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::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.pat.span.from_expansion();
  96             if has_no_effect(cx, init);
  97             if let PatKind::Binding(_, _, ident, _) = local.pat.kind;
  98             if ident.name.to_ident_string().starts_with('_');
  99             then {
 100                 span_lint_hir(
 101                     cx,
 102                     NO_EFFECT_UNDERSCORE_BINDING,
 103                     init.hir_id,
 104                     stmt.span,
 105                     "binding to `_` prefixed variable with no side-effect"
 106                 );
 107                 return true;
 108             }
 109         }
 110     }
 111     false
 112 }
 113
 114 fn has_no_effect(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
 115     if expr.span.from_expansion() {
 116         return false;
 117     }
 118     match peel_blocks(expr).kind {
 119         ExprKind::Lit(..) | ExprKind::Closure(..) => true,
 120         ExprKind::Path(..) => !has_drop(cx, cx.typeck_results().expr_ty(expr)),
 121         ExprKind::Index(a, b) | ExprKind::Binary(_, a, b) => has_no_effect(cx, a) && has_no_effect(cx, b),
 122         ExprKind::Array(v) | ExprKind::Tup(v) => v.iter().all(|val| has_no_effect(cx, val)),
 123         ExprKind::Repeat(inner, _)
 124         | ExprKind::Cast(inner, _)
 125         | ExprKind::Type(inner, _)
 126         | ExprKind::Unary(_, inner)
 127         | ExprKind::Field(inner, _)
 128         | ExprKind::AddrOf(_, _, inner)
 129         | ExprKind::Box(inner) => has_no_effect(cx, inner),
 130         ExprKind::Struct(_, fields, ref base) => {
 131             !has_drop(cx, cx.typeck_results().expr_ty(expr))
 132                 && fields.iter().all(|field| has_no_effect(cx, field.expr))
 133                 && base.as_ref().map_or(true, |base| has_no_effect(cx, base))
 134         },
 135         ExprKind::Call(callee, args) => {
 136             if let ExprKind::Path(ref qpath) = callee.kind {
 137                 if cx.typeck_results().type_dependent_def(expr.hir_id).is_some() {
 138                     // type-dependent function call like `impl FnOnce for X`
 139                     return false;
 140                 }
 141                 let def_matched = matches!(
 142                     cx.qpath_res(qpath, callee.hir_id),
 143                     Res::Def(DefKind::Struct | DefKind::Variant | DefKind::Ctor(..), ..)
 144                 );
 145                 if def_matched || is_range_literal(expr) {
 146                     !has_drop(cx, cx.typeck_results().expr_ty(expr)) && args.iter().all(|arg| has_no_effect(cx, arg))
 147                 } else {
 148                     false
 149                 }
 150             } else {
 151                 false
 152             }
 153         },
 154         _ => false,
 155     }
 156 }
 157
 158 fn check_unnecessary_operation(cx: &LateContext<'_>, stmt: &Stmt<'_>) {
 159     if_chain! {
 160         if let StmtKind::Semi(expr) = stmt.kind;
 161         if let Some(reduced) = reduce_expression(cx, expr);
 162         if !&reduced.iter().any(|e| e.span.from_expansion());
 163         then {
 164             if let ExprKind::Index(..) = &expr.kind {
 165                 let snippet = if let (Some(arr), Some(func)) =
 166                     (snippet_opt(cx, reduced[0].span), snippet_opt(cx, reduced[1].span))
 167                 {
 168                     format!("assert!({}.len() > {});", &arr, &func)
 169                 } else {
 170                     return;
 171                 };
 172                 span_lint_hir_and_then(
 173                     cx,
 174                     UNNECESSARY_OPERATION,
 175                     expr.hir_id,
 176                     stmt.span,
 177                     "unnecessary operation",
 178                     |diag| {
 179                         diag.span_suggestion(
 180                             stmt.span,
 181                             "statement can be written as",
 182                             snippet,
 183                             Applicability::MaybeIncorrect,
 184                         );
 185                     },
 186                 );
 187             } else {
 188                 let mut snippet = String::new();
 189                 for e in reduced {
 190                     if let Some(snip) = snippet_opt(cx, e.span) {
 191                         snippet.push_str(&snip);
 192                         snippet.push(';');
 193                     } else {
 194                         return;
 195                     }
 196                 }
 197                 span_lint_hir_and_then(
 198                     cx,
 199                     UNNECESSARY_OPERATION,
 200                     expr.hir_id,
 201                     stmt.span,
 202                     "unnecessary operation",
 203                     |diag| {
 204                         diag.span_suggestion(
 205                             stmt.span,
 206                             "statement can be reduced to",
 207                             snippet,
 208                             Applicability::MachineApplicable,
 209                         );
 210                     },
 211                 );
 212             }
 213         }
 214     }
 215 }
 216
 217 fn reduce_expression<'a>(cx: &LateContext<'_>, expr: &'a Expr<'a>) -> Option<Vec<&'a Expr<'a>>> {
 218     if expr.span.from_expansion() {
 219         return None;
 220     }
 221     match expr.kind {
 222         ExprKind::Index(a, b) => Some(vec![a, b]),
 223         ExprKind::Binary(ref binop, a, b) if binop.node != BinOpKind::And && binop.node != BinOpKind::Or => {
 224             Some(vec![a, b])
 225         },
 226         ExprKind::Array(v) | ExprKind::Tup(v) => Some(v.iter().collect()),
 227         ExprKind::Repeat(inner, _)
 228         | ExprKind::Cast(inner, _)
 229         | ExprKind::Type(inner, _)
 230         | ExprKind::Unary(_, inner)
 231         | ExprKind::Field(inner, _)
 232         | ExprKind::AddrOf(_, _, inner)
 233         | ExprKind::Box(inner) => reduce_expression(cx, inner).or_else(|| Some(vec![inner])),
 234         ExprKind::Struct(_, fields, ref base) => {
 235             if has_drop(cx, cx.typeck_results().expr_ty(expr)) {
 236                 None
 237             } else {
 238                 Some(fields.iter().map(|f| &f.expr).chain(base).map(Deref::deref).collect())
 239             }
 240         },
 241         ExprKind::Call(callee, args) => {
 242             if let ExprKind::Path(ref qpath) = callee.kind {
 243                 if cx.typeck_results().type_dependent_def(expr.hir_id).is_some() {
 244                     // type-dependent function call like `impl FnOnce for X`
 245                     return None;
 246                 }
 247                 let res = cx.qpath_res(qpath, callee.hir_id);
 248                 match res {
 249                     Res::Def(DefKind::Struct | DefKind::Variant | DefKind::Ctor(..), ..)
 250                         if !has_drop(cx, cx.typeck_results().expr_ty(expr)) =>
 251                     {
 252                         Some(args.iter().collect())
 253                     },
 254                     _ => None,
 255                 }
 256             } else {
 257                 None
 258             }
 259         },
 260         ExprKind::Block(block, _) => {
 261             if block.stmts.is_empty() {
 262                 block.expr.as_ref().and_then(|e| {
 263                     match block.rules {
 264                         BlockCheckMode::UnsafeBlock(UnsafeSource::UserProvided) => None,
 265                         BlockCheckMode::DefaultBlock => Some(vec![&**e]),
 266                         // in case of compiler-inserted signaling blocks
 267                         BlockCheckMode::UnsafeBlock(_) => reduce_expression(cx, e),
 268                     }
 269                 })
 270             } else {
 271                 None
 272             }
 273         },
 274         _ => None,
 275     }
 276 }