner),
 129         ExprKind::Struct(_, fields, ref base) => {
 130             !has_drop(cx, cx.typeck_results().expr_ty(expr))
 131                 && fields.iter().all(|field| has_no_effect(cx, field.expr))
 132                 && base.as_ref().map_or(true, |base| has_no_effect(cx, base))
 133         },
 134         ExprKind::Call(callee, args) => {
 135             if let ExprKind::Path(ref qpath) = callee.kind {
 136                 if cx.typeck_results().type_dependent_def(expr.hir_id).is_some() {
 137                     // type-dependent function call like `impl FnOnce for X`
 138                     return false;
 139                 }
 140                 let def_matched = matches!(
 141                     cx.qpath_res(qpath, callee.hir_id),
 142                     Res::Def(DefKind::Struct | DefKind::Variant | DefKind::Ctor(..), ..)
 143                 );
 144                 if def_matched || is_range_literal(expr) {
 145                     !has_drop(cx, cx.typeck_results().expr_ty(expr)) && args.iter().all(|arg| has_no_effect(cx, arg))
 146                 } else {
 147                     false
 148                 }
 149             } else {
 150                 false
 151             }
 152         },
 153         ExprKind::Block(block, _) => {
 154             block.stmts.is_empty() && block.expr.as_ref().map_or(false, |expr| has_no_effect(cx, expr))
 155         },
 156         _ => false,
 157     }
 158 }
 159
 160 fn check_unnecessary_operation(cx: &LateContext<'tcx>, stmt: &'tcx Stmt<'_>) {
 161     if_chain! {
 162         if let StmtKind::Semi(expr) = stmt.kind;
 163         if let Some(reduced) = reduce_expression(cx, expr);
 164         if !&reduced.iter().any(|e| e.span.from_expansion());
 165         then {
 166             if let ExprKind::Index(..) = &expr.kind {
 167                 let snippet = if let (Some(arr), Some(func)) =
 168                     (snippet_opt(cx, reduced[0].span), snippet_opt(cx, reduced[1].span))
 169                 {
 170                     format!("assert!({}.len() > {});", &arr, &func)
 171                 } else {
 172                     return;
 173                 };
 174                 span_lint_hir_and_then(
 175                     cx,
 176                     UNNECESSARY_OPERATION,
 177                     expr.hir_id,
 178                     stmt.span,
 179                     "unnecessary operation",
 180                     |diag| {
 181                         diag.span_suggestion(
 182                             stmt.span,
 183                             "statement can be written as",
 184                             snippet,
 185                             Applicability::MaybeIncorrect,
 186                         );
 187                     },
 188                 );
 189             } else {
 190                 let mut snippet = String::new();
 191                 for e in reduced {
 192                     if let Some(snip) = snippet_opt(cx, e.span) {
 193                         snippet.push_str(&snip);
 194                         snippet.push(';');
 195                     } else {
 196                         return;
 197                     }
 198                 }
 199                 span_lint_hir_and_then(
 200                     cx,
 201                     UNNECESSARY_OPERATION,
 202                     expr.hir_id,
 203                     stmt.span,
 204                     "unnecessary operation",
 205                     |diag| {
 206                         diag.span_suggestion(
 207                             stmt.span,
 208                             "statement can be reduced to",
 209                             snippet,
 210                             Applicability::MachineApplicable,
 211                         );
 212                     },
 213                 );
 214             }
 215         }
 216     }
 217 }
 218
 219 fn reduce_expression<'a>(cx: &LateContext<'_>, expr: &'a Expr<'a>) -> Option<Vec<&'a Expr<'a>>> {
 220     if expr.span.from_expansion() {
 221         return None;
 222     }
 223     match expr.kind {
 224         ExprKind::Index(a, b) => Some(vec![a, b]),
 225         ExprKind::Binary(ref binop, a, b) if binop.node != BinOpKind::And && binop.node != BinOpKind::Or => {
 226             Some(vec![a, b])
 227         },
 228         ExprKind::Array(v) | ExprKind::Tup(v) => Some(v.iter().collect()),
 229         ExprKind::Repeat(inner, _)
 230         | ExprKind::Cast(inner, _)
 231         | ExprKind::Type(inner, _)
 232         | ExprKind::Unary(_, inner)
 233         | ExprKind::Field(inner, _)
 234         | ExprKind::AddrOf(_, _, inner)
 235         | ExprKind::Box(inner) => reduce_expression(cx, inner).or_else(|| Some(vec![inner])),
 236         ExprKind::Struct(_, fields, ref base) => {
 237             if has_drop(cx, cx.typeck_results().expr_ty(expr)) {
 238                 None
 239             } else {
 240                 Some(fields.iter().map(|f| &f.expr).chain(base).map(Deref::deref).collect())
 241             }
 242         },
 243         ExprKind::Call(callee, args) => {
 244             if let ExprKind::Path(ref qpath) = callee.kind {
 245                 if cx.typeck_results().type_dependent_def(expr.hir_id).is_some() {
 246                     // type-dependent function call like `impl FnOnce for X`
 247                     return None;
 248                 }
 249                 let res = cx.qpath_res(qpath, callee.hir_id);
 250                 match res {
 251                     Res::Def(DefKind::Struct | DefKind::Variant | DefKind::Ctor(..), ..)
 252                         if !has_drop(cx, cx.typeck_results().expr_ty(expr)) =>
 253                     {
 254                         Some(args.iter().collect())
 255                     },
 256                     _ => None,
 257                 }
 258             } else {
 259                 None
 260             }
 261         },
 262         ExprKind::Block(block, _) => {
 263             if block.stmts.is_empty() {
 264                 block.expr.as_ref().and_then(|e| {
 265                     match block.rules {
 266                         BlockCheckMode::UnsafeBlock(UnsafeSource::UserProvided) => None,
 267                         BlockCheckMode::DefaultBlock => Some(vec![&**e]),
 268                         // in case of compiler-inserted signaling blocks
 269                         BlockCheckMode::UnsafeBlock(_) => reduce_expression(cx, e),
 270                     }
 271                 })
 272             } else {
 273                 None
 274             }
 275         },
 276         _ => None,
 277     }
 278 }