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