clippy_lints/src/transmute/mod.rs

   1 mod useless_transmute;
   2 mod utils;
   3
   4 use utils::*;
   5
   6 use crate::utils::{
   7     in_constant, match_def_path, paths, snippet, span_lint, span_lint_and_sugg, span_lint_and_then, sugg,
   8 };
   9 use if_chain::if_chain;
  10 use rustc_ast as ast;
  11 use rustc_errors::Applicability;
  12 use rustc_hir::{Expr, ExprKind, Mutability, UnOp};
  13 use rustc_lint::{LateContext, LateLintPass};
  14 use rustc_middle::ty;
  15 use rustc_session::{declare_lint_pass, declare_tool_lint};
  16 use std::borrow::Cow;
  17
  18 declare_clippy_lint! {
  19     /// **What it does:** Checks for transmutes that can't ever be correct on any
  20     /// architecture.
  21     ///
  22     /// **Why is this bad?** It's basically guaranteed to be undefined behaviour.
  23     ///
  24     /// **Known problems:** When accessing C, users might want to store pointer
  25     /// sized objects in `extradata` arguments to save an allocation.
  26     ///
  27     /// **Example:**
  28     /// ```ignore
  29     /// let ptr: *const T = core::intrinsics::transmute('x')
  30     /// ```
  31     pub WRONG_TRANSMUTE,
  32     correctness,
  33     "transmutes that are confusing at best, undefined behaviour at worst and always useless"
  34 }
  35
  36 // FIXME: Move this to `complexity` again, after #5343 is fixed
  37 declare_clippy_lint! {
  38     /// **What it does:** Checks for transmutes to the original type of the object
  39     /// and transmutes that could be a cast.
  40     ///
  41     /// **Why is this bad?** Readability. The code tricks people into thinking that
  42     /// something complex is going on.
  43     ///
  44     /// **Known problems:** None.
  45     ///
  46     /// **Example:**
  47     /// ```rust,ignore
  48     /// core::intrinsics::transmute(t); // where the result type is the same as `t`'s
  49     /// ```
  50     pub USELESS_TRANSMUTE,
  51     nursery,
  52     "transmutes that have the same to and from types or could be a cast/coercion"
  53 }
  54
  55 // FIXME: Merge this lint with USELESS_TRANSMUTE once that is out of the nursery.
  56 declare_clippy_lint! {
  57     /// **What it does:**Checks for transmutes that could be a pointer cast.
  58     ///
  59     /// **Why is this bad?** Readability. The code tricks people into thinking that
  60     /// something complex is going on.
  61     ///
  62     /// **Known problems:** None.
  63     ///
  64     /// **Example:**
  65     ///
  66     /// ```rust
  67     /// # let p: *const [i32] = &[];
  68     /// unsafe { std::mem::transmute::<*const [i32], *const [u16]>(p) };
  69     /// ```
  70     /// Use instead:
  71     /// ```rust
  72     /// # let p: *const [i32] = &[];
  73     /// p as *const [u16];
  74     /// ```
  75     pub TRANSMUTES_EXPRESSIBLE_AS_PTR_CASTS,
  76     complexity,
  77     "transmutes that could be a pointer cast"
  78 }
  79
  80 declare_clippy_lint! {
  81     /// **What it does:** Checks for transmutes between a type `T` and `*T`.
  82     ///
  83     /// **Why is this bad?** It's easy to mistakenly transmute between a type and a
  84     /// pointer to that type.
  85     ///
  86     /// **Known problems:** None.
  87     ///
  88     /// **Example:**
  89     /// ```rust,ignore
  90     /// core::intrinsics::transmute(t) // where the result type is the same as
  91     ///                                // `*t` or `&t`'s
  92     /// ```
  93     pub CROSSPOINTER_TRANSMUTE,
  94     complexity,
  95     "transmutes that have to or from types that are a pointer to the other"
  96 }
  97
  98 declare_clippy_lint! {
  99     /// **What it does:** Checks for transmutes from a pointer to a reference.
 100     ///
 101     /// **Why is this bad?** This can always be rewritten with `&` and `*`.
 102     ///
 103     /// **Known problems:**
 104     /// - `mem::transmute` in statics and constants is stable from Rust 1.46.0,
 105     /// while dereferencing raw pointer is not stable yet.
 106     /// If you need to do this in those places,
 107     /// you would have to use `transmute` instead.
 108     ///
 109     /// **Example:**
 110     /// ```rust,ignore
 111     /// unsafe {
 112     ///     let _: &T = std::mem::transmute(p); // where p: *const T
 113     /// }
 114     ///
 115     /// // can be written:
 116     /// let _: &T = &*p;
 117     /// ```
 118     pub TRANSMUTE_PTR_TO_REF,
 119     complexity,
 120     "transmutes from a pointer to a reference type"
 121 }
 122
 123 declare_clippy_lint! {
 124     /// **What it does:** Checks for transmutes from an integer to a `char`.
 125     ///
 126     /// **Why is this bad?** Not every integer is a Unicode scalar value.
 127     ///
 128     /// **Known problems:**
 129     /// - [`from_u32`] which this lint suggests using is slower than `transmute`
 130     /// as it needs to validate the input.
 131     /// If you are certain that the input is always a valid Unicode scalar value,
 132     /// use [`from_u32_unchecked`] which is as fast as `transmute`
 133     /// but has a semantically meaningful name.
 134     /// - You might want to handle `None` returned from [`from_u32`] instead of calling `unwrap`.
 135     ///
 136     /// [`from_u32`]: https://doc.rust-lang.org/std/char/fn.from_u32.html
 137     /// [`from_u32_unchecked`]: https://doc.rust-lang.org/std/char/fn.from_u32_unchecked.html
 138     ///
 139     /// **Example:**
 140     /// ```rust
 141     /// let x = 1_u32;
 142     /// unsafe {
 143     ///     let _: char = std::mem::transmute(x); // where x: u32
 144     /// }
 145     ///
 146     /// // should be:
 147     /// let _ = std::char::from_u32(x).unwrap();
 148     /// ```
 149     pub TRANSMUTE_INT_TO_CHAR,
 150     complexity,
 151     "transmutes from an integer to a `char`"
 152 }
 153
 154 declare_clippy_lint! {
 155     /// **What it does:** Checks for transmutes from a `&[u8]` to a `&str`.
 156     ///
 157     /// **Why is this bad?** Not every byte slice is a valid UTF-8 string.
 158     ///
 159     /// **Known problems:**
 160     /// - [`from_utf8`] which this lint suggests using is slower than `transmute`
 161     /// as it needs to validate the input.
 162     /// If you are certain that the input is always a valid UTF-8,
 163     /// use [`from_utf8_unchecked`] which is as fast as `transmute`
 164     /// but has a semantically meaningful name.
 165     /// - You might want to handle errors returned from [`from_utf8`] instead of calling `unwrap`.
 166     ///
 167     /// [`from_utf8`]: https://doc.rust-lang.org/std/str/fn.from_utf8.html
 168     /// [`from_utf8_unchecked`]: https://doc.rust-lang.org/std/str/fn.from_utf8_unchecked.html
 169     ///
 170     /// **Example:**
 171     /// ```rust
 172     /// let b: &[u8] = &[1_u8, 2_u8];
 173     /// unsafe {
 174     ///     let _: &str = std::mem::transmute(b); // where b: &[u8]
 175     /// }
 176     ///
 177     /// // should be:
 178     /// let _ = std::str::from_utf8(b).unwrap();
 179     /// ```
 180     pub TRANSMUTE_BYTES_TO_STR,
 181     complexity,
 182     "transmutes from a `&[u8]` to a `&str`"
 183 }
 184
 185 declare_clippy_lint! {
 186     /// **What it does:** Checks for transmutes from an integer to a `bool`.
 187     ///
 188     /// **Why is this bad?** This might result in an invalid in-memory representation of a `bool`.
 189     ///
 190     /// **Known problems:** None.
 191     ///
 192     /// **Example:**
 193     /// ```rust
 194     /// let x = 1_u8;
 195     /// unsafe {
 196     ///     let _: bool = std::mem::transmute(x); // where x: u8
 197     /// }
 198     ///
 199     /// // should be:
 200     /// let _: bool = x != 0;
 201     /// ```
 202     pub TRANSMUTE_INT_TO_BOOL,
 203     complexity,
 204     "transmutes from an integer to a `bool`"
 205 }
 206
 207 declare_clippy_lint! {
 208     /// **What it does:** Checks for transmutes from an integer to a float.
 209     ///
 210     /// **Why is this bad?** Transmutes are dangerous and error-prone, whereas `from_bits` is intuitive
 211     /// and safe.
 212     ///
 213     /// **Known problems:** None.
 214     ///
 215     /// **Example:**
 216     /// ```rust
 217     /// unsafe {
 218     ///     let _: f32 = std::mem::transmute(1_u32); // where x: u32
 219     /// }
 220     ///
 221     /// // should be:
 222     /// let _: f32 = f32::from_bits(1_u32);
 223     /// ```
 224     pub TRANSMUTE_INT_TO_FLOAT,
 225     complexity,
 226     "transmutes from an integer to a float"
 227 }
 228
 229 declare_clippy_lint! {
 230     /// **What it does:** Checks for transmutes from a float to an integer.
 231     ///
 232     /// **Why is this bad?** Transmutes are dangerous and error-prone, whereas `to_bits` is intuitive
 233     /// and safe.
 234     ///
 235     /// **Known problems:** None.
 236     ///
 237     /// **Example:**
 238     /// ```rust
 239     /// unsafe {
 240     ///     let _: u32 = std::mem::transmute(1f32);
 241     /// }
 242     ///
 243     /// // should be:
 244     /// let _: u32 = 1f32.to_bits();
 245     /// ```
 246     pub TRANSMUTE_FLOAT_TO_INT,
 247     complexity,
 248     "transmutes from a float to an integer"
 249 }
 250
 251 declare_clippy_lint! {
 252     /// **What it does:** Checks for transmutes from a pointer to a pointer, or
 253     /// from a reference to a reference.
 254     ///
 255     /// **Why is this bad?** Transmutes are dangerous, and these can instead be
 256     /// written as casts.
 257     ///
 258     /// **Known problems:** None.
 259     ///
 260     /// **Example:**
 261     /// ```rust
 262     /// let ptr = &1u32 as *const u32;
 263     /// unsafe {
 264     ///     // pointer-to-pointer transmute
 265     ///     let _: *const f32 = std::mem::transmute(ptr);
 266     ///     // ref-ref transmute
 267     ///     let _: &f32 = std::mem::transmute(&1u32);
 268     /// }
 269     /// // These can be respectively written:
 270     /// let _ = ptr as *const f32;
 271     /// let _ = unsafe{ &*(&1u32 as *const u32 as *const f32) };
 272     /// ```
 273     pub TRANSMUTE_PTR_TO_PTR,
 274     complexity,
 275     "transmutes from a pointer to a pointer / a reference to a reference"
 276 }
 277
 278 declare_clippy_lint! {
 279     /// **What it does:** Checks for transmutes between collections whose
 280     /// types have different ABI, size or alignment.
 281     ///
 282     /// **Why is this bad?** This is undefined behavior.
 283     ///
 284     /// **Known problems:** Currently, we cannot know whether a type is a
 285     /// collection, so we just lint the ones that come with `std`.
 286     ///
 287     /// **Example:**
 288     /// ```rust
 289     /// // different size, therefore likely out-of-bounds memory access
 290     /// // You absolutely do not want this in your code!
 291     /// unsafe {
 292     ///     std::mem::transmute::<_, Vec<u32>>(vec![2_u16])
 293     /// };
 294     /// ```
 295     ///
 296     /// You must always iterate, map and collect the values:
 297     ///
 298     /// ```rust
 299     /// vec![2_u16].into_iter().map(u32::from).collect::<Vec<_>>();
 300     /// ```
 301     pub UNSOUND_COLLECTION_TRANSMUTE,
 302     correctness,
 303     "transmute between collections of layout-incompatible types"
 304 }
 305
 306 declare_lint_pass!(Transmute => [
 307     CROSSPOINTER_TRANSMUTE,
 308     TRANSMUTE_PTR_TO_REF,
 309     TRANSMUTE_PTR_TO_PTR,
 310     USELESS_TRANSMUTE,
 311     WRONG_TRANSMUTE,
 312     TRANSMUTE_INT_TO_CHAR,
 313     TRANSMUTE_BYTES_TO_STR,
 314     TRANSMUTE_INT_TO_BOOL,
 315     TRANSMUTE_INT_TO_FLOAT,
 316     TRANSMUTE_FLOAT_TO_INT,
 317     UNSOUND_COLLECTION_TRANSMUTE,
 318     TRANSMUTES_EXPRESSIBLE_AS_PTR_CASTS,
 319 ]);
 320
 321 // used to check for UNSOUND_COLLECTION_TRANSMUTE
 322 static COLLECTIONS: &[&[&str]] = &[
 323     &paths::VEC,
 324     &paths::VEC_DEQUE,
 325     &paths::BINARY_HEAP,
 326     &paths::BTREESET,
 327     &paths::BTREEMAP,
 328     &paths::HASHSET,
 329     &paths::HASHMAP,
 330 ];
 331
 332 impl<'tcx> LateLintPass<'tcx> for Transmute {
 333     #[allow(clippy::similar_names, clippy::too_many_lines)]
 334     fn check_expr(&mut self, cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) {
 335         if_chain! {
 336             if let ExprKind::Call(ref path_expr, ref args) = e.kind;
 337             if let ExprKind::Path(ref qpath) = path_expr.kind;
 338             if let Some(def_id) = cx.qpath_res(qpath, path_expr.hir_id).opt_def_id();
 339             if match_def_path(cx, def_id, &paths::TRANSMUTE);
 340             then {
 341                 // Avoid suggesting from/to bits and dereferencing raw pointers in const contexts.
 342                 // See https://github.com/rust-lang/rust/issues/73736 for progress on making them `const fn`.
 343                 // And see https://github.com/rust-lang/rust/issues/51911 for dereferencing raw pointers.
 344                 let const_context = in_constant(cx, e.hir_id);
 345
 346                 let from_ty = cx.typeck_results().expr_ty(&args[0]);
 347                 let to_ty = cx.typeck_results().expr_ty(e);
 348
 349                 let triggered = useless_transmute::check(cx, e, from_ty, to_ty, args);
 350                 if triggered {
 351                     return;
 352                 }
 353
 354                 match (&from_ty.kind(), &to_ty.kind()) {
 355                     (ty::Float(_) | ty::Char, ty::Ref(..) | ty::RawPtr(_)) => span_lint(
 356                         cx,
 357                         WRONG_TRANSMUTE,
 358                         e.span,
 359                         &format!("transmute from a `{}` to a pointer", from_ty),
 360                     ),
 361                     (ty::RawPtr(from_ptr), _) if from_ptr.ty == to_ty => span_lint(
 362                         cx,
 363                         CROSSPOINTER_TRANSMUTE,
 364                         e.span,
 365                         &format!(
 366                             "transmute from a type (`{}`) to the type that it points to (`{}`)",
 367                             from_ty, to_ty
 368                         ),
 369                     ),
 370                     (_, ty::RawPtr(to_ptr)) if to_ptr.ty == from_ty => span_lint(
 371                         cx,
 372                         CROSSPOINTER_TRANSMUTE,
 373                         e.span,
 374                         &format!(
 375                             "transmute from a type (`{}`) to a pointer to that type (`{}`)",
 376                             from_ty, to_ty
 377                         ),
 378                     ),
 379                     (ty::RawPtr(from_pty), ty::Ref(_, to_ref_ty, mutbl)) => span_lint_and_then(
 380                         cx,
 381                         TRANSMUTE_PTR_TO_REF,
 382                         e.span,
 383                         &format!(
 384                             "transmute from a pointer type (`{}`) to a reference type \
 385                              (`{}`)",
 386                             from_ty, to_ty
 387                         ),
 388                         |diag| {
 389                             let arg = sugg::Sugg::hir(cx, &args[0], "..");
 390                             let (deref, cast) = if *mutbl == Mutability::Mut {
 391                                 ("&mut *", "*mut")
 392                             } else {
 393                                 ("&*", "*const")
 394                             };
 395
 396                             let arg = if from_pty.ty == *to_ref_ty {
 397                                 arg
 398                             } else {
 399                                 arg.as_ty(&format!("{} {}", cast, get_type_snippet(cx, qpath, to_ref_ty)))
 400                             };
 401
 402                             diag.span_suggestion(
 403                                 e.span,
 404                                 "try",
 405                                 sugg::make_unop(deref, arg).to_string(),
 406                                 Applicability::Unspecified,
 407                             );
 408                         },
 409                     ),
 410                     (ty::Int(ty::IntTy::I32) | ty::Uint(ty::UintTy::U32), &ty::Char) => {
 411                         span_lint_and_then(
 412                             cx,
 413                             TRANSMUTE_INT_TO_CHAR,
 414                             e.span,
 415                             &format!("transmute from a `{}` to a `char`", from_ty),
 416                             |diag| {
 417                                 let arg = sugg::Sugg::hir(cx, &args[0], "..");
 418                                 let arg = if let ty::Int(_) = from_ty.kind() {
 419                                     arg.as_ty(ast::UintTy::U32.name_str())
 420                                 } else {
 421                                     arg
 422                                 };
 423                                 diag.span_suggestion(
 424                                     e.span,
 425                                     "consider using",
 426                                     format!("std::char::from_u32({}).unwrap()", arg.to_string()),
 427                                     Applicability::Unspecified,
 428                                 );
 429                             },
 430                         )
 431                     },
 432                     (ty::Ref(_, ty_from, from_mutbl), ty::Ref(_, ty_to, to_mutbl)) => {
 433                         if_chain! {
 434                             if let (&ty::Slice(slice_ty), &ty::Str) = (&ty_from.kind(), &ty_to.kind());
 435                             if let ty::Uint(ty::UintTy::U8) = slice_ty.kind();
 436                             if from_mutbl == to_mutbl;
 437                             then {
 438                                 let postfix = if *from_mutbl == Mutability::Mut {
 439                                     "_mut"
 440                                 } else {
 441                                     ""
 442                                 };
 443
 444                                 span_lint_and_sugg(
 445                                     cx,
 446                                     TRANSMUTE_BYTES_TO_STR,
 447                                     e.span,
 448                                     &format!("transmute from a `{}` to a `{}`", from_ty, to_ty),
 449                                     "consider using",
 450                                     format!(
 451                                         "std::str::from_utf8{}({}).unwrap()",
 452                                         postfix,
 453                                         snippet(cx, args[0].span, ".."),
 454                                     ),
 455                                     Applicability::Unspecified,
 456                                 );
 457                             } else {
 458                                 if (cx.tcx.erase_regions(from_ty) != cx.tcx.erase_regions(to_ty))
 459                                     && !const_context {
 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(ty::IntTy::I8) | ty::Uint(ty::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::Uint(_), ty::Float(_)) if !const_context => 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::Uint(_)) if !const_context => 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::Neg, 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(from_adt, from_substs), ty::Adt(to_adt, 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                     (_, _) if can_be_expressed_as_pointer_cast(cx, e, from_ty, to_ty) => span_lint_and_then(
 608                         cx,
 609                         TRANSMUTES_EXPRESSIBLE_AS_PTR_CASTS,
 610                         e.span,
 611                         &format!(
 612                             "transmute from `{}` to `{}` which could be expressed as a pointer cast instead",
 613                             from_ty,
 614                             to_ty
 615                         ),
 616                         |diag| {
 617                             if let Some(arg) = sugg::Sugg::hir_opt(cx, &args[0]) {
 618                                 let sugg = arg.as_ty(&to_ty.to_string()).to_string();
 619                                 diag.span_suggestion(e.span, "try", sugg, Applicability::MachineApplicable);
 620                             }
 621                         }
 622                     ),
 623                     _ => {
 624                         return;
 625                     },
 626                 }
 627             }
 628         }
 629     }
 630 }