clippy_lints/src/transmute.rs

   1 use crate::utils::{last_path_segment, match_def_path, paths, snippet, span_lint, span_lint_and_then, sugg};
   2 use if_chain::if_chain;
   3 use rustc::declare_lint_pass;
   4 use rustc::hir::*;
   5 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
   6 use rustc::ty::{self, Ty};
   7 use rustc_errors::Applicability;
   8 use rustc_session::declare_tool_lint;
   9 use std::borrow::Cow;
  10 use syntax::ast;
  11
  12 declare_clippy_lint! {
  13     /// **What it does:** Checks for transmutes that can't ever be correct on any
  14     /// architecture.
  15     ///
  16     /// **Why is this bad?** It's basically guaranteed to be undefined behaviour.
  17     ///
  18     /// **Known problems:** When accessing C, users might want to store pointer
  19     /// sized objects in `extradata` arguments to save an allocation.
  20     ///
  21     /// **Example:**
  22     /// ```ignore
  23     /// let ptr: *const T = core::intrinsics::transmute('x')
  24     /// ```
  25     pub WRONG_TRANSMUTE,
  26     correctness,
  27     "transmutes that are confusing at best, undefined behaviour at worst and always useless"
  28 }
  29
  30 declare_clippy_lint! {
  31     /// **What it does:** Checks for transmutes to the original type of the object
  32     /// and transmutes that could be a cast.
  33     ///
  34     /// **Why is this bad?** Readability. The code tricks people into thinking that
  35     /// something complex is going on.
  36     ///
  37     /// **Known problems:** None.
  38     ///
  39     /// **Example:**
  40     /// ```rust,ignore
  41     /// core::intrinsics::transmute(t); // where the result type is the same as `t`'s
  42     /// ```
  43     pub USELESS_TRANSMUTE,
  44     complexity,
  45     "transmutes that have the same to and from types or could be a cast/coercion"
  46 }
  47
  48 declare_clippy_lint! {
  49     /// **What it does:** Checks for transmutes between a type `T` and `*T`.
  50     ///
  51     /// **Why is this bad?** It's easy to mistakenly transmute between a type and a
  52     /// pointer to that type.
  53     ///
  54     /// **Known problems:** None.
  55     ///
  56     /// **Example:**
  57     /// ```rust,ignore
  58     /// core::intrinsics::transmute(t) // where the result type is the same as
  59     ///                                // `*t` or `&t`'s
  60     /// ```
  61     pub CROSSPOINTER_TRANSMUTE,
  62     complexity,
  63     "transmutes that have to or from types that are a pointer to the other"
  64 }
  65
  66 declare_clippy_lint! {
  67     /// **What it does:** Checks for transmutes from a pointer to a reference.
  68     ///
  69     /// **Why is this bad?** This can always be rewritten with `&` and `*`.
  70     ///
  71     /// **Known problems:** None.
  72     ///
  73     /// **Example:**
  74     /// ```rust,ignore
  75     /// unsafe {
  76     ///     let _: &T = std::mem::transmute(p); // where p: *const T
  77     /// }
  78     ///
  79     /// // can be written:
  80     /// let _: &T = &*p;
  81     /// ```
  82     pub TRANSMUTE_PTR_TO_REF,
  83     complexity,
  84     "transmutes from a pointer to a reference type"
  85 }
  86
  87 declare_clippy_lint! {
  88     /// **What it does:** Checks for transmutes from an integer to a `char`.
  89     ///
  90     /// **Why is this bad?** Not every integer is a Unicode scalar value.
  91     ///
  92     /// **Known problems:**
  93     /// - [`from_u32`] which this lint suggests using is slower than `transmute`
  94     /// as it needs to validate the input.
  95     /// If you are certain that the input is always a valid Unicode scalar value,
  96     /// use [`from_u32_unchecked`] which is as fast as `transmute`
  97     /// but has a semantically meaningful name.
  98     /// - You might want to handle `None` returned from [`from_u32`] instead of calling `unwrap`.
  99     ///
 100     /// [`from_u32`]: https://doc.rust-lang.org/std/char/fn.from_u32.html
 101     /// [`from_u32_unchecked`]: https://doc.rust-lang.org/std/char/fn.from_u32_unchecked.html
 102     ///
 103     /// **Example:**
 104     /// ```rust
 105     /// let x = 1_u32;
 106     /// unsafe {
 107     ///     let _: char = std::mem::transmute(x); // where x: u32
 108     /// }
 109     ///
 110     /// // should be:
 111     /// let _ = std::char::from_u32(x).unwrap();
 112     /// ```
 113     pub TRANSMUTE_INT_TO_CHAR,
 114     complexity,
 115     "transmutes from an integer to a `char`"
 116 }
 117
 118 declare_clippy_lint! {
 119     /// **What it does:** Checks for transmutes from a `&[u8]` to a `&str`.
 120     ///
 121     /// **Why is this bad?** Not every byte slice is a valid UTF-8 string.
 122     ///
 123     /// **Known problems:**
 124     /// - [`from_utf8`] which this lint suggests using is slower than `transmute`
 125     /// as it needs to validate the input.
 126     /// If you are certain that the input is always a valid UTF-8,
 127     /// use [`from_utf8_unchecked`] which is as fast as `transmute`
 128     /// but has a semantically meaningful name.
 129     /// - You might want to handle errors returned from [`from_utf8`] instead of calling `unwrap`.
 130     ///
 131     /// [`from_utf8`]: https://doc.rust-lang.org/std/str/fn.from_utf8.html
 132     /// [`from_utf8_unchecked`]: https://doc.rust-lang.org/std/str/fn.from_utf8_unchecked.html
 133     ///
 134     /// **Example:**
 135     /// ```rust
 136     /// let b: &[u8] = &[1_u8, 2_u8];
 137     /// unsafe {
 138     ///     let _: &str = std::mem::transmute(b); // where b: &[u8]
 139     /// }
 140     ///
 141     /// // should be:
 142     /// let _ = std::str::from_utf8(b).unwrap();
 143     /// ```
 144     pub TRANSMUTE_BYTES_TO_STR,
 145     complexity,
 146     "transmutes from a `&[u8]` to a `&str`"
 147 }
 148
 149 declare_clippy_lint! {
 150     /// **What it does:** Checks for transmutes from an integer to a `bool`.
 151     ///
 152     /// **Why is this bad?** This might result in an invalid in-memory representation of a `bool`.
 153     ///
 154     /// **Known problems:** None.
 155     ///
 156     /// **Example:**
 157     /// ```rust
 158     /// let x = 1_u8;
 159     /// unsafe {
 160     ///     let _: bool = std::mem::transmute(x); // where x: u8
 161     /// }
 162     ///
 163     /// // should be:
 164     /// let _: bool = x != 0;
 165     /// ```
 166     pub TRANSMUTE_INT_TO_BOOL,
 167     complexity,
 168     "transmutes from an integer to a `bool`"
 169 }
 170
 171 declare_clippy_lint! {
 172     /// **What it does:** Checks for transmutes from an integer to a float.
 173     ///
 174     /// **Why is this bad?** Transmutes are dangerous and error-prone, whereas `from_bits` is intuitive
 175     /// and safe.
 176     ///
 177     /// **Known problems:** None.
 178     ///
 179     /// **Example:**
 180     /// ```rust
 181     /// unsafe {
 182     ///     let _: f32 = std::mem::transmute(1_u32); // where x: u32
 183     /// }
 184     ///
 185     /// // should be:
 186     /// let _: f32 = f32::from_bits(1_u32);
 187     /// ```
 188     pub TRANSMUTE_INT_TO_FLOAT,
 189     complexity,
 190     "transmutes from an integer to a float"
 191 }
 192
 193 declare_clippy_lint! {
 194     /// **What it does:** Checks for transmutes from a float to an integer.
 195     ///
 196     /// **Why is this bad?** Transmutes are dangerous and error-prone, whereas `to_bits` is intuitive
 197     /// and safe.
 198     ///
 199     /// **Known problems:** None.
 200     ///
 201     /// **Example:**
 202     /// ```rust
 203     /// unsafe {
 204     ///     let _: u32 = std::mem::transmute(1f32);
 205     /// }
 206     ///
 207     /// // should be:
 208     /// let _: u32 = 1f32.to_bits();
 209     /// ```
 210     pub TRANSMUTE_FLOAT_TO_INT,
 211     nursery,
 212     "transmutes from a float to an integer"
 213 }
 214
 215 declare_clippy_lint! {
 216     /// **What it does:** Checks for transmutes from a pointer to a pointer, or
 217     /// from a reference to a reference.
 218     ///
 219     /// **Why is this bad?** Transmutes are dangerous, and these can instead be
 220     /// written as casts.
 221     ///
 222     /// **Known problems:** None.
 223     ///
 224     /// **Example:**
 225     /// ```rust
 226     /// let ptr = &1u32 as *const u32;
 227     /// unsafe {
 228     ///     // pointer-to-pointer transmute
 229     ///     let _: *const f32 = std::mem::transmute(ptr);
 230     ///     // ref-ref transmute
 231     ///     let _: &f32 = std::mem::transmute(&1u32);
 232     /// }
 233     /// // These can be respectively written:
 234     /// let _ = ptr as *const f32;
 235     /// let _ = unsafe{ &*(&1u32 as *const u32 as *const f32) };
 236     /// ```
 237     pub TRANSMUTE_PTR_TO_PTR,
 238     complexity,
 239     "transmutes from a pointer to a pointer / a reference to a reference"
 240 }
 241
 242 declare_clippy_lint! {
 243     /// **What it does:** Checks for transmutes between collections whose
 244     /// types have different ABI, size or alignment.
 245     ///
 246     /// **Why is this bad?** This is undefined behavior.
 247     ///
 248     /// **Known problems:** Currently, we cannot know whether a type is a
 249     /// collection, so we just lint the ones that come with `std`.
 250     ///
 251     /// **Example:**
 252     /// ```rust
 253     /// // different size, therefore likely out-of-bounds memory access
 254     /// // You absolutely do not want this in your code!
 255     /// unsafe {
 256     ///     std::mem::transmute::<_, Vec<u32>>(vec![2_u16])
 257     /// };
 258     /// ```
 259     ///
 260     /// You must always iterate, map and collect the values:
 261     ///
 262     /// ```rust
 263     /// vec![2_u16].into_iter().map(u32::from).collect::<Vec<_>>();
 264     /// ```
 265     pub UNSOUND_COLLECTION_TRANSMUTE,
 266     correctness,
 267     "transmute between collections of layout-incompatible types"
 268 }
 269 declare_lint_pass!(Transmute => [
 270     CROSSPOINTER_TRANSMUTE,
 271     TRANSMUTE_PTR_TO_REF,
 272     TRANSMUTE_PTR_TO_PTR,
 273     USELESS_TRANSMUTE,
 274     WRONG_TRANSMUTE,
 275     TRANSMUTE_INT_TO_CHAR,
 276     TRANSMUTE_BYTES_TO_STR,
 277     TRANSMUTE_INT_TO_BOOL,
 278     TRANSMUTE_INT_TO_FLOAT,
 279     TRANSMUTE_FLOAT_TO_INT,
 280     UNSOUND_COLLECTION_TRANSMUTE,
 281 ]);
 282
 283 // used to check for UNSOUND_COLLECTION_TRANSMUTE
 284 static COLLECTIONS: &[&[&str]] = &[
 285     &paths::VEC,
 286     &paths::VEC_DEQUE,
 287     &paths::BINARY_HEAP,
 288     &paths::BTREESET,
 289     &paths::BTREEMAP,
 290     &paths::HASHSET,
 291     &paths::HASHMAP,
 292 ];
 293 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Transmute {
 294     #[allow(clippy::similar_names, clippy::too_many_lines)]
 295     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, e: &'tcx Expr) {
 296         if_chain! {
 297             if let ExprKind::Call(ref path_expr, ref args) = e.kind;
 298             if let ExprKind::Path(ref qpath) = path_expr.kind;
 299             if let Some(def_id) = cx.tables.qpath_res(qpath, path_expr.hir_id).opt_def_id();
 300             if match_def_path(cx, def_id, &paths::TRANSMUTE);
 301             then {
 302                 let from_ty = cx.tables.expr_ty(&args[0]);
 303                 let to_ty = cx.tables.expr_ty(e);
 304
 305                 match (&from_ty.kind, &to_ty.kind) {
 306                     _ if from_ty == to_ty => span_lint(
 307                         cx,
 308                         USELESS_TRANSMUTE,
 309                         e.span,
 310                         &format!("transmute from a type (`{}`) to itself", from_ty),
 311                     ),
 312                     (&ty::Ref(_, rty, rty_mutbl), &ty::RawPtr(ptr_ty)) => span_lint_and_then(
 313                         cx,
 314                         USELESS_TRANSMUTE,
 315                         e.span,
 316                         "transmute from a reference to a pointer",
 317                         |db| {
 318                             if let Some(arg) = sugg::Sugg::hir_opt(cx, &args[0]) {
 319                                 let rty_and_mut = ty::TypeAndMut {
 320                                     ty: rty,
 321                                     mutbl: rty_mutbl,
 322                                 };
 323
 324                                 let sugg = if ptr_ty == rty_and_mut {
 325                                     arg.as_ty(to_ty)
 326                                 } else {
 327                                     arg.as_ty(cx.tcx.mk_ptr(rty_and_mut)).as_ty(to_ty)
 328                                 };
 329
 330                                 db.span_suggestion(e.span, "try", sugg.to_string(), Applicability::Unspecified);
 331                             }
 332                         },
 333                     ),
 334                     (&ty::Int(_), &ty::RawPtr(_)) | (&ty::Uint(_), &ty::RawPtr(_)) => span_lint_and_then(
 335                         cx,
 336                         USELESS_TRANSMUTE,
 337                         e.span,
 338                         "transmute from an integer to a pointer",
 339                         |db| {
 340                             if let Some(arg) = sugg::Sugg::hir_opt(cx, &args[0]) {
 341                                 db.span_suggestion(
 342                                     e.span,
 343                                     "try",
 344                                     arg.as_ty(&to_ty.to_string()).to_string(),
 345                                     Applicability::Unspecified,
 346                                 );
 347                             }
 348                         },
 349                     ),
 350                     (&ty::Float(_), &ty::Ref(..))
 351                     | (&ty::Float(_), &ty::RawPtr(_))
 352                     | (&ty::Char, &ty::Ref(..))
 353                     | (&ty::Char, &ty::RawPtr(_)) => span_lint(
 354                         cx,
 355                         WRONG_TRANSMUTE,
 356                         e.span,
 357                         &format!("transmute from a `{}` to a pointer", from_ty),
 358                     ),
 359                     (&ty::RawPtr(from_ptr), _) if from_ptr.ty == to_ty => span_lint(
 360                         cx,
 361                         CROSSPOINTER_TRANSMUTE,
 362                         e.span,
 363                         &format!(
 364                             "transmute from a type (`{}`) to the type that it points to (`{}`)",
 365                             from_ty, to_ty
 366                         ),
 367                     ),
 368                     (_, &ty::RawPtr(to_ptr)) if to_ptr.ty == from_ty => span_lint(
 369                         cx,
 370                         CROSSPOINTER_TRANSMUTE,
 371                         e.span,
 372                         &format!(
 373                             "transmute from a type (`{}`) to a pointer to that type (`{}`)",
 374                             from_ty, to_ty
 375                         ),
 376                     ),
 377                     (&ty::RawPtr(from_pty), &ty::Ref(_, to_ref_ty, mutbl)) => span_lint_and_then(
 378                         cx,
 379                         TRANSMUTE_PTR_TO_REF,
 380                         e.span,
 381                         &format!(
 382                             "transmute from a pointer type (`{}`) to a reference type \
 383                              (`{}`)",
 384                             from_ty, to_ty
 385                         ),
 386                         |db| {
 387                             let arg = sugg::Sugg::hir(cx, &args[0], "..");
 388                             let (deref, cast) = if mutbl == Mutability::Mutable {
 389                                 ("&mut *", "*mut")
 390                             } else {
 391                                 ("&*", "*const")
 392                             };
 393
 394                             let arg = if from_pty.ty == to_ref_ty {
 395                                 arg
 396                             } else {
 397                                 arg.as_ty(&format!("{} {}", cast, get_type_snippet(cx, qpath, to_ref_ty)))
 398                             };
 399
 400                             db.span_suggestion(
 401                                 e.span,
 402                                 "try",
 403                                 sugg::make_unop(deref, arg).to_string(),
 404                                 Applicability::Unspecified,
 405                             );
 406                         },
 407                     ),
 408                     (&ty::Int(ast::IntTy::I32), &ty::Char) | (&ty::Uint(ast::UintTy::U32), &ty::Char) => {
 409                         span_lint_and_then(
 410                             cx,
 411                             TRANSMUTE_INT_TO_CHAR,
 412                             e.span,
 413                             &format!("transmute from a `{}` to a `char`", from_ty),
 414                             |db| {
 415                                 let arg = sugg::Sugg::hir(cx, &args[0], "..");
 416                                 let arg = if let ty::Int(_) = from_ty.kind {
 417                                     arg.as_ty(ast::UintTy::U32.name_str())
 418                                 } else {
 419                                     arg
 420                                 };
 421                                 db.span_suggestion(
 422                                     e.span,
 423                                     "consider using",
 424                                     format!("std::char::from_u32({}).unwrap()", arg.to_string()),
 425                                     Applicability::Unspecified,
 426                                 );
 427                             },
 428                         )
 429                     },
 430                     (&ty::Ref(_, ty_from, from_mutbl), &ty::Ref(_, ty_to, to_mutbl)) => {
 431                         if_chain! {
 432                             if let (&ty::Slice(slice_ty), &ty::Str) = (&ty_from.kind, &ty_to.kind);
 433                             if let ty::Uint(ast::UintTy::U8) = slice_ty.kind;
 434                             if from_mutbl == to_mutbl;
 435                             then {
 436                                 let postfix = if from_mutbl == Mutability::Mutable {
 437                                     "_mut"
 438                                 } else {
 439                                     ""
 440                                 };
 441
 442                                 span_lint_and_then(
 443                                     cx,
 444                                     TRANSMUTE_BYTES_TO_STR,
 445                                     e.span,
 446                                     &format!("transmute from a `{}` to a `{}`", from_ty, to_ty),
 447                                     |db| {
 448                                         db.span_suggestion(
 449                                             e.span,
 450                                             "consider using",
 451                                             format!(
 452                                                 "std::str::from_utf8{}({}).unwrap()",
 453                                                 postfix,
 454                                                 snippet(cx, args[0].span, ".."),
 455                                             ),
 456                                             Applicability::Unspecified,
 457                                         );
 458                                     }
 459                                 )
 460                             } else {
 461                                 if cx.tcx.erase_regions(&from_ty) != cx.tcx.erase_regions(&to_ty) {
 462                                     span_lint_and_then(
 463                                         cx,
 464                                         TRANSMUTE_PTR_TO_PTR,
 465                                         e.span,
 466                                         "transmute from a reference to a reference",
 467                                         |db| if let Some(arg) = sugg::Sugg::hir_opt(cx, &args[0]) {
 468                                             let ty_from_and_mut = ty::TypeAndMut {
 469                                                 ty: ty_from,
 470                                                 mutbl: from_mutbl
 471                                             };
 472                                             let ty_to_and_mut = ty::TypeAndMut { ty: ty_to, mutbl: to_mutbl };
 473                                             let sugg_paren = arg
 474                                                 .as_ty(cx.tcx.mk_ptr(ty_from_and_mut))
 475                                                 .as_ty(cx.tcx.mk_ptr(ty_to_and_mut));
 476                                             let sugg = if to_mutbl == Mutability::Mutable {
 477                                                 sugg_paren.mut_addr_deref()
 478                                             } else {
 479                                                 sugg_paren.addr_deref()
 480                                             };
 481                                             db.span_suggestion(
 482                                                 e.span,
 483                                                 "try",
 484                                                 sugg.to_string(),
 485                                                 Applicability::Unspecified,
 486                                             );
 487                                         },
 488                                     )
 489                                 }
 490                             }
 491                         }
 492                     },
 493                     (&ty::RawPtr(_), &ty::RawPtr(to_ty)) => span_lint_and_then(
 494                         cx,
 495                         TRANSMUTE_PTR_TO_PTR,
 496                         e.span,
 497                         "transmute from a pointer to a pointer",
 498                         |db| {
 499                             if let Some(arg) = sugg::Sugg::hir_opt(cx, &args[0]) {
 500                                 let sugg = arg.as_ty(cx.tcx.mk_ptr(to_ty));
 501                                 db.span_suggestion(e.span, "try", sugg.to_string(), Applicability::Unspecified);
 502                             }
 503                         },
 504                     ),
 505                     (&ty::Int(ast::IntTy::I8), &ty::Bool) | (&ty::Uint(ast::UintTy::U8), &ty::Bool) => {
 506                         span_lint_and_then(
 507                             cx,
 508                             TRANSMUTE_INT_TO_BOOL,
 509                             e.span,
 510                             &format!("transmute from a `{}` to a `bool`", from_ty),
 511                             |db| {
 512                                 let arg = sugg::Sugg::hir(cx, &args[0], "..");
 513                                 let zero = sugg::Sugg::NonParen(Cow::from("0"));
 514                                 db.span_suggestion(
 515                                     e.span,
 516                                     "consider using",
 517                                     sugg::make_binop(ast::BinOpKind::Ne, &arg, &zero).to_string(),
 518                                     Applicability::Unspecified,
 519                                 );
 520                             },
 521                         )
 522                     },
 523                     (&ty::Int(_), &ty::Float(_)) | (&ty::Uint(_), &ty::Float(_)) => span_lint_and_then(
 524                         cx,
 525                         TRANSMUTE_INT_TO_FLOAT,
 526                         e.span,
 527                         &format!("transmute from a `{}` to a `{}`", from_ty, to_ty),
 528                         |db| {
 529                             let arg = sugg::Sugg::hir(cx, &args[0], "..");
 530                             let arg = if let ty::Int(int_ty) = from_ty.kind {
 531                                 arg.as_ty(format!(
 532                                     "u{}",
 533                                     int_ty.bit_width().map_or_else(|| "size".to_string(), |v| v.to_string())
 534                                 ))
 535                             } else {
 536                                 arg
 537                             };
 538                             db.span_suggestion(
 539                                 e.span,
 540                                 "consider using",
 541                                 format!("{}::from_bits({})", to_ty, arg.to_string()),
 542                                 Applicability::Unspecified,
 543                             );
 544                         },
 545                     ),
 546                     (&ty::Float(float_ty), &ty::Int(_)) | (&ty::Float(float_ty), &ty::Uint(_)) => span_lint_and_then(
 547                         cx,
 548                         TRANSMUTE_FLOAT_TO_INT,
 549                         e.span,
 550                         &format!("transmute from a `{}` to a `{}`", from_ty, to_ty),
 551                         |db| {
 552                             let mut expr = &args[0];
 553                             let mut arg = sugg::Sugg::hir(cx, expr, "..");
 554
 555                             if let ExprKind::Unary(UnOp::UnNeg, inner_expr) = &expr.kind {
 556                                 expr = &inner_expr;
 557                             }
 558
 559                             if_chain! {
 560                                 // if the expression is a float literal and it is unsuffixed then
 561                                 // add a suffix so the suggestion is valid and unambiguous
 562                                 let op = format!("{}{}", arg, float_ty.name_str()).into();
 563                                 if let ExprKind::Lit(lit) = &expr.kind;
 564                                 if let ast::LitKind::Float(_, ast::LitFloatType::Unsuffixed) = lit.node;
 565                                 then {
 566                                     match arg {
 567                                         sugg::Sugg::MaybeParen(_) => arg = sugg::Sugg::MaybeParen(op),
 568                                         _ => arg = sugg::Sugg::NonParen(op)
 569                                     }
 570                                 }
 571                             }
 572
 573                             arg = sugg::Sugg::NonParen(format!("{}.to_bits()", arg.maybe_par()).into());
 574
 575                             // cast the result of `to_bits` if `to_ty` is signed
 576                             arg = if let ty::Int(int_ty) = to_ty.kind {
 577                                 arg.as_ty(int_ty.name_str().to_string())
 578                             } else {
 579                                 arg
 580                             };
 581
 582                             db.span_suggestion(
 583                                 e.span,
 584                                 "consider using",
 585                                 arg.to_string(),
 586                                 Applicability::Unspecified,
 587                             );
 588                         },
 589                     ),
 590                     (&ty::Adt(ref from_adt, ref from_substs), &ty::Adt(ref to_adt, ref to_substs)) => {
 591                         if from_adt.did != to_adt.did ||
 592                                 !COLLECTIONS.iter().any(|path| match_def_path(cx, to_adt.did, path)) {
 593                             return;
 594                         }
 595                         if from_substs.types().zip(to_substs.types())
 596                                               .any(|(from_ty, to_ty)| is_layout_incompatible(cx, from_ty, to_ty)) {
 597                             span_lint(
 598                                 cx,
 599                                 UNSOUND_COLLECTION_TRANSMUTE,
 600                                 e.span,
 601                                 &format!(
 602                                     "transmute from `{}` to `{}` with mismatched layout is unsound",
 603                                     from_ty,
 604                                     to_ty
 605                                 )
 606                             );
 607                         }
 608                     },
 609                     _ => return,
 610                 }
 611             }
 612         }
 613     }
 614 }
 615
 616 /// Gets the snippet of `Bar` in `…::transmute<Foo, &Bar>`. If that snippet is
 617 /// not available , use
 618 /// the type's `ToString` implementation. In weird cases it could lead to types
 619 /// with invalid `'_`
 620 /// lifetime, but it should be rare.
 621 fn get_type_snippet(cx: &LateContext<'_, '_>, path: &QPath, to_ref_ty: Ty<'_>) -> String {
 622     let seg = last_path_segment(path);
 623     if_chain! {
 624         if let Some(ref params) = seg.args;
 625         if !params.parenthesized;
 626         if let Some(to_ty) = params.args.iter().filter_map(|arg| match arg {
 627             GenericArg::Type(ty) => Some(ty),
 628             _ => None,
 629         }).nth(1);
 630         if let TyKind::Rptr(_, ref to_ty) = to_ty.kind;
 631         then {
 632             return snippet(cx, to_ty.ty.span, &to_ref_ty.to_string()).to_string();
 633         }
 634     }
 635
 636     to_ref_ty.to_string()
 637 }
 638
 639 // check if the component types of the transmuted collection and the result have different ABI,
 640 // size or alignment
 641 fn is_layout_incompatible<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, from: Ty<'tcx>, to: Ty<'tcx>) -> bool {
 642     let from_ty_layout = cx.tcx.layout_of(ty::ParamEnv::empty().and(from));
 643     let to_ty_layout = cx.tcx.layout_of(ty::ParamEnv::empty().and(to));
 644     if let (Ok(from_layout), Ok(to_layout)) = (from_ty_layout, to_ty_layout) {
 645         from_layout.size != to_layout.size || from_layout.align != to_layout.align || from_layout.abi != to_layout.abi
 646     } else {
 647         // no idea about layout, so don't lint
 648         false
 649     }
 650 }