clippy_lints/src/transmute.rs

   1 use crate::utils::{
   2     is_normalizable, last_path_segment, match_def_path, paths, snippet, span_lint, span_lint_and_sugg,
   3     span_lint_and_then, sugg,
   4 };
   5 use if_chain::if_chain;
   6 use rustc_ast::ast;
   7 use rustc_errors::Applicability;
   8 use rustc_hir::{Expr, ExprKind, GenericArg, Mutability, QPath, TyKind, UnOp};
   9 use rustc_lint::{LateContext, LateLintPass};
  10 use rustc_middle::ty::{self, Ty};
  11 use rustc_session::{declare_lint_pass, declare_tool_lint};
  12 use std::borrow::Cow;
  13
  14 declare_clippy_lint! {
  15     /// **What it does:** Checks for transmutes that can't ever be correct on any
  16     /// architecture.
  17     ///
  18     /// **Why is this bad?** It's basically guaranteed to be undefined behaviour.
  19     ///
  20     /// **Known problems:** When accessing C, users might want to store pointer
  21     /// sized objects in `extradata` arguments to save an allocation.
  22     ///
  23     /// **Example:**
  24     /// ```ignore
  25     /// let ptr: *const T = core::intrinsics::transmute('x')
  26     /// ```
  27     pub WRONG_TRANSMUTE,
  28     correctness,
  29     "transmutes that are confusing at best, undefined behaviour at worst and always useless"
  30 }
  31
  32 // FIXME: Move this to `complexity` again, after #5343 is fixed
  33 declare_clippy_lint! {
  34     /// **What it does:** Checks for transmutes to the original type of the object
  35     /// and transmutes that could be a cast.
  36     ///
  37     /// **Why is this bad?** Readability. The code tricks people into thinking that
  38     /// something complex is going on.
  39     ///
  40     /// **Known problems:** None.
  41     ///
  42     /// **Example:**
  43     /// ```rust,ignore
  44     /// core::intrinsics::transmute(t); // where the result type is the same as `t`'s
  45     /// ```
  46     pub USELESS_TRANSMUTE,
  47     nursery,
  48     "transmutes that have the same to and from types or could be a cast/coercion"
  49 }
  50
  51 declare_clippy_lint! {
  52     /// **What it does:** Checks for transmutes between a type `T` and `*T`.
  53     ///
  54     /// **Why is this bad?** It's easy to mistakenly transmute between a type and a
  55     /// pointer to that type.
  56     ///
  57     /// **Known problems:** None.
  58     ///
  59     /// **Example:**
  60     /// ```rust,ignore
  61     /// core::intrinsics::transmute(t) // where the result type is the same as
  62     ///                                // `*t` or `&t`'s
  63     /// ```
  64     pub CROSSPOINTER_TRANSMUTE,
  65     complexity,
  66     "transmutes that have to or from types that are a pointer to the other"
  67 }
  68
  69 declare_clippy_lint! {
  70     /// **What it does:** Checks for transmutes from a pointer to a reference.
  71     ///
  72     /// **Why is this bad?** This can always be rewritten with `&` and `*`.
  73     ///
  74     /// **Known problems:** None.
  75     ///
  76     /// **Example:**
  77     /// ```rust,ignore
  78     /// unsafe {
  79     ///     let _: &T = std::mem::transmute(p); // where p: *const T
  80     /// }
  81     ///
  82     /// // can be written:
  83     /// let _: &T = &*p;
  84     /// ```
  85     pub TRANSMUTE_PTR_TO_REF,
  86     complexity,
  87     "transmutes from a pointer to a reference type"
  88 }
  89
  90 declare_clippy_lint! {
  91     /// **What it does:** Checks for transmutes from an integer to a `char`.
  92     ///
  93     /// **Why is this bad?** Not every integer is a Unicode scalar value.
  94     ///
  95     /// **Known problems:**
  96     /// - [`from_u32`] which this lint suggests using is slower than `transmute`
  97     /// as it needs to validate the input.
  98     /// If you are certain that the input is always a valid Unicode scalar value,
  99     /// use [`from_u32_unchecked`] which is as fast as `transmute`
 100     /// but has a semantically meaningful name.
 101     /// - You might want to handle `None` returned from [`from_u32`] instead of calling `unwrap`.
 102     ///
 103     /// [`from_u32`]: https://doc.rust-lang.org/std/char/fn.from_u32.html
 104     /// [`from_u32_unchecked`]: https://doc.rust-lang.org/std/char/fn.from_u32_unchecked.html
 105     ///
 106     /// **Example:**
 107     /// ```rust
 108     /// let x = 1_u32;
 109     /// unsafe {
 110     ///     let _: char = std::mem::transmute(x); // where x: u32
 111     /// }
 112     ///
 113     /// // should be:
 114     /// let _ = std::char::from_u32(x).unwrap();
 115     /// ```
 116     pub TRANSMUTE_INT_TO_CHAR,
 117     complexity,
 118     "transmutes from an integer to a `char`"
 119 }
 120
 121 declare_clippy_lint! {
 122     /// **What it does:** Checks for transmutes from a `&[u8]` to a `&str`.
 123     ///
 124     /// **Why is this bad?** Not every byte slice is a valid UTF-8 string.
 125     ///
 126     /// **Known problems:**
 127     /// - [`from_utf8`] which this lint suggests using is slower than `transmute`
 128     /// as it needs to validate the input.
 129     /// If you are certain that the input is always a valid UTF-8,
 130     /// use [`from_utf8_unchecked`] which is as fast as `transmute`
 131     /// but has a semantically meaningful name.
 132     /// - You might want to handle errors returned from [`from_utf8`] instead of calling `unwrap`.
 133     ///
 134     /// [`from_utf8`]: https://doc.rust-lang.org/std/str/fn.from_utf8.html
 135     /// [`from_utf8_unchecked`]: https://doc.rust-lang.org/std/str/fn.from_utf8_unchecked.html
 136     ///
 137     /// **Example:**
 138     /// ```rust
 139     /// let b: &[u8] = &[1_u8, 2_u8];
 140     /// unsafe {
 141     ///     let _: &str = std::mem::transmute(b); // where b: &[u8]
 142     /// }
 143     ///
 144     /// // should be:
 145     /// let _ = std::str::from_utf8(b).unwrap();
 146     /// ```
 147     pub TRANSMUTE_BYTES_TO_STR,
 148     complexity,
 149     "transmutes from a `&[u8]` to a `&str`"
 150 }
 151
 152 declare_clippy_lint! {
 153     /// **What it does:** Checks for transmutes from an integer to a `bool`.
 154     ///
 155     /// **Why is this bad?** This might result in an invalid in-memory representation of a `bool`.
 156     ///
 157     /// **Known problems:** None.
 158     ///
 159     /// **Example:**
 160     /// ```rust
 161     /// let x = 1_u8;
 162     /// unsafe {
 163     ///     let _: bool = std::mem::transmute(x); // where x: u8
 164     /// }
 165     ///
 166     /// // should be:
 167     /// let _: bool = x != 0;
 168     /// ```
 169     pub TRANSMUTE_INT_TO_BOOL,
 170     complexity,
 171     "transmutes from an integer to a `bool`"
 172 }
 173
 174 declare_clippy_lint! {
 175     /// **What it does:** Checks for transmutes from an integer to a float.
 176     ///
 177     /// **Why is this bad?** Transmutes are dangerous and error-prone, whereas `from_bits` is intuitive
 178     /// and safe.
 179     ///
 180     /// **Known problems:** None.
 181     ///
 182     /// **Example:**
 183     /// ```rust
 184     /// unsafe {
 185     ///     let _: f32 = std::mem::transmute(1_u32); // where x: u32
 186     /// }
 187     ///
 188     /// // should be:
 189     /// let _: f32 = f32::from_bits(1_u32);
 190     /// ```
 191     pub TRANSMUTE_INT_TO_FLOAT,
 192     complexity,
 193     "transmutes from an integer to a float"
 194 }
 195
 196 declare_clippy_lint! {
 197     /// **What it does:** Checks for transmutes from a float to an integer.
 198     ///
 199     /// **Why is this bad?** Transmutes are dangerous and error-prone, whereas `to_bits` is intuitive
 200     /// and safe.
 201     ///
 202     /// **Known problems:** None.
 203     ///
 204     /// **Example:**
 205     /// ```rust
 206     /// unsafe {
 207     ///     let _: u32 = std::mem::transmute(1f32);
 208     /// }
 209     ///
 210     /// // should be:
 211     /// let _: u32 = 1f32.to_bits();
 212     /// ```
 213     pub TRANSMUTE_FLOAT_TO_INT,
 214     complexity,
 215     "transmutes from a float to an integer"
 216 }
 217
 218 declare_clippy_lint! {
 219     /// **What it does:** Checks for transmutes from a pointer to a pointer, or
 220     /// from a reference to a reference.
 221     ///
 222     /// **Why is this bad?** Transmutes are dangerous, and these can instead be
 223     /// written as casts.
 224     ///
 225     /// **Known problems:** None.
 226     ///
 227     /// **Example:**
 228     /// ```rust
 229     /// let ptr = &1u32 as *const u32;
 230     /// unsafe {
 231     ///     // pointer-to-pointer transmute
 232     ///     let _: *const f32 = std::mem::transmute(ptr);
 233     ///     // ref-ref transmute
 234     ///     let _: &f32 = std::mem::transmute(&1u32);
 235     /// }
 236     /// // These can be respectively written:
 237     /// let _ = ptr as *const f32;
 238     /// let _ = unsafe{ &*(&1u32 as *const u32 as *const f32) };
 239     /// ```
 240     pub TRANSMUTE_PTR_TO_PTR,
 241     complexity,
 242     "transmutes from a pointer to a pointer / a reference to a reference"
 243 }
 244
 245 declare_clippy_lint! {
 246     /// **What it does:** Checks for transmutes between collections whose
 247     /// types have different ABI, size or alignment.
 248     ///
 249     /// **Why is this bad?** This is undefined behavior.
 250     ///
 251     /// **Known problems:** Currently, we cannot know whether a type is a
 252     /// collection, so we just lint the ones that come with `std`.
 253     ///
 254     /// **Example:**
 255     /// ```rust
 256     /// // different size, therefore likely out-of-bounds memory access
 257     /// // You absolutely do not want this in your code!
 258     /// unsafe {
 259     ///     std::mem::transmute::<_, Vec<u32>>(vec![2_u16])
 260     /// };
 261     /// ```
 262     ///
 263     /// You must always iterate, map and collect the values:
 264     ///
 265     /// ```rust
 266     /// vec![2_u16].into_iter().map(u32::from).collect::<Vec<_>>();
 267     /// ```
 268     pub UNSOUND_COLLECTION_TRANSMUTE,
 269     correctness,
 270     "transmute between collections of layout-incompatible types"
 271 }
 272 declare_lint_pass!(Transmute => [
 273     CROSSPOINTER_TRANSMUTE,
 274     TRANSMUTE_PTR_TO_REF,
 275     TRANSMUTE_PTR_TO_PTR,
 276     USELESS_TRANSMUTE,
 277     WRONG_TRANSMUTE,
 278     TRANSMUTE_INT_TO_CHAR,
 279     TRANSMUTE_BYTES_TO_STR,
 280     TRANSMUTE_INT_TO_BOOL,
 281     TRANSMUTE_INT_TO_FLOAT,
 282     TRANSMUTE_FLOAT_TO_INT,
 283     UNSOUND_COLLECTION_TRANSMUTE,
 284 ]);
 285
 286 // used to check for UNSOUND_COLLECTION_TRANSMUTE
 287 static COLLECTIONS: &[&[&str]] = &[
 288     &paths::VEC,
 289     &paths::VEC_DEQUE,
 290     &paths::BINARY_HEAP,
 291     &paths::BTREESET,
 292     &paths::BTREEMAP,
 293     &paths::HASHSET,
 294     &paths::HASHMAP,
 295 ];
 296 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Transmute {
 297     #[allow(clippy::similar_names, clippy::too_many_lines)]
 298     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, e: &'tcx Expr<'_>) {
 299         if_chain! {
 300             if let ExprKind::Call(ref path_expr, ref args) = e.kind;
 301             if let ExprKind::Path(ref qpath) = path_expr.kind;
 302             if let Some(def_id) = cx.tables.qpath_res(qpath, path_expr.hir_id).opt_def_id();
 303             if match_def_path(cx, def_id, &paths::TRANSMUTE);
 304             then {
 305                 let from_ty = cx.tables.expr_ty(&args[0]);
 306                 let to_ty = cx.tables.expr_ty(e);
 307
 308                 match (&from_ty.kind, &to_ty.kind) {
 309                     _ if from_ty == to_ty => span_lint(
 310                         cx,
 311                         USELESS_TRANSMUTE,
 312                         e.span,
 313                         &format!("transmute from a type (`{}`) to itself", from_ty),
 314                     ),
 315                     (&ty::Ref(_, rty, rty_mutbl), &ty::RawPtr(ptr_ty)) => span_lint_and_then(
 316                         cx,
 317                         USELESS_TRANSMUTE,
 318                         e.span,
 319                         "transmute from a reference to a pointer",
 320                         |diag| {
 321                             if let Some(arg) = sugg::Sugg::hir_opt(cx, &args[0]) {
 322                                 let rty_and_mut = ty::TypeAndMut {
 323                                     ty: rty,
 324                                     mutbl: rty_mutbl,
 325                                 };
 326
 327                                 let sugg = if ptr_ty == rty_and_mut {
 328                                     arg.as_ty(to_ty)
 329                                 } else {
 330                                     arg.as_ty(cx.tcx.mk_ptr(rty_and_mut)).as_ty(to_ty)
 331                                 };
 332
 333                                 diag.span_suggestion(e.span, "try", sugg.to_string(), Applicability::Unspecified);
 334                             }
 335                         },
 336                     ),
 337                     (&ty::Int(_), &ty::RawPtr(_)) | (&ty::Uint(_), &ty::RawPtr(_)) => span_lint_and_then(
 338                         cx,
 339                         USELESS_TRANSMUTE,
 340                         e.span,
 341                         "transmute from an integer to a pointer",
 342                         |diag| {
 343                             if let Some(arg) = sugg::Sugg::hir_opt(cx, &args[0]) {
 344                                 diag.span_suggestion(
 345                                     e.span,
 346                                     "try",
 347                                     arg.as_ty(&to_ty.to_string()).to_string(),
 348                                     Applicability::Unspecified,
 349                                 );
 350                             }
 351                         },
 352                     ),
 353                     (&ty::Float(_), &ty::Ref(..))
 354                     | (&ty::Float(_), &ty::RawPtr(_))
 355                     | (&ty::Char, &ty::Ref(..))
 356                     | (&ty::Char, &ty::RawPtr(_)) => span_lint(
 357                         cx,
 358                         WRONG_TRANSMUTE,
 359                         e.span,
 360                         &format!("transmute from a `{}` to a pointer", from_ty),
 361                     ),
 362                     (&ty::RawPtr(from_ptr), _) if from_ptr.ty == to_ty => span_lint(
 363                         cx,
 364                         CROSSPOINTER_TRANSMUTE,
 365                         e.span,
 366                         &format!(
 367                             "transmute from a type (`{}`) to the type that it points to (`{}`)",
 368                             from_ty, to_ty
 369                         ),
 370                     ),
 371                     (_, &ty::RawPtr(to_ptr)) if to_ptr.ty == from_ty => span_lint(
 372                         cx,
 373                         CROSSPOINTER_TRANSMUTE,
 374                         e.span,
 375                         &format!(
 376                             "transmute from a type (`{}`) to a pointer to that type (`{}`)",
 377                             from_ty, to_ty
 378                         ),
 379                     ),
 380                     (&ty::RawPtr(from_pty), &ty::Ref(_, to_ref_ty, mutbl)) => span_lint_and_then(
 381                         cx,
 382                         TRANSMUTE_PTR_TO_REF,
 383                         e.span,
 384                         &format!(
 385                             "transmute from a pointer type (`{}`) to a reference type \
 386                              (`{}`)",
 387                             from_ty, to_ty
 388                         ),
 389                         |diag| {
 390                             let arg = sugg::Sugg::hir(cx, &args[0], "..");
 391                             let (deref, cast) = if mutbl == Mutability::Mut {
 392                                 ("&mut *", "*mut")
 393                             } else {
 394                                 ("&*", "*const")
 395                             };
 396
 397                             let arg = if from_pty.ty == to_ref_ty {
 398                                 arg
 399                             } else {
 400                                 arg.as_ty(&format!("{} {}", cast, get_type_snippet(cx, qpath, to_ref_ty)))
 401                             };
 402
 403                             diag.span_suggestion(
 404                                 e.span,
 405                                 "try",
 406                                 sugg::make_unop(deref, arg).to_string(),
 407                                 Applicability::Unspecified,
 408                             );
 409                         },
 410                     ),
 411                     (&ty::Int(ast::IntTy::I32), &ty::Char) | (&ty::Uint(ast::UintTy::U32), &ty::Char) => {
 412                         span_lint_and_then(
 413                             cx,
 414                             TRANSMUTE_INT_TO_CHAR,
 415                             e.span,
 416                             &format!("transmute from a `{}` to a `char`", from_ty),
 417                             |diag| {
 418                                 let arg = sugg::Sugg::hir(cx, &args[0], "..");
 419                                 let arg = if let ty::Int(_) = from_ty.kind {
 420                                     arg.as_ty(ast::UintTy::U32.name_str())
 421                                 } else {
 422                                     arg
 423                                 };
 424                                 diag.span_suggestion(
 425                                     e.span,
 426                                     "consider using",
 427                                     format!("std::char::from_u32({}).unwrap()", arg.to_string()),
 428                                     Applicability::Unspecified,
 429                                 );
 430                             },
 431                         )
 432                     },
 433                     (&ty::Ref(_, ty_from, from_mutbl), &ty::Ref(_, ty_to, to_mutbl)) => {
 434                         if_chain! {
 435                             if let (&ty::Slice(slice_ty), &ty::Str) = (&ty_from.kind, &ty_to.kind);
 436                             if let ty::Uint(ast::UintTy::U8) = slice_ty.kind;
 437                             if from_mutbl == to_mutbl;
 438                             then {
 439                                 let postfix = if from_mutbl == Mutability::Mut {
 440                                     "_mut"
 441                                 } else {
 442                                     ""
 443                                 };
 444
 445                                 span_lint_and_sugg(
 446                                     cx,
 447                                     TRANSMUTE_BYTES_TO_STR,
 448                                     e.span,
 449                                     &format!("transmute from a `{}` to a `{}`", from_ty, to_ty),
 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                             } else {
 459                                 if cx.tcx.erase_regions(&from_ty) != cx.tcx.erase_regions(&to_ty) {
 460                                     span_lint_and_then(
 461                                         cx,
 462                                         TRANSMUTE_PTR_TO_PTR,
 463                                         e.span,
 464                                         "transmute from a reference to a reference",
 465                                         |diag| if let Some(arg) = sugg::Sugg::hir_opt(cx, &args[0]) {
 466                                             let ty_from_and_mut = ty::TypeAndMut {
 467                                                 ty: ty_from,
 468                                                 mutbl: from_mutbl
 469                                             };
 470                                             let ty_to_and_mut = ty::TypeAndMut { ty: ty_to, mutbl: to_mutbl };
 471                                             let sugg_paren = arg
 472                                                 .as_ty(cx.tcx.mk_ptr(ty_from_and_mut))
 473                                                 .as_ty(cx.tcx.mk_ptr(ty_to_and_mut));
 474                                             let sugg = if to_mutbl == Mutability::Mut {
 475                                                 sugg_paren.mut_addr_deref()
 476                                             } else {
 477                                                 sugg_paren.addr_deref()
 478                                             };
 479                                             diag.span_suggestion(
 480                                                 e.span,
 481                                                 "try",
 482                                                 sugg.to_string(),
 483                                                 Applicability::Unspecified,
 484                                             );
 485                                         },
 486                                     )
 487                                 }
 488                             }
 489                         }
 490                     },
 491                     (&ty::RawPtr(_), &ty::RawPtr(to_ty)) => span_lint_and_then(
 492                         cx,
 493                         TRANSMUTE_PTR_TO_PTR,
 494                         e.span,
 495                         "transmute from a pointer to a pointer",
 496                         |diag| {
 497                             if let Some(arg) = sugg::Sugg::hir_opt(cx, &args[0]) {
 498                                 let sugg = arg.as_ty(cx.tcx.mk_ptr(to_ty));
 499                                 diag.span_suggestion(e.span, "try", sugg.to_string(), Applicability::Unspecified);
 500                             }
 501                         },
 502                     ),
 503                     (&ty::Int(ast::IntTy::I8), &ty::Bool) | (&ty::Uint(ast::UintTy::U8), &ty::Bool) => {
 504                         span_lint_and_then(
 505                             cx,
 506                             TRANSMUTE_INT_TO_BOOL,
 507                             e.span,
 508                             &format!("transmute from a `{}` to a `bool`", from_ty),
 509                             |diag| {
 510                                 let arg = sugg::Sugg::hir(cx, &args[0], "..");
 511                                 let zero = sugg::Sugg::NonParen(Cow::from("0"));
 512                                 diag.span_suggestion(
 513                                     e.span,
 514                                     "consider using",
 515                                     sugg::make_binop(ast::BinOpKind::Ne, &arg, &zero).to_string(),
 516                                     Applicability::Unspecified,
 517                                 );
 518                             },
 519                         )
 520                     },
 521                     (&ty::Int(_), &ty::Float(_)) | (&ty::Uint(_), &ty::Float(_)) => span_lint_and_then(
 522                         cx,
 523                         TRANSMUTE_INT_TO_FLOAT,
 524                         e.span,
 525                         &format!("transmute from a `{}` to a `{}`", from_ty, to_ty),
 526                         |diag| {
 527                             let arg = sugg::Sugg::hir(cx, &args[0], "..");
 528                             let arg = if let ty::Int(int_ty) = from_ty.kind {
 529                                 arg.as_ty(format!(
 530                                     "u{}",
 531                                     int_ty.bit_width().map_or_else(|| "size".to_string(), |v| v.to_string())
 532                                 ))
 533                             } else {
 534                                 arg
 535                             };
 536                             diag.span_suggestion(
 537                                 e.span,
 538                                 "consider using",
 539                                 format!("{}::from_bits({})", to_ty, arg.to_string()),
 540                                 Applicability::Unspecified,
 541                             );
 542                         },
 543                     ),
 544                     (&ty::Float(float_ty), &ty::Int(_)) | (&ty::Float(float_ty), &ty::Uint(_)) => span_lint_and_then(
 545                         cx,
 546                         TRANSMUTE_FLOAT_TO_INT,
 547                         e.span,
 548                         &format!("transmute from a `{}` to a `{}`", from_ty, to_ty),
 549                         |diag| {
 550                             let mut expr = &args[0];
 551                             let mut arg = sugg::Sugg::hir(cx, expr, "..");
 552
 553                             if let ExprKind::Unary(UnOp::UnNeg, inner_expr) = &expr.kind {
 554                                 expr = &inner_expr;
 555                             }
 556
 557                             if_chain! {
 558                                 // if the expression is a float literal and it is unsuffixed then
 559                                 // add a suffix so the suggestion is valid and unambiguous
 560                                 let op = format!("{}{}", arg, float_ty.name_str()).into();
 561                                 if let ExprKind::Lit(lit) = &expr.kind;
 562                                 if let ast::LitKind::Float(_, ast::LitFloatType::Unsuffixed) = lit.node;
 563                                 then {
 564                                     match arg {
 565                                         sugg::Sugg::MaybeParen(_) => arg = sugg::Sugg::MaybeParen(op),
 566                                         _ => arg = sugg::Sugg::NonParen(op)
 567                                     }
 568                                 }
 569                             }
 570
 571                             arg = sugg::Sugg::NonParen(format!("{}.to_bits()", arg.maybe_par()).into());
 572
 573                             // cast the result of `to_bits` if `to_ty` is signed
 574                             arg = if let ty::Int(int_ty) = to_ty.kind {
 575                                 arg.as_ty(int_ty.name_str().to_string())
 576                             } else {
 577                                 arg
 578                             };
 579
 580                             diag.span_suggestion(
 581                                 e.span,
 582                                 "consider using",
 583                                 arg.to_string(),
 584                                 Applicability::Unspecified,
 585                             );
 586                         },
 587                     ),
 588                     (&ty::Adt(ref from_adt, ref from_substs), &ty::Adt(ref to_adt, ref to_substs)) => {
 589                         if from_adt.did != to_adt.did ||
 590                                 !COLLECTIONS.iter().any(|path| match_def_path(cx, to_adt.did, path)) {
 591                             return;
 592                         }
 593                         if from_substs.types().zip(to_substs.types())
 594                                               .any(|(from_ty, to_ty)| is_layout_incompatible(cx, from_ty, to_ty)) {
 595                             span_lint(
 596                                 cx,
 597                                 UNSOUND_COLLECTION_TRANSMUTE,
 598                                 e.span,
 599                                 &format!(
 600                                     "transmute from `{}` to `{}` with mismatched layout is unsound",
 601                                     from_ty,
 602                                     to_ty
 603                                 )
 604                             );
 605                         }
 606                     },
 607                     _ => return,
 608                 }
 609             }
 610         }
 611     }
 612 }
 613
 614 /// Gets the snippet of `Bar` in `…::transmute<Foo, &Bar>`. If that snippet is
 615 /// not available , use
 616 /// the type's `ToString` implementation. In weird cases it could lead to types
 617 /// with invalid `'_`
 618 /// lifetime, but it should be rare.
 619 fn get_type_snippet(cx: &LateContext<'_, '_>, path: &QPath<'_>, to_ref_ty: Ty<'_>) -> String {
 620     let seg = last_path_segment(path);
 621     if_chain! {
 622         if let Some(ref params) = seg.args;
 623         if !params.parenthesized;
 624         if let Some(to_ty) = params.args.iter().filter_map(|arg| match arg {
 625             GenericArg::Type(ty) => Some(ty),
 626             _ => None,
 627         }).nth(1);
 628         if let TyKind::Rptr(_, ref to_ty) = to_ty.kind;
 629         then {
 630             return snippet(cx, to_ty.ty.span, &to_ref_ty.to_string()).to_string();
 631         }
 632     }
 633
 634     to_ref_ty.to_string()
 635 }
 636
 637 // check if the component types of the transmuted collection and the result have different ABI,
 638 // size or alignment
 639 fn is_layout_incompatible<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, from: Ty<'tcx>, to: Ty<'tcx>) -> bool {
 640     let empty_param_env = ty::ParamEnv::empty();
 641     // check if `from` and `to` are normalizable to avoid ICE (#4968)
 642     if !(is_normalizable(cx, empty_param_env, from) && is_normalizable(cx, empty_param_env, to)) {
 643         return false;
 644     }
 645     let from_ty_layout = cx.tcx.layout_of(empty_param_env.and(from));
 646     let to_ty_layout = cx.tcx.layout_of(empty_param_env.and(to));
 647     if let (Ok(from_layout), Ok(to_layout)) = (from_ty_layout, to_ty_layout) {
 648         from_layout.size != to_layout.size || from_layout.align != to_layout.align || from_layout.abi != to_layout.abi
 649     } else {
 650         // no idea about layout, so don't lint
 651         false
 652     }
 653 }