src/tools/clippy/clippy_lints/src/misc.rs

   1 use reexport::*;
   2 use rustc::hir::*;
   3 use rustc::hir::intravisit::FnKind;
   4 use rustc::lint::*;
   5 use rustc::ty;
   6 use syntax::codemap::{ExpnFormat, Span};
   7 use utils::{get_item_name, get_parent_expr, implements_trait, in_constant, in_macro, is_integer_literal,
   8             iter_input_pats, last_path_segment, match_qpath, match_trait_method, paths, snippet, span_lint,
   9             span_lint_and_then, walk_ptrs_ty};
  10 use utils::sugg::Sugg;
  11 use syntax::ast::{LitKind, CRATE_NODE_ID};
  12 use consts::{constant, Constant};
  13
  14 /// **What it does:** Checks for function arguments and let bindings denoted as
  15 /// `ref`.
  16 ///
  17 /// **Why is this bad?** The `ref` declaration makes the function take an owned
  18 /// value, but turns the argument into a reference (which means that the value
  19 /// is destroyed when exiting the function). This adds not much value: either
  20 /// take a reference type, or take an owned value and create references in the
  21 /// body.
  22 ///
  23 /// For let bindings, `let x = &foo;` is preferred over `let ref x = foo`. The
  24 /// type of `x` is more obvious with the former.
  25 ///
  26 /// **Known problems:** If the argument is dereferenced within the function,
  27 /// removing the `ref` will lead to errors. This can be fixed by removing the
  28 /// dereferences, e.g. changing `*x` to `x` within the function.
  29 ///
  30 /// **Example:**
  31 /// ```rust
  32 /// fn foo(ref x: u8) -> bool { .. }
  33 /// ```
  34 declare_clippy_lint! {
  35     pub TOPLEVEL_REF_ARG,
  36     style,
  37     "an entire binding declared as `ref`, in a function argument or a `let` statement"
  38 }
  39
  40 /// **What it does:** Checks for comparisons to NaN.
  41 ///
  42 /// **Why is this bad?** NaN does not compare meaningfully to anything – not
  43 /// even itself – so those comparisons are simply wrong.
  44 ///
  45 /// **Known problems:** None.
  46 ///
  47 /// **Example:**
  48 /// ```rust
  49 /// x == NAN
  50 /// ```
  51 declare_clippy_lint! {
  52     pub CMP_NAN,
  53     correctness,
  54     "comparisons to NAN, which will always return false, probably not intended"
  55 }
  56
  57 /// **What it does:** Checks for (in-)equality comparisons on floating-point
  58 /// values (apart from zero), except in functions called `*eq*` (which probably
  59 /// implement equality for a type involving floats).
  60 ///
  61 /// **Why is this bad?** Floating point calculations are usually imprecise, so
  62 /// asking if two values are *exactly* equal is asking for trouble. For a good
  63 /// guide on what to do, see [the floating point
  64 /// guide](http://www.floating-point-gui.de/errors/comparison).
  65 ///
  66 /// **Known problems:** None.
  67 ///
  68 /// **Example:**
  69 /// ```rust
  70 /// y == 1.23f64
  71 /// y != x  // where both are floats
  72 /// ```
  73 declare_clippy_lint! {
  74     pub FLOAT_CMP,
  75     correctness,
  76     "using `==` or `!=` on float values instead of comparing difference with an epsilon"
  77 }
  78
  79 /// **What it does:** Checks for conversions to owned values just for the sake
  80 /// of a comparison.
  81 ///
  82 /// **Why is this bad?** The comparison can operate on a reference, so creating
  83 /// an owned value effectively throws it away directly afterwards, which is
  84 /// needlessly consuming code and heap space.
  85 ///
  86 /// **Known problems:** None.
  87 ///
  88 /// **Example:**
  89 /// ```rust
  90 /// x.to_owned() == y
  91 /// ```
  92 declare_clippy_lint! {
  93     pub CMP_OWNED,
  94     perf,
  95     "creating owned instances for comparing with others, e.g. `x == \"foo\".to_string()`"
  96 }
  97
  98 /// **What it does:** Checks for getting the remainder of a division by one.
  99 ///
 100 /// **Why is this bad?** The result can only ever be zero. No one will write
 101 /// such code deliberately, unless trying to win an Underhanded Rust
 102 /// Contest. Even for that contest, it's probably a bad idea. Use something more
 103 /// underhanded.
 104 ///
 105 /// **Known problems:** None.
 106 ///
 107 /// **Example:**
 108 /// ```rust
 109 /// x % 1
 110 /// ```
 111 declare_clippy_lint! {
 112     pub MODULO_ONE,
 113     correctness,
 114     "taking a number modulo 1, which always returns 0"
 115 }
 116
 117 /// **What it does:** Checks for patterns in the form `name @ _`.
 118 ///
 119 /// **Why is this bad?** It's almost always more readable to just use direct
 120 /// bindings.
 121 ///
 122 /// **Known problems:** None.
 123 ///
 124 /// **Example:**
 125 /// ```rust
 126 /// match v {
 127 ///     Some(x) => (),
 128 ///     y @ _   => (), // easier written as `y`,
 129 /// }
 130 /// ```
 131 declare_clippy_lint! {
 132     pub REDUNDANT_PATTERN,
 133     style,
 134     "using `name @ _` in a pattern"
 135 }
 136
 137 /// **What it does:** Checks for the use of bindings with a single leading
 138 /// underscore.
 139 ///
 140 /// **Why is this bad?** A single leading underscore is usually used to indicate
 141 /// that a binding will not be used. Using such a binding breaks this
 142 /// expectation.
 143 ///
 144 /// **Known problems:** The lint does not work properly with desugaring and
 145 /// macro, it has been allowed in the mean time.
 146 ///
 147 /// **Example:**
 148 /// ```rust
 149 /// let _x = 0;
 150 /// let y = _x + 1; // Here we are using `_x`, even though it has a leading
 151 ///                 // underscore. We should rename `_x` to `x`
 152 /// ```
 153 declare_clippy_lint! {
 154     pub USED_UNDERSCORE_BINDING,
 155     pedantic,
 156     "using a binding which is prefixed with an underscore"
 157 }
 158
 159 /// **What it does:** Checks for the use of short circuit boolean conditions as
 160 /// a
 161 /// statement.
 162 ///
 163 /// **Why is this bad?** Using a short circuit boolean condition as a statement
 164 /// may hide the fact that the second part is executed or not depending on the
 165 /// outcome of the first part.
 166 ///
 167 /// **Known problems:** None.
 168 ///
 169 /// **Example:**
 170 /// ```rust
 171 /// f() && g();  // We should write `if f() { g(); }`.
 172 /// ```
 173 declare_clippy_lint! {
 174     pub SHORT_CIRCUIT_STATEMENT,
 175     complexity,
 176     "using a short circuit boolean condition as a statement"
 177 }
 178
 179 /// **What it does:** Catch casts from `0` to some pointer type
 180 ///
 181 /// **Why is this bad?** This generally means `null` and is better expressed as
 182 /// {`std`, `core`}`::ptr::`{`null`, `null_mut`}.
 183 ///
 184 /// **Known problems:** None.
 185 ///
 186 /// **Example:**
 187 ///
 188 /// ```rust
 189 /// 0 as *const u32
 190 /// ```
 191 declare_clippy_lint! {
 192     pub ZERO_PTR,
 193     style,
 194     "using 0 as *{const, mut} T"
 195 }
 196
 197 /// **What it does:** Checks for (in-)equality comparisons on floating-point
 198 /// value and constant, except in functions called `*eq*` (which probably
 199 /// implement equality for a type involving floats).
 200 ///
 201 /// **Why is this bad?** Floating point calculations are usually imprecise, so
 202 /// asking if two values are *exactly* equal is asking for trouble. For a good
 203 /// guide on what to do, see [the floating point
 204 /// guide](http://www.floating-point-gui.de/errors/comparison).
 205 ///
 206 /// **Known problems:** None.
 207 ///
 208 /// **Example:**
 209 /// ```rust
 210 /// const ONE == 1.00f64
 211 /// x == ONE  // where both are floats
 212 /// ```
 213 declare_clippy_lint! {
 214     pub FLOAT_CMP_CONST,
 215     restriction,
 216     "using `==` or `!=` on float constants instead of comparing difference with an epsilon"
 217 }
 218
 219 #[derive(Copy, Clone)]
 220 pub struct Pass;
 221
 222 impl LintPass for Pass {
 223     fn get_lints(&self) -> LintArray {
 224         lint_array!(
 225             TOPLEVEL_REF_ARG,
 226             CMP_NAN,
 227             FLOAT_CMP,
 228             CMP_OWNED,
 229             MODULO_ONE,
 230             REDUNDANT_PATTERN,
 231             USED_UNDERSCORE_BINDING,
 232             SHORT_CIRCUIT_STATEMENT,
 233             ZERO_PTR,
 234             FLOAT_CMP_CONST
 235         )
 236     }
 237 }
 238
 239 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Pass {
 240     fn check_fn(
 241         &mut self,
 242         cx: &LateContext<'a, 'tcx>,
 243         k: FnKind<'tcx>,
 244         decl: &'tcx FnDecl,
 245         body: &'tcx Body,
 246         _: Span,
 247         _: NodeId,
 248     ) {
 249         if let FnKind::Closure(_) = k {
 250             // Does not apply to closures
 251             return;
 252         }
 253         for arg in iter_input_pats(decl, body) {
 254             match arg.pat.node {
 255                 PatKind::Binding(BindingAnnotation::Ref, _, _, _) |
 256                 PatKind::Binding(BindingAnnotation::RefMut, _, _, _) => {
 257                     span_lint(
 258                         cx,
 259                         TOPLEVEL_REF_ARG,
 260                         arg.pat.span,
 261                         "`ref` directly on a function argument is ignored. Consider using a reference type \
 262                          instead.",
 263                     );
 264                 },
 265                 _ => {},
 266             }
 267         }
 268     }
 269
 270     fn check_stmt(&mut self, cx: &LateContext<'a, 'tcx>, s: &'tcx Stmt) {
 271         if_chain! {
 272             if let StmtDecl(ref d, _) = s.node;
 273             if let DeclLocal(ref l) = d.node;
 274             if let PatKind::Binding(an, _, i, None) = l.pat.node;
 275             if let Some(ref init) = l.init;
 276             then {
 277                 if an == BindingAnnotation::Ref || an == BindingAnnotation::RefMut {
 278                     let init = Sugg::hir(cx, init, "..");
 279                     let (mutopt,initref) = if an == BindingAnnotation::RefMut {
 280                         ("mut ", init.mut_addr())
 281                     } else {
 282                         ("", init.addr())
 283                     };
 284                     let tyopt = if let Some(ref ty) = l.ty {
 285                         format!(": &{mutopt}{ty}", mutopt=mutopt, ty=snippet(cx, ty.span, "_"))
 286                     } else {
 287                         "".to_owned()
 288                     };
 289                     span_lint_and_then(cx,
 290                         TOPLEVEL_REF_ARG,
 291                         l.pat.span,
 292                         "`ref` on an entire `let` pattern is discouraged, take a reference with `&` instead",
 293                         |db| {
 294                             db.span_suggestion(s.span,
 295                                                "try",
 296                                                format!("let {name}{tyopt} = {initref};",
 297                                                        name=snippet(cx, i.span, "_"),
 298                                                        tyopt=tyopt,
 299                                                        initref=initref));
 300                         }
 301                     );
 302                 }
 303             }
 304         };
 305         if_chain! {
 306             if let StmtSemi(ref expr, _) = s.node;
 307             if let Expr_::ExprBinary(ref binop, ref a, ref b) = expr.node;
 308             if binop.node == BiAnd || binop.node == BiOr;
 309             if let Some(sugg) = Sugg::hir_opt(cx, a);
 310             then {
 311                 span_lint_and_then(cx,
 312                     SHORT_CIRCUIT_STATEMENT,
 313                     s.span,
 314                     "boolean short circuit operator in statement may be clearer using an explicit test",
 315                     |db| {
 316                         let sugg = if binop.node == BiOr { !sugg } else { sugg };
 317                         db.span_suggestion(s.span, "replace it with",
 318                                            format!("if {} {{ {}; }}", sugg, &snippet(cx, b.span, "..")));
 319                     });
 320             }
 321         };
 322     }
 323
 324     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
 325         match expr.node {
 326             ExprCast(ref e, ref ty) => {
 327                 check_cast(cx, expr.span, e, ty);
 328                 return;
 329             },
 330             ExprBinary(ref cmp, ref left, ref right) => {
 331                 let op = cmp.node;
 332                 if op.is_comparison() {
 333                     if let ExprPath(QPath::Resolved(_, ref path)) = left.node {
 334                         check_nan(cx, path, expr);
 335                     }
 336                     if let ExprPath(QPath::Resolved(_, ref path)) = right.node {
 337                         check_nan(cx, path, expr);
 338                     }
 339                     check_to_owned(cx, left, right);
 340                     check_to_owned(cx, right, left);
 341                 }
 342                 if (op == BiEq || op == BiNe) && (is_float(cx, left) || is_float(cx, right)) {
 343                     if is_allowed(cx, left) || is_allowed(cx, right) {
 344                         return;
 345                     }
 346                     if let Some(name) = get_item_name(cx, expr) {
 347                         let name = name.as_str();
 348                         if name == "eq" || name == "ne" || name == "is_nan" || name.starts_with("eq_")
 349                             || name.ends_with("_eq")
 350                         {
 351                             return;
 352                         }
 353                     }
 354                     let (lint, msg) = if is_named_constant(cx, left) || is_named_constant(cx, right) {
 355                         (FLOAT_CMP_CONST, "strict comparison of f32 or f64 constant")
 356                     } else {
 357                         (FLOAT_CMP, "strict comparison of f32 or f64")
 358                     };
 359                     span_lint_and_then(cx, lint, expr.span, msg, |db| {
 360                         let lhs = Sugg::hir(cx, left, "..");
 361                         let rhs = Sugg::hir(cx, right, "..");
 362
 363                         db.span_suggestion(
 364                             expr.span,
 365                             "consider comparing them within some error",
 366                             format!("({}).abs() < error", lhs - rhs),
 367                         );
 368                         db.span_note(expr.span, "std::f32::EPSILON and std::f64::EPSILON are available.");
 369                     });
 370                 } else if op == BiRem && is_integer_literal(right, 1) {
 371                     span_lint(cx, MODULO_ONE, expr.span, "any number modulo 1 will be 0");
 372                 }
 373             },
 374             _ => {},
 375         }
 376         if in_attributes_expansion(expr) {
 377             // Don't lint things expanded by #[derive(...)], etc
 378             return;
 379         }
 380         let binding = match expr.node {
 381             ExprPath(ref qpath) => {
 382                 let binding = last_path_segment(qpath).name.as_str();
 383                 if binding.starts_with('_') &&
 384                     !binding.starts_with("__") &&
 385                     binding != "_result" && // FIXME: #944
 386                     is_used(cx, expr) &&
 387                     // don't lint if the declaration is in a macro
 388                     non_macro_local(cx, &cx.tables.qpath_def(qpath, expr.hir_id))
 389                 {
 390                     Some(binding)
 391                 } else {
 392                     None
 393                 }
 394             },
 395             ExprField(_, spanned) => {
 396                 let name = spanned.node.as_str();
 397                 if name.starts_with('_') && !name.starts_with("__") {
 398                     Some(name)
 399                 } else {
 400                     None
 401                 }
 402             },
 403             _ => None,
 404         };
 405         if let Some(binding) = binding {
 406             span_lint(
 407                 cx,
 408                 USED_UNDERSCORE_BINDING,
 409                 expr.span,
 410                 &format!(
 411                     "used binding `{}` which is prefixed with an underscore. A leading \
 412                      underscore signals that a binding will not be used.",
 413                     binding
 414                 ),
 415             );
 416         }
 417     }
 418
 419     fn check_pat(&mut self, cx: &LateContext<'a, 'tcx>, pat: &'tcx Pat) {
 420         if let PatKind::Binding(_, _, ref ident, Some(ref right)) = pat.node {
 421             if right.node == PatKind::Wild {
 422                 span_lint(
 423                     cx,
 424                     REDUNDANT_PATTERN,
 425                     pat.span,
 426                     &format!("the `{} @ _` pattern can be written as just `{}`", ident.node, ident.node),
 427                 );
 428             }
 429         }
 430     }
 431 }
 432
 433 fn check_nan(cx: &LateContext, path: &Path, expr: &Expr) {
 434     if !in_constant(cx, expr.id) {
 435         if let Some(seg) = path.segments.last() {
 436             if seg.name == "NAN" {
 437                 span_lint(
 438                     cx,
 439                     CMP_NAN,
 440                     expr.span,
 441                     "doomed comparison with NAN, use `std::{f32,f64}::is_nan()` instead",
 442                     );
 443             }
 444         }
 445     }
 446 }
 447
 448 fn is_named_constant<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) -> bool {
 449     if let Some((_, res)) = constant(cx, cx.tables, expr) {
 450         res
 451     } else {
 452        false
 453     }
 454 }
 455
 456 fn is_allowed<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) -> bool {
 457     match constant(cx, cx.tables, expr) {
 458         Some((Constant::F32(f), _)) => f == 0.0 || f.is_infinite(),
 459         Some((Constant::F64(f), _)) => f == 0.0 || f.is_infinite(),
 460         _ => false,
 461     }
 462 }
 463
 464 fn is_float(cx: &LateContext, expr: &Expr) -> bool {
 465     matches!(walk_ptrs_ty(cx.tables.expr_ty(expr)).sty, ty::TyFloat(_))
 466 }
 467
 468 fn check_to_owned(cx: &LateContext, expr: &Expr, other: &Expr) {
 469     let (arg_ty, snip) = match expr.node {
 470         ExprMethodCall(.., ref args) if args.len() == 1 => {
 471             if match_trait_method(cx, expr, &paths::TO_STRING) || match_trait_method(cx, expr, &paths::TO_OWNED) {
 472                 (cx.tables.expr_ty_adjusted(&args[0]), snippet(cx, args[0].span, ".."))
 473             } else {
 474                 return;
 475             }
 476         },
 477         ExprCall(ref path, ref v) if v.len() == 1 => if let ExprPath(ref path) = path.node {
 478             if match_qpath(path, &["String", "from_str"]) || match_qpath(path, &["String", "from"]) {
 479                 (cx.tables.expr_ty_adjusted(&v[0]), snippet(cx, v[0].span, ".."))
 480             } else {
 481                 return;
 482             }
 483         } else {
 484             return;
 485         },
 486         _ => return,
 487     };
 488
 489     let other_ty = cx.tables.expr_ty_adjusted(other);
 490     let partial_eq_trait_id = match cx.tcx.lang_items().eq_trait() {
 491         Some(id) => id,
 492         None => return,
 493     };
 494
 495     // *arg impls PartialEq<other>
 496     if !arg_ty
 497         .builtin_deref(true)
 498         .map_or(false, |tam| implements_trait(cx, tam.ty, partial_eq_trait_id, &[other_ty]))
 499         // arg impls PartialEq<*other>
 500         && !other_ty
 501         .builtin_deref(true)
 502         .map_or(false, |tam| implements_trait(cx, arg_ty, partial_eq_trait_id, &[tam.ty]))
 503         // arg impls PartialEq<other>
 504         && !implements_trait(cx, arg_ty, partial_eq_trait_id, &[other_ty])
 505     {
 506         return;
 507     }
 508
 509     span_lint_and_then(
 510         cx,
 511         CMP_OWNED,
 512         expr.span,
 513         "this creates an owned instance just for comparison",
 514         |db| {
 515             // this is as good as our recursion check can get, we can't prove that the
 516             // current function is
 517             // called by
 518             // PartialEq::eq, but we can at least ensure that this code is not part of it
 519             let parent_fn = cx.tcx.hir.get_parent(expr.id);
 520             let parent_impl = cx.tcx.hir.get_parent(parent_fn);
 521             if parent_impl != CRATE_NODE_ID {
 522                 if let map::NodeItem(item) = cx.tcx.hir.get(parent_impl) {
 523                     if let ItemImpl(.., Some(ref trait_ref), _, _) = item.node {
 524                         if trait_ref.path.def.def_id() == partial_eq_trait_id {
 525                             // we are implementing PartialEq, don't suggest not doing `to_owned`, otherwise
 526                             // we go into
 527                             // recursion
 528                             db.span_label(expr.span, "try calling implementing the comparison without allocating");
 529                             return;
 530                         }
 531                     }
 532                 }
 533             }
 534             db.span_suggestion(expr.span, "try", snip.to_string());
 535         },
 536     );
 537 }
 538
 539 /// Heuristic to see if an expression is used. Should be compatible with
 540 /// `unused_variables`'s idea
 541 /// of what it means for an expression to be "used".
 542 fn is_used(cx: &LateContext, expr: &Expr) -> bool {
 543     if let Some(parent) = get_parent_expr(cx, expr) {
 544         match parent.node {
 545             ExprAssign(_, ref rhs) | ExprAssignOp(_, _, ref rhs) => **rhs == *expr,
 546             _ => is_used(cx, parent),
 547         }
 548     } else {
 549         true
 550     }
 551 }
 552
 553 /// Test whether an expression is in a macro expansion (e.g. something
 554 /// generated by
 555 /// `#[derive(...)`] or the like).
 556 fn in_attributes_expansion(expr: &Expr) -> bool {
 557     expr.span
 558         .ctxt()
 559         .outer()
 560         .expn_info()
 561         .map_or(false, |info| matches!(info.callee.format, ExpnFormat::MacroAttribute(_)))
 562 }
 563
 564 /// Test whether `def` is a variable defined outside a macro.
 565 fn non_macro_local(cx: &LateContext, def: &def::Def) -> bool {
 566     match *def {
 567         def::Def::Local(id) | def::Def::Upvar(id, _, _) => !in_macro(cx.tcx.hir.span(id)),
 568         _ => false,
 569     }
 570 }
 571
 572 fn check_cast(cx: &LateContext, span: Span, e: &Expr, ty: &Ty) {
 573     if_chain! {
 574         if let TyPtr(MutTy { mutbl, .. }) = ty.node;
 575         if let ExprLit(ref lit) = e.node;
 576         if let LitKind::Int(value, ..) = lit.node;
 577         if value == 0;
 578         if !in_constant(cx, e.id);
 579         then {
 580             let msg = match mutbl {
 581                 Mutability::MutMutable => "`0 as *mut _` detected. Consider using `ptr::null_mut()`",
 582                 Mutability::MutImmutable => "`0 as *const _` detected. Consider using `ptr::null()`",
 583             };
 584             span_lint(cx, ZERO_PTR, span, msg);
 585         }
 586     }
 587 }