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, LintContext};
  10 use rustc_middle::lint::in_external_macro;
  11 use rustc_session::{declare_lint_pass, declare_tool_lint};
  12 use std::ops::Deref;
  13
  14 declare_clippy_lint! {
  15     /// ### What it does
  16     /// Checks for statements which have no effect.
  17     ///
  18     /// ### Why is this bad?
  19     /// Unlike dead code, these statements are actually
  20     /// executed. However, as they have no effect, all they do is make the code less
  21     /// readable.
  22     ///
  23     /// ### Example
  24     /// ```rust
  25     /// 0;
  26     /// ```
  27     #[clippy::version = "pre 1.29.0"]
  28     pub NO_EFFECT,
  29     complexity,
  30     "statements with no effect"
  31 }
  32
  33 declare_clippy_lint! {
  34     /// ### What it does
  35     /// Checks for binding to underscore prefixed variable without side-effects.
  36     ///
  37     /// ### Why is this bad?
  38     /// Unlike dead code, these bindings are actually
  39     /// executed. However, as they have no effect and shouldn't be used further on, all they
  40     /// do is make the code less readable.
  41     ///
  42     /// ### Known problems
  43     /// Further usage of this variable is not checked, which can lead to false positives if it is
  44     /// used later in the code.
  45     ///
  46     /// ### Example
  47     /// ```rust,ignore
  48     /// let _i_serve_no_purpose = 1;
  49     /// ```
  50     #[clippy::version = "1.58.0"]
  51     pub NO_EFFECT_UNDERSCORE_BINDING,
  52     pedantic,
  53     "binding to `_` prefixed variable with no side-effect"
  54 }
  55
  56 declare_clippy_lint! {
  57     /// ### What it does
  58     /// Checks for expression statements that can be reduced to a
  59     /// sub-expression.
  60     ///
  61     /// ### Why is this bad?
  62     /// Expressions by themselves often have no side-effects.
  63     /// Having such expressions reduces readability.
  64     ///
  65     /// ### Example
  66     /// ```rust,ignore
  67     /// compute_array()[0];
  68     /// ```
  69     #[clippy::version = "pre 1.29.0"]
  70     pub UNNECESSARY_OPERATION,
  71     complexity,
  72     "outer expressions with no effect"
  73 }
  74
  75 declare_lint_pass!(NoEffect => [NO_EFFECT, UNNECESSARY_OPERATION, NO_EFFECT_UNDERSCORE_BINDING]);
  76
  77 impl<'tcx> LateLintPass<'tcx> for NoEffect {
  78     fn check_stmt(&mut self, cx: &LateContext<'tcx>, stmt: &'tcx Stmt<'_>) {
  79         if check_no_effect(cx, stmt) {
  80             return;
  81         }
  82         check_unnecessary_operation(cx, stmt);
  83     }
  84 }
  85
  86 fn check_no_effect(cx: &LateContext<'_>, stmt: &Stmt<'_>) -> bool {
  87     if let StmtKind::Semi(expr) = stmt.kind {
  88         if has_no_effect(cx, expr) {
  89             span_lint_hir(cx, NO_EFFECT, expr.hir_id, stmt.span, "statement with no effect");
  90             return true;
  91         }
  92     } else if let StmtKind::Local(local) = stmt.kind {
  93         if_chain! {
  94             if !is_lint_allowed(cx, NO_EFFECT_UNDERSCORE_BINDING, local.hir_id);
  95             if let Some(init) = local.init;
  96             if local.els.is_none();
  97             if !local.pat.span.from_expansion();
  98             if has_no_effect(cx, init);
  99             if let PatKind::Binding(_, _, ident, _) = local.pat.kind;
 100             if ident.name.to_ident_string().starts_with('_');
 101             then {
 102                 span_lint_hir(
 103                     cx,
 104                     NO_EFFECT_UNDERSCORE_BINDING,
 105                     init.hir_id,
 106                     stmt.span,
 107                     "binding to `_` prefixed variable with no side-effect"
 108                 );
 109                 return true;
 110             }
 111         }
 112     }
 113     false
 114 }
 115
 116 fn has_no_effect(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
 117     if expr.span.from_expansion() {
 118         return false;
 119     }
 120     match peel_blocks(expr).kind {
 121         ExprKind::Lit(..) | ExprKind::Closure { .. } => true,
 122         ExprKind::Path(..) => !has_drop(cx, cx.typeck_results().expr_ty(expr)),
 123         ExprKind::Index(a, b) | ExprKind::Binary(_, a, b) => has_no_effect(cx, a) && has_no_effect(cx, b),
 124         ExprKind::Array(v) | ExprKind::Tup(v) => v.iter().all(|val| has_no_effect(cx, val)),
 125         ExprKind::Repeat(inner, _)
 126         | ExprKind::Cast(inner, _)
 127         | ExprKind::Type(inner, _)
 128         | ExprKind::Unary(_, inner)
 129         | ExprKind::Field(inner, _)
 130         | ExprKind::AddrOf(_, _, inner)
 131         | ExprKind::Box(inner) => has_no_effect(cx, inner),
 132         ExprKind::Struct(_, fields, ref base) => {
 133             !has_drop(cx, cx.typeck_results().expr_ty(expr))
 134                 && fields.iter().all(|field| has_no_effect(cx, field.expr))
 135                 && base.as_ref().map_or(true, |base| has_no_effect(cx, base))
 136         },
 137         ExprKind::Call(callee, args) => {
 138             if let ExprKind::Path(ref qpath) = callee.kind {
 139                 if cx.typeck_results().type_dependent_def(expr.hir_id).is_some() {
 140                     // type-dependent function call like `impl FnOnce for X`
 141                     return false;
 142                 }
 143                 let def_matched = matches!(
 144                     cx.qpath_res(qpath, callee.hir_id),
 145                     Res::Def(DefKind::Struct | DefKind::Variant | DefKind::Ctor(..), ..)
 146                 );
 147                 if def_matched || is_range_literal(expr) {
 148                     !has_drop(cx, cx.typeck_results().expr_ty(expr)) && args.iter().all(|arg| has_no_effect(cx, arg))
 149                 } else {
 150                     false
 151                 }
 152             } else {
 153                 false
 154             }
 155         },
 156         _ => false,
 157     }
 158 }
 159
 160 fn check_unnecessary_operation(cx: &LateContext<'_>, stmt: &Stmt<'_>) {
 161     if_chain! {
 162         if let StmtKind::Semi(expr) = stmt.kind;
 163         let ctxt = stmt.span.ctxt();
 164         if expr.span.ctxt() == ctxt;
 165         if let Some(reduced) = reduce_expression(cx, expr);
 166         if !in_external_macro(cx.sess(), stmt.span);
 167         if reduced.iter().all(|e| e.span.ctxt() == ctxt);
 168         then {
 169             if let ExprKind::Index(..) = &expr.kind {
 170                 let snippet = if let (Some(arr), Some(func)) =
 171                     (snippet_opt(cx, reduced[0].span), snippet_opt(cx, reduced[1].span))
 172                 {
 173                     format!("assert!({}.len() > {});", &arr, &func)
 174                 } else {
 175                     return;
 176                 };
 177                 span_lint_hir_and_then(
 178                     cx,
 179                     UNNECESSARY_OPERATION,
 180                     expr.hir_id,
 181                     stmt.span,
 182                     "unnecessary operation",
 183                     |diag| {
 184                         diag.span_suggestion(
 185                             stmt.span,
 186                             "statement can be written as",
 187                             snippet,
 188                             Applicability::MaybeIncorrect,
 189                         );
 190                     },
 191                 );
 192             } else {
 193                 let mut snippet = String::new();
 194                 for e in reduced {
 195                     if let Some(snip) = snippet_opt(cx, e.span) {
 196                         snippet.push_str(&snip);
 197                         snippet.push(';');
 198                     } else {
 199                         return;
 200                     }
 201                 }
 202                 span_lint_hir_and_then(
 203                     cx,
 204                     UNNECESSARY_OPERATION,
 205                     expr.hir_id,
 206                     stmt.span,
 207                     "unnecessary operation",
 208                     |diag| {
 209                         diag.span_suggestion(
 210                             stmt.span,
 211                             "statement can be reduced to",
 212                             snippet,
 213                             Applicability::MachineApplicable,
 214                         );
 215                     },
 216                 );
 217             }
 218         }
 219     }
 220 }
 221
 222 fn reduce_expression<'a>(cx: &LateContext<'_>, expr: &'a Expr<'a>) -> Option<Vec<&'a Expr<'a>>> {
 223     if expr.span.from_expansion() {
 224         return None;
 225     }
 226     match expr.kind {
 227         ExprKind::Index(a, b) => Some(vec![a, b]),
 228         ExprKind::Binary(ref binop, a, b) if binop.node != BinOpKind::And && binop.node != BinOpKind::Or => {
 229             Some(vec![a, b])
 230         },
 231         ExprKind::Array(v) | ExprKind::Tup(v) => Some(v.iter().collect()),
 232         ExprKind::Repeat(inner, _)
 233         | ExprKind::Cast(inner, _)
 234         | ExprKind::Type(inner, _)
 235         | ExprKind::Unary(_, inner)
 236         | ExprKind::Field(inner, _)
 237         | ExprKind::AddrOf(_, _, inner)
 238         | ExprKind::Box(inner) => reduce_expression(cx, inner).or_else(|| Some(vec![inner])),
 239         ExprKind::Struct(_, fields, ref base) => {
 240             if has_drop(cx, cx.typeck_results().expr_ty(expr)) {
 241                 None
 242             } else {
 243                 Some(fields.iter().map(|f| &f.expr).chain(base).map(Deref::deref).collect())
 244             }
 245         },
 246         ExprKind::Call(callee, args) => {
 247             if let ExprKind::Path(ref qpath) = callee.kind {
 248                 if cx.typeck_results().type_dependent_def(expr.hir_id).is_some() {
 249                     // type-dependent function call like `impl FnOnce for X`
 250                     return None;
 251                 }
 252                 let res = cx.qpath_res(qpath, callee.hir_id);
 253                 match res {
 254                     Res::Def(DefKind::Struct | DefKind::Variant | DefKind::Ctor(..), ..)
 255                         if !has_drop(cx, cx.typeck_results().expr_ty(expr)) =>
 256                     {
 257                         Some(args.iter().collect())
 258                     },
 259                     _ => None,
 260                 }
 261             } else {
 262                 None
 263             }
 264         },
 265         ExprKind::Block(block, _) => {
 266             if block.stmts.is_empty() {
 267                 block.expr.as_ref().and_then(|e| {
 268                     match block.rules {
 269                         BlockCheckMode::UnsafeBlock(UnsafeSource::UserProvided) => None,
 270                         BlockCheckMode::DefaultBlock => Some(vec![&**e]),
 271                         // in case of compiler-inserted signaling blocks
 272                         BlockCheckMode::UnsafeBlock(_) => reduce_expression(cx, e),
 273                     }
 274                 })
 275             } else {
 276                 None
 277             }
 278         },
 279         _ => None,
 280     }
 281 }