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
 273 declare_clippy_lint! {
 274     /// **What it does:**
 275     ///
 276     /// **Why is this bad?**
 277     ///
 278     /// **Known problems:** None.
 279     ///
 280     /// **Example:**
 281     ///
 282     /// ```rust
 283     /// // example code where clippy issues a warning
 284     /// ```
 285     /// Use instead:
 286     /// ```rust
 287     /// // example code which does not raise clippy warning
 288     /// ```
 289     pub TRANSMUTES_EXPRESSIBLE_AS_PTR_CASTS,
 290     complexity,
 291     "default lint description"
 292 }
 293
 294 declare_lint_pass!(Transmute => [
 295     CROSSPOINTER_TRANSMUTE,
 296     TRANSMUTE_PTR_TO_REF,
 297     TRANSMUTE_PTR_TO_PTR,
 298     USELESS_TRANSMUTE,
 299     WRONG_TRANSMUTE,
 300     TRANSMUTE_INT_TO_CHAR,
 301     TRANSMUTE_BYTES_TO_STR,
 302     TRANSMUTE_INT_TO_BOOL,
 303     TRANSMUTE_INT_TO_FLOAT,
 304     TRANSMUTE_FLOAT_TO_INT,
 305     UNSOUND_COLLECTION_TRANSMUTE,
 306     TRANSMUTES_EXPRESSIBLE_AS_PTR_CASTS,
 307 ]);
 308
 309 // used to check for UNSOUND_COLLECTION_TRANSMUTE
 310 static COLLECTIONS: &[&[&str]] = &[
 311     &paths::VEC,
 312     &paths::VEC_DEQUE,
 313     &paths::BINARY_HEAP,
 314     &paths::BTREESET,
 315     &paths::BTREEMAP,
 316     &paths::HASHSET,
 317     &paths::HASHMAP,
 318 ];
 319 impl<'tcx> LateLintPass<'tcx> for Transmute {
 320     #[allow(clippy::similar_names, clippy::too_many_lines)]
 321     fn check_expr(&mut self, cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) {
 322         if_chain! {
 323             if let ExprKind::Call(ref path_expr, ref args) = e.kind;
 324             if let ExprKind::Path(ref qpath) = path_expr.kind;
 325             if let Some(def_id) = cx.qpath_res(qpath, path_expr.hir_id).opt_def_id();
 326             if match_def_path(cx, def_id, &paths::TRANSMUTE);
 327             then {
 328                 let from_ty = cx.typeck_results().expr_ty(&args[0]);
 329                 let to_ty = cx.typeck_results().expr_ty(e);
 330
 331                 match (&from_ty.kind, &to_ty.kind) {
 332                     _ if from_ty == to_ty => span_lint(
 333                         cx,
 334                         USELESS_TRANSMUTE,
 335                         e.span,
 336                         &format!("transmute from a type (`{}`) to itself", from_ty),
 337                     ),
 338                     (ty::Ref(_, rty, rty_mutbl), ty::RawPtr(ptr_ty)) => span_lint_and_then(
 339                         cx,
 340                         USELESS_TRANSMUTE,
 341                         e.span,
 342                         "transmute from a reference to a pointer",
 343                         |diag| {
 344                             if let Some(arg) = sugg::Sugg::hir_opt(cx, &args[0]) {
 345                                 let rty_and_mut = ty::TypeAndMut {
 346                                     ty: rty,
 347                                     mutbl: *rty_mutbl,
 348                                 };
 349
 350                                 let sugg = if *ptr_ty == rty_and_mut {
 351                                     arg.as_ty(to_ty)
 352                                 } else {
 353                                     arg.as_ty(cx.tcx.mk_ptr(rty_and_mut)).as_ty(to_ty)
 354                                 };
 355
 356                                 diag.span_suggestion(e.span, "try", sugg.to_string(), Applicability::Unspecified);
 357                             }
 358                         },
 359                     ),
 360                     (ty::Int(_) | ty::Uint(_), ty::RawPtr(_)) => span_lint_and_then(
 361                         cx,
 362                         USELESS_TRANSMUTE,
 363                         e.span,
 364                         "transmute from an integer to a pointer",
 365                         |diag| {
 366                             if let Some(arg) = sugg::Sugg::hir_opt(cx, &args[0]) {
 367                                 diag.span_suggestion(
 368                                     e.span,
 369                                     "try",
 370                                     arg.as_ty(&to_ty.to_string()).to_string(),
 371                                     Applicability::Unspecified,
 372                                 );
 373                             }
 374                         },
 375                     ),
 376                     (ty::Float(_) | ty::Char, ty::Ref(..) | ty::RawPtr(_)) => span_lint(
 377                         cx,
 378                         WRONG_TRANSMUTE,
 379                         e.span,
 380                         &format!("transmute from a `{}` to a pointer", from_ty),
 381                     ),
 382                     (ty::RawPtr(from_ptr), _) if from_ptr.ty == to_ty => span_lint(
 383                         cx,
 384                         CROSSPOINTER_TRANSMUTE,
 385                         e.span,
 386                         &format!(
 387                             "transmute from a type (`{}`) to the type that it points to (`{}`)",
 388                             from_ty, to_ty
 389                         ),
 390                     ),
 391                     (_, ty::RawPtr(to_ptr)) if to_ptr.ty == from_ty => span_lint(
 392                         cx,
 393                         CROSSPOINTER_TRANSMUTE,
 394                         e.span,
 395                         &format!(
 396                             "transmute from a type (`{}`) to a pointer to that type (`{}`)",
 397                             from_ty, to_ty
 398                         ),
 399                     ),
 400                     (ty::RawPtr(from_pty), ty::Ref(_, to_ref_ty, mutbl)) => span_lint_and_then(
 401                         cx,
 402                         TRANSMUTE_PTR_TO_REF,
 403                         e.span,
 404                         &format!(
 405                             "transmute from a pointer type (`{}`) to a reference type \
 406                              (`{}`)",
 407                             from_ty, to_ty
 408                         ),
 409                         |diag| {
 410                             let arg = sugg::Sugg::hir(cx, &args[0], "..");
 411                             let (deref, cast) = if *mutbl == Mutability::Mut {
 412                                 ("&mut *", "*mut")
 413                             } else {
 414                                 ("&*", "*const")
 415                             };
 416
 417                             let arg = if from_pty.ty == *to_ref_ty {
 418                                 arg
 419                             } else {
 420                                 arg.as_ty(&format!("{} {}", cast, get_type_snippet(cx, qpath, to_ref_ty)))
 421                             };
 422
 423                             diag.span_suggestion(
 424                                 e.span,
 425                                 "try",
 426                                 sugg::make_unop(deref, arg).to_string(),
 427                                 Applicability::Unspecified,
 428                             );
 429                         },
 430                     ),
 431                     (ty::Int(ast::IntTy::I32) | ty::Uint(ast::UintTy::U32), &ty::Char) => {
 432                         span_lint_and_then(
 433                             cx,
 434                             TRANSMUTE_INT_TO_CHAR,
 435                             e.span,
 436                             &format!("transmute from a `{}` to a `char`", from_ty),
 437                             |diag| {
 438                                 let arg = sugg::Sugg::hir(cx, &args[0], "..");
 439                                 let arg = if let ty::Int(_) = from_ty.kind {
 440                                     arg.as_ty(ast::UintTy::U32.name_str())
 441                                 } else {
 442                                     arg
 443                                 };
 444                                 diag.span_suggestion(
 445                                     e.span,
 446                                     "consider using",
 447                                     format!("std::char::from_u32({}).unwrap()", arg.to_string()),
 448                                     Applicability::Unspecified,
 449                                 );
 450                             },
 451                         )
 452                     },
 453                     (ty::Ref(_, ty_from, from_mutbl), ty::Ref(_, ty_to, to_mutbl)) => {
 454                         if_chain! {
 455                             if let (&ty::Slice(slice_ty), &ty::Str) = (&ty_from.kind, &ty_to.kind);
 456                             if let ty::Uint(ast::UintTy::U8) = slice_ty.kind;
 457                             if from_mutbl == to_mutbl;
 458                             then {
 459                                 let postfix = if *from_mutbl == Mutability::Mut {
 460                                     "_mut"
 461                                 } else {
 462                                     ""
 463                                 };
 464
 465                                 span_lint_and_sugg(
 466                                     cx,
 467                                     TRANSMUTE_BYTES_TO_STR,
 468                                     e.span,
 469                                     &format!("transmute from a `{}` to a `{}`", from_ty, to_ty),
 470                                     "consider using",
 471                                     format!(
 472                                         "std::str::from_utf8{}({}).unwrap()",
 473                                         postfix,
 474                                         snippet(cx, args[0].span, ".."),
 475                                     ),
 476                                     Applicability::Unspecified,
 477                                 );
 478                             } else {
 479                                 if cx.tcx.erase_regions(&from_ty) != cx.tcx.erase_regions(&to_ty) {
 480                                     span_lint_and_then(
 481                                         cx,
 482                                         TRANSMUTE_PTR_TO_PTR,
 483                                         e.span,
 484                                         "transmute from a reference to a reference",
 485                                         |diag| if let Some(arg) = sugg::Sugg::hir_opt(cx, &args[0]) {
 486                                             let ty_from_and_mut = ty::TypeAndMut {
 487                                                 ty: ty_from,
 488                                                 mutbl: *from_mutbl
 489                                             };
 490                                             let ty_to_and_mut = ty::TypeAndMut { ty: ty_to, mutbl: *to_mutbl };
 491                                             let sugg_paren = arg
 492                                                 .as_ty(cx.tcx.mk_ptr(ty_from_and_mut))
 493                                                 .as_ty(cx.tcx.mk_ptr(ty_to_and_mut));
 494                                             let sugg = if *to_mutbl == Mutability::Mut {
 495                                                 sugg_paren.mut_addr_deref()
 496                                             } else {
 497                                                 sugg_paren.addr_deref()
 498                                             };
 499                                             diag.span_suggestion(
 500                                                 e.span,
 501                                                 "try",
 502                                                 sugg.to_string(),
 503                                                 Applicability::Unspecified,
 504                                             );
 505                                         },
 506                                     )
 507                                 }
 508                             }
 509                         }
 510                     },
 511                     (ty::RawPtr(_), ty::RawPtr(to_ty)) => span_lint_and_then(
 512                         cx,
 513                         TRANSMUTE_PTR_TO_PTR,
 514                         e.span,
 515                         "transmute from a pointer to a pointer",
 516                         |diag| {
 517                             if let Some(arg) = sugg::Sugg::hir_opt(cx, &args[0]) {
 518                                 let sugg = arg.as_ty(cx.tcx.mk_ptr(*to_ty));
 519                                 diag.span_suggestion(e.span, "try", sugg.to_string(), Applicability::Unspecified);
 520                             }
 521                         },
 522                     ),
 523                     (ty::Int(ast::IntTy::I8) | ty::Uint(ast::UintTy::U8), ty::Bool) => {
 524                         span_lint_and_then(
 525                             cx,
 526                             TRANSMUTE_INT_TO_BOOL,
 527                             e.span,
 528                             &format!("transmute from a `{}` to a `bool`", from_ty),
 529                             |diag| {
 530                                 let arg = sugg::Sugg::hir(cx, &args[0], "..");
 531                                 let zero = sugg::Sugg::NonParen(Cow::from("0"));
 532                                 diag.span_suggestion(
 533                                     e.span,
 534                                     "consider using",
 535                                     sugg::make_binop(ast::BinOpKind::Ne, &arg, &zero).to_string(),
 536                                     Applicability::Unspecified,
 537                                 );
 538                             },
 539                         )
 540                     },
 541                     (ty::Int(_) | ty::Uint(_), ty::Float(_)) => span_lint_and_then(
 542                         cx,
 543                         TRANSMUTE_INT_TO_FLOAT,
 544                         e.span,
 545                         &format!("transmute from a `{}` to a `{}`", from_ty, to_ty),
 546                         |diag| {
 547                             let arg = sugg::Sugg::hir(cx, &args[0], "..");
 548                             let arg = if let ty::Int(int_ty) = from_ty.kind {
 549                                 arg.as_ty(format!(
 550                                     "u{}",
 551                                     int_ty.bit_width().map_or_else(|| "size".to_string(), |v| v.to_string())
 552                                 ))
 553                             } else {
 554                                 arg
 555                             };
 556                             diag.span_suggestion(
 557                                 e.span,
 558                                 "consider using",
 559                                 format!("{}::from_bits({})", to_ty, arg.to_string()),
 560                                 Applicability::Unspecified,
 561                             );
 562                         },
 563                     ),
 564                     (ty::Float(float_ty), ty::Int(_) | ty::Uint(_)) => span_lint_and_then(
 565                         cx,
 566                         TRANSMUTE_FLOAT_TO_INT,
 567                         e.span,
 568                         &format!("transmute from a `{}` to a `{}`", from_ty, to_ty),
 569                         |diag| {
 570                             let mut expr = &args[0];
 571                             let mut arg = sugg::Sugg::hir(cx, expr, "..");
 572
 573                             if let ExprKind::Unary(UnOp::UnNeg, inner_expr) = &expr.kind {
 574                                 expr = &inner_expr;
 575                             }
 576
 577                             if_chain! {
 578                                 // if the expression is a float literal and it is unsuffixed then
 579                                 // add a suffix so the suggestion is valid and unambiguous
 580                                 let op = format!("{}{}", arg, float_ty.name_str()).into();
 581                                 if let ExprKind::Lit(lit) = &expr.kind;
 582                                 if let ast::LitKind::Float(_, ast::LitFloatType::Unsuffixed) = lit.node;
 583                                 then {
 584                                     match arg {
 585                                         sugg::Sugg::MaybeParen(_) => arg = sugg::Sugg::MaybeParen(op),
 586                                         _ => arg = sugg::Sugg::NonParen(op)
 587                                     }
 588                                 }
 589                             }
 590
 591                             arg = sugg::Sugg::NonParen(format!("{}.to_bits()", arg.maybe_par()).into());
 592
 593                             // cast the result of `to_bits` if `to_ty` is signed
 594                             arg = if let ty::Int(int_ty) = to_ty.kind {
 595                                 arg.as_ty(int_ty.name_str().to_string())
 596                             } else {
 597                                 arg
 598                             };
 599
 600                             diag.span_suggestion(
 601                                 e.span,
 602                                 "consider using",
 603                                 arg.to_string(),
 604                                 Applicability::Unspecified,
 605                             );
 606                         },
 607                     ),
 608                     (ty::Adt(from_adt, from_substs), ty::Adt(to_adt, to_substs)) => {
 609                         if from_adt.did != to_adt.did ||
 610                                 !COLLECTIONS.iter().any(|path| match_def_path(cx, to_adt.did, path)) {
 611                             return;
 612                         }
 613                         if from_substs.types().zip(to_substs.types())
 614                                               .any(|(from_ty, to_ty)| is_layout_incompatible(cx, from_ty, to_ty)) {
 615                             span_lint(
 616                                 cx,
 617                                 UNSOUND_COLLECTION_TRANSMUTE,
 618                                 e.span,
 619                                 &format!(
 620                                     "transmute from `{}` to `{}` with mismatched layout is unsound",
 621                                     from_ty,
 622                                     to_ty
 623                                 )
 624                             );
 625                         }
 626                     },
 627                     _ => return,
 628                 }
 629             }
 630         }
 631     }
 632 }
 633
 634 /// Gets the snippet of `Bar` in `…::transmute<Foo, &Bar>`. If that snippet is
 635 /// not available , use
 636 /// the type's `ToString` implementation. In weird cases it could lead to types
 637 /// with invalid `'_`
 638 /// lifetime, but it should be rare.
 639 fn get_type_snippet(cx: &LateContext<'_>, path: &QPath<'_>, to_ref_ty: Ty<'_>) -> String {
 640     let seg = last_path_segment(path);
 641     if_chain! {
 642         if let Some(ref params) = seg.args;
 643         if !params.parenthesized;
 644         if let Some(to_ty) = params.args.iter().filter_map(|arg| match arg {
 645             GenericArg::Type(ty) => Some(ty),
 646             _ => None,
 647         }).nth(1);
 648         if let TyKind::Rptr(_, ref to_ty) = to_ty.kind;
 649         then {
 650             return snippet(cx, to_ty.ty.span, &to_ref_ty.to_string()).to_string();
 651         }
 652     }
 653
 654     to_ref_ty.to_string()
 655 }
 656
 657 // check if the component types of the transmuted collection and the result have different ABI,
 658 // size or alignment
 659 fn is_layout_incompatible<'tcx>(cx: &LateContext<'tcx>, from: Ty<'tcx>, to: Ty<'tcx>) -> bool {
 660     let empty_param_env = ty::ParamEnv::empty();
 661     // check if `from` and `to` are normalizable to avoid ICE (#4968)
 662     if !(is_normalizable(cx, empty_param_env, from) && is_normalizable(cx, empty_param_env, to)) {
 663         return false;
 664     }
 665     let from_ty_layout = cx.tcx.layout_of(empty_param_env.and(from));
 666     let to_ty_layout = cx.tcx.layout_of(empty_param_env.and(to));
 667     if let (Ok(from_layout), Ok(to_layout)) = (from_ty_layout, to_ty_layout) {
 668         from_layout.size != to_layout.size || from_layout.align != to_layout.align || from_layout.abi != to_layout.abi
 669     } else {
 670         // no idea about layout, so don't lint
 671         false
 672     }
 673 }