Auto merge of #6716 - magurotuna:refactor-transmute-mod, r=flip1995

Refactor: arrange transmute lints

This PR arranges `transmute` lints so that they can be accessed more easily.
Basically, I followed the instruction described in #6680 as to how to do the refactoring.

- `declare_clippy_lint!` and `impl LintPass` is placed in `transmute/mod.rs`
- Uitlity functions is placed in `transmute/utils.rs`
- Each lint function about `transmute` is moved into its own module, like `transmute/useless_transmute.rs`

For ease of review, I refactored step by step, keeping each commit small. For instance, all I did in
2451781 was to move  `useless_transmute` into its own module.

---

changelog: Refactor `transmute.rs` file into `transmute` module.

15 files changed, 1062 insertions, 763 deletions

diff --git a/clippy_lints/src/transmute.rs b/clippy_lints/src/transmute.rs
deleted file mode 100644
index dc938ed0238..00000000000
--- a/clippy_lints/src/transmute.rs
+++ /dev/null
@@ -1,763 +0,0 @@
-use crate::utils::{
-    in_constant, is_normalizable, last_path_segment, match_def_path, paths, snippet, span_lint, span_lint_and_sugg,
-    span_lint_and_then, sugg,
-};
-use if_chain::if_chain;
-use rustc_ast as ast;
-use rustc_errors::Applicability;
-use rustc_hir::{Expr, ExprKind, GenericArg, Mutability, QPath, TyKind, UnOp};
-use rustc_lint::{LateContext, LateLintPass};
-use rustc_middle::ty::{self, cast::CastKind, Ty};
-use rustc_session::{declare_lint_pass, declare_tool_lint};
-use rustc_span::DUMMY_SP;
-use rustc_typeck::check::{cast::CastCheck, FnCtxt, Inherited};
-use std::borrow::Cow;
-
-declare_clippy_lint! {
-    /// **What it does:** Checks for transmutes that can't ever be correct on any
-    /// architecture.
-    ///
-    /// **Why is this bad?** It's basically guaranteed to be undefined behaviour.
-    ///
-    /// **Known problems:** When accessing C, users might want to store pointer
-    /// sized objects in `extradata` arguments to save an allocation.
-    ///
-    /// **Example:**
-    /// ```ignore
-    /// let ptr: *const T = core::intrinsics::transmute('x')
-    /// ```
-    pub WRONG_TRANSMUTE,
-    correctness,
-    "transmutes that are confusing at best, undefined behaviour at worst and always useless"
-}
-
-// FIXME: Move this to `complexity` again, after #5343 is fixed
-declare_clippy_lint! {
-    /// **What it does:** Checks for transmutes to the original type of the object
-    /// and transmutes that could be a cast.
-    ///
-    /// **Why is this bad?** Readability. The code tricks people into thinking that
-    /// something complex is going on.
-    ///
-    /// **Known problems:** None.
-    ///
-    /// **Example:**
-    /// ```rust,ignore
-    /// core::intrinsics::transmute(t); // where the result type is the same as `t`'s
-    /// ```
-    pub USELESS_TRANSMUTE,
-    nursery,
-    "transmutes that have the same to and from types or could be a cast/coercion"
-}
-
-// FIXME: Merge this lint with USELESS_TRANSMUTE once that is out of the nursery.
-declare_clippy_lint! {
-    /// **What it does:**Checks for transmutes that could be a pointer cast.
-    ///
-    /// **Why is this bad?** Readability. The code tricks people into thinking that
-    /// something complex is going on.
-    ///
-    /// **Known problems:** None.
-    ///
-    /// **Example:**
-    ///
-    /// ```rust
-    /// # let p: *const [i32] = &[];
-    /// unsafe { std::mem::transmute::<*const [i32], *const [u16]>(p) };
-    /// ```
-    /// Use instead:
-    /// ```rust
-    /// # let p: *const [i32] = &[];
-    /// p as *const [u16];
-    /// ```
-    pub TRANSMUTES_EXPRESSIBLE_AS_PTR_CASTS,
-    complexity,
-    "transmutes that could be a pointer cast"
-}
-
-declare_clippy_lint! {
-    /// **What it does:** Checks for transmutes between a type `T` and `*T`.
-    ///
-    /// **Why is this bad?** It's easy to mistakenly transmute between a type and a
-    /// pointer to that type.
-    ///
-    /// **Known problems:** None.
-    ///
-    /// **Example:**
-    /// ```rust,ignore
-    /// core::intrinsics::transmute(t) // where the result type is the same as
-    ///                                // `*t` or `&t`'s
-    /// ```
-    pub CROSSPOINTER_TRANSMUTE,
-    complexity,
-    "transmutes that have to or from types that are a pointer to the other"
-}
-
-declare_clippy_lint! {
-    /// **What it does:** Checks for transmutes from a pointer to a reference.
-    ///
-    /// **Why is this bad?** This can always be rewritten with `&` and `*`.
-    ///
-    /// **Known problems:**
-    /// - `mem::transmute` in statics and constants is stable from Rust 1.46.0,
-    /// while dereferencing raw pointer is not stable yet.
-    /// If you need to do this in those places,
-    /// you would have to use `transmute` instead.
-    ///
-    /// **Example:**
-    /// ```rust,ignore
-    /// unsafe {
-    ///     let _: &T = std::mem::transmute(p); // where p: *const T
-    /// }
-    ///
-    /// // can be written:
-    /// let _: &T = &*p;
-    /// ```
-    pub TRANSMUTE_PTR_TO_REF,
-    complexity,
-    "transmutes from a pointer to a reference type"
-}
-
-declare_clippy_lint! {
-    /// **What it does:** Checks for transmutes from an integer to a `char`.
-    ///
-    /// **Why is this bad?** Not every integer is a Unicode scalar value.
-    ///
-    /// **Known problems:**
-    /// - [`from_u32`] which this lint suggests using is slower than `transmute`
-    /// as it needs to validate the input.
-    /// If you are certain that the input is always a valid Unicode scalar value,
-    /// use [`from_u32_unchecked`] which is as fast as `transmute`
-    /// but has a semantically meaningful name.
-    /// - You might want to handle `None` returned from [`from_u32`] instead of calling `unwrap`.
-    ///
-    /// [`from_u32`]: https://doc.rust-lang.org/std/char/fn.from_u32.html
-    /// [`from_u32_unchecked`]: https://doc.rust-lang.org/std/char/fn.from_u32_unchecked.html
-    ///
-    /// **Example:**
-    /// ```rust
-    /// let x = 1_u32;
-    /// unsafe {
-    ///     let _: char = std::mem::transmute(x); // where x: u32
-    /// }
-    ///
-    /// // should be:
-    /// let _ = std::char::from_u32(x).unwrap();
-    /// ```
-    pub TRANSMUTE_INT_TO_CHAR,
-    complexity,
-    "transmutes from an integer to a `char`"
-}
-
-declare_clippy_lint! {
-    /// **What it does:** Checks for transmutes from a `&[u8]` to a `&str`.
-    ///
-    /// **Why is this bad?** Not every byte slice is a valid UTF-8 string.
-    ///
-    /// **Known problems:**
-    /// - [`from_utf8`] which this lint suggests using is slower than `transmute`
-    /// as it needs to validate the input.
-    /// If you are certain that the input is always a valid UTF-8,
-    /// use [`from_utf8_unchecked`] which is as fast as `transmute`
-    /// but has a semantically meaningful name.
-    /// - You might want to handle errors returned from [`from_utf8`] instead of calling `unwrap`.
-    ///
-    /// [`from_utf8`]: https://doc.rust-lang.org/std/str/fn.from_utf8.html
-    /// [`from_utf8_unchecked`]: https://doc.rust-lang.org/std/str/fn.from_utf8_unchecked.html
-    ///
-    /// **Example:**
-    /// ```rust
-    /// let b: &[u8] = &[1_u8, 2_u8];
-    /// unsafe {
-    ///     let _: &str = std::mem::transmute(b); // where b: &[u8]
-    /// }
-    ///
-    /// // should be:
-    /// let _ = std::str::from_utf8(b).unwrap();
-    /// ```
-    pub TRANSMUTE_BYTES_TO_STR,
-    complexity,
-    "transmutes from a `&[u8]` to a `&str`"
-}
-
-declare_clippy_lint! {
-    /// **What it does:** Checks for transmutes from an integer to a `bool`.
-    ///
-    /// **Why is this bad?** This might result in an invalid in-memory representation of a `bool`.
-    ///
-    /// **Known problems:** None.
-    ///
-    /// **Example:**
-    /// ```rust
-    /// let x = 1_u8;
-    /// unsafe {
-    ///     let _: bool = std::mem::transmute(x); // where x: u8
-    /// }
-    ///
-    /// // should be:
-    /// let _: bool = x != 0;
-    /// ```
-    pub TRANSMUTE_INT_TO_BOOL,
-    complexity,
-    "transmutes from an integer to a `bool`"
-}
-
-declare_clippy_lint! {
-    /// **What it does:** Checks for transmutes from an integer to a float.
-    ///
-    /// **Why is this bad?** Transmutes are dangerous and error-prone, whereas `from_bits` is intuitive
-    /// and safe.
-    ///
-    /// **Known problems:** None.
-    ///
-    /// **Example:**
-    /// ```rust
-    /// unsafe {
-    ///     let _: f32 = std::mem::transmute(1_u32); // where x: u32
-    /// }
-    ///
-    /// // should be:
-    /// let _: f32 = f32::from_bits(1_u32);
-    /// ```
-    pub TRANSMUTE_INT_TO_FLOAT,
-    complexity,
-    "transmutes from an integer to a float"
-}
-
-declare_clippy_lint! {
-    /// **What it does:** Checks for transmutes from a float to an integer.
-    ///
-    /// **Why is this bad?** Transmutes are dangerous and error-prone, whereas `to_bits` is intuitive
-    /// and safe.
-    ///
-    /// **Known problems:** None.
-    ///
-    /// **Example:**
-    /// ```rust
-    /// unsafe {
-    ///     let _: u32 = std::mem::transmute(1f32);
-    /// }
-    ///
-    /// // should be:
-    /// let _: u32 = 1f32.to_bits();
-    /// ```
-    pub TRANSMUTE_FLOAT_TO_INT,
-    complexity,
-    "transmutes from a float to an integer"
-}
-
-declare_clippy_lint! {
-    /// **What it does:** Checks for transmutes from a pointer to a pointer, or
-    /// from a reference to a reference.
-    ///
-    /// **Why is this bad?** Transmutes are dangerous, and these can instead be
-    /// written as casts.
-    ///
-    /// **Known problems:** None.
-    ///
-    /// **Example:**
-    /// ```rust
-    /// let ptr = &1u32 as *const u32;
-    /// unsafe {
-    ///     // pointer-to-pointer transmute
-    ///     let _: *const f32 = std::mem::transmute(ptr);
-    ///     // ref-ref transmute
-    ///     let _: &f32 = std::mem::transmute(&1u32);
-    /// }
-    /// // These can be respectively written:
-    /// let _ = ptr as *const f32;
-    /// let _ = unsafe{ &*(&1u32 as *const u32 as *const f32) };
-    /// ```
-    pub TRANSMUTE_PTR_TO_PTR,
-    complexity,
-    "transmutes from a pointer to a pointer / a reference to a reference"
-}
-
-declare_clippy_lint! {
-    /// **What it does:** Checks for transmutes between collections whose
-    /// types have different ABI, size or alignment.
-    ///
-    /// **Why is this bad?** This is undefined behavior.
-    ///
-    /// **Known problems:** Currently, we cannot know whether a type is a
-    /// collection, so we just lint the ones that come with `std`.
-    ///
-    /// **Example:**
-    /// ```rust
-    /// // different size, therefore likely out-of-bounds memory access
-    /// // You absolutely do not want this in your code!
-    /// unsafe {
-    ///     std::mem::transmute::<_, Vec<u32>>(vec![2_u16])
-    /// };
-    /// ```
-    ///
-    /// You must always iterate, map and collect the values:
-    ///
-    /// ```rust
-    /// vec![2_u16].into_iter().map(u32::from).collect::<Vec<_>>();
-    /// ```
-    pub UNSOUND_COLLECTION_TRANSMUTE,
-    correctness,
-    "transmute between collections of layout-incompatible types"
-}
-
-declare_lint_pass!(Transmute => [
-    CROSSPOINTER_TRANSMUTE,
-    TRANSMUTE_PTR_TO_REF,
-    TRANSMUTE_PTR_TO_PTR,
-    USELESS_TRANSMUTE,
-    WRONG_TRANSMUTE,
-    TRANSMUTE_INT_TO_CHAR,
-    TRANSMUTE_BYTES_TO_STR,
-    TRANSMUTE_INT_TO_BOOL,
-    TRANSMUTE_INT_TO_FLOAT,
-    TRANSMUTE_FLOAT_TO_INT,
-    UNSOUND_COLLECTION_TRANSMUTE,
-    TRANSMUTES_EXPRESSIBLE_AS_PTR_CASTS,
-]);
-
-// used to check for UNSOUND_COLLECTION_TRANSMUTE
-static COLLECTIONS: &[&[&str]] = &[
-    &paths::VEC,
-    &paths::VEC_DEQUE,
-    &paths::BINARY_HEAP,
-    &paths::BTREESET,
-    &paths::BTREEMAP,
-    &paths::HASHSET,
-    &paths::HASHMAP,
-];
-impl<'tcx> LateLintPass<'tcx> for Transmute {
-    #[allow(clippy::similar_names, clippy::too_many_lines)]
-    fn check_expr(&mut self, cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) {
-        if_chain! {
-            if let ExprKind::Call(ref path_expr, ref args) = e.kind;
-            if let ExprKind::Path(ref qpath) = path_expr.kind;
-            if let Some(def_id) = cx.qpath_res(qpath, path_expr.hir_id).opt_def_id();
-            if match_def_path(cx, def_id, &paths::TRANSMUTE);
-            then {
-                // Avoid suggesting from/to bits and dereferencing raw pointers in const contexts.
-                // See https://github.com/rust-lang/rust/issues/73736 for progress on making them `const fn`.
-                // And see https://github.com/rust-lang/rust/issues/51911 for dereferencing raw pointers.
-                let const_context = in_constant(cx, e.hir_id);
-
-                let from_ty = cx.typeck_results().expr_ty(&args[0]);
-                let to_ty = cx.typeck_results().expr_ty(e);
-
-                match (&from_ty.kind(), &to_ty.kind()) {
-                    _ if from_ty == to_ty => span_lint(
-                        cx,
-                        USELESS_TRANSMUTE,
-                        e.span,
-                        &format!("transmute from a type (`{}`) to itself", from_ty),
-                    ),
-                    (ty::Ref(_, rty, rty_mutbl), ty::RawPtr(ptr_ty)) => span_lint_and_then(
-                        cx,
-                        USELESS_TRANSMUTE,
-                        e.span,
-                        "transmute from a reference to a pointer",
-                        |diag| {
-                            if let Some(arg) = sugg::Sugg::hir_opt(cx, &args[0]) {
-                                let rty_and_mut = ty::TypeAndMut {
-                                    ty: rty,
-                                    mutbl: *rty_mutbl,
-                                };
-
-                                let sugg = if *ptr_ty == rty_and_mut {
-                                    arg.as_ty(to_ty)
-                                } else {
-                                    arg.as_ty(cx.tcx.mk_ptr(rty_and_mut)).as_ty(to_ty)
-                                };
-
-                                diag.span_suggestion(e.span, "try", sugg.to_string(), Applicability::Unspecified);
-                            }
-                        },
-                    ),
-                    (ty::Int(_) | ty::Uint(_), ty::RawPtr(_)) => span_lint_and_then(
-                        cx,
-                        USELESS_TRANSMUTE,
-                        e.span,
-                        "transmute from an integer to a pointer",
-                        |diag| {
-                            if let Some(arg) = sugg::Sugg::hir_opt(cx, &args[0]) {
-                                diag.span_suggestion(
-                                    e.span,
-                                    "try",
-                                    arg.as_ty(&to_ty.to_string()).to_string(),
-                                    Applicability::Unspecified,
-                                );
-                            }
-                        },
-                    ),
-                    (ty::Float(_) | ty::Char, ty::Ref(..) | ty::RawPtr(_)) => span_lint(
-                        cx,
-                        WRONG_TRANSMUTE,
-                        e.span,
-                        &format!("transmute from a `{}` to a pointer", from_ty),
-                    ),
-                    (ty::RawPtr(from_ptr), _) if from_ptr.ty == to_ty => span_lint(
-                        cx,
-                        CROSSPOINTER_TRANSMUTE,
-                        e.span,
-                        &format!(
-                            "transmute from a type (`{}`) to the type that it points to (`{}`)",
-                            from_ty, to_ty
-                        ),
-                    ),
-                    (_, ty::RawPtr(to_ptr)) if to_ptr.ty == from_ty => span_lint(
-                        cx,
-                        CROSSPOINTER_TRANSMUTE,
-                        e.span,
-                        &format!(
-                            "transmute from a type (`{}`) to a pointer to that type (`{}`)",
-                            from_ty, to_ty
-                        ),
-                    ),
-                    (ty::RawPtr(from_pty), ty::Ref(_, to_ref_ty, mutbl)) => span_lint_and_then(
-                        cx,
-                        TRANSMUTE_PTR_TO_REF,
-                        e.span,
-                        &format!(
-                            "transmute from a pointer type (`{}`) to a reference type \
-                             (`{}`)",
-                            from_ty, to_ty
-                        ),
-                        |diag| {
-                            let arg = sugg::Sugg::hir(cx, &args[0], "..");
-                            let (deref, cast) = if *mutbl == Mutability::Mut {
-                                ("&mut *", "*mut")
-                            } else {
-                                ("&*", "*const")
-                            };
-
-                            let arg = if from_pty.ty == *to_ref_ty {
-                                arg
-                            } else {
-                                arg.as_ty(&format!("{} {}", cast, get_type_snippet(cx, qpath, to_ref_ty)))
-                            };
-
-                            diag.span_suggestion(
-                                e.span,
-                                "try",
-                                sugg::make_unop(deref, arg).to_string(),
-                                Applicability::Unspecified,
-                            );
-                        },
-                    ),
-                    (ty::Int(ty::IntTy::I32) | ty::Uint(ty::UintTy::U32), &ty::Char) => {
-                        span_lint_and_then(
-                            cx,
-                            TRANSMUTE_INT_TO_CHAR,
-                            e.span,
-                            &format!("transmute from a `{}` to a `char`", from_ty),
-                            |diag| {
-                                let arg = sugg::Sugg::hir(cx, &args[0], "..");
-                                let arg = if let ty::Int(_) = from_ty.kind() {
-                                    arg.as_ty(ast::UintTy::U32.name_str())
-                                } else {
-                                    arg
-                                };
-                                diag.span_suggestion(
-                                    e.span,
-                                    "consider using",
-                                    format!("std::char::from_u32({}).unwrap()", arg.to_string()),
-                                    Applicability::Unspecified,
-                                );
-                            },
-                        )
-                    },
-                    (ty::Ref(_, ty_from, from_mutbl), ty::Ref(_, ty_to, to_mutbl)) => {
-                        if_chain! {
-                            if let (&ty::Slice(slice_ty), &ty::Str) = (&ty_from.kind(), &ty_to.kind());
-                            if let ty::Uint(ty::UintTy::U8) = slice_ty.kind();
-                            if from_mutbl == to_mutbl;
-                            then {
-                                let postfix = if *from_mutbl == Mutability::Mut {
-                                    "_mut"
-                                } else {
-                                    ""
-                                };
-
-                                span_lint_and_sugg(
-                                    cx,
-                                    TRANSMUTE_BYTES_TO_STR,
-                                    e.span,
-                                    &format!("transmute from a `{}` to a `{}`", from_ty, to_ty),
-                                    "consider using",
-                                    format!(
-                                        "std::str::from_utf8{}({}).unwrap()",
-                                        postfix,
-                                        snippet(cx, args[0].span, ".."),
-                                    ),
-                                    Applicability::Unspecified,
-                                );
-                            } else {
-                                if (cx.tcx.erase_regions(from_ty) != cx.tcx.erase_regions(to_ty))
-                                    && !const_context {
-                                    span_lint_and_then(
-                                        cx,
-                                        TRANSMUTE_PTR_TO_PTR,
-                                        e.span,
-                                        "transmute from a reference to a reference",
-                                        |diag| if let Some(arg) = sugg::Sugg::hir_opt(cx, &args[0]) {
-                                            let ty_from_and_mut = ty::TypeAndMut {
-                                                ty: ty_from,
-                                                mutbl: *from_mutbl
-                                            };
-                                            let ty_to_and_mut = ty::TypeAndMut { ty: ty_to, mutbl: *to_mutbl };
-                                            let sugg_paren = arg
-                                                .as_ty(cx.tcx.mk_ptr(ty_from_and_mut))
-                                                .as_ty(cx.tcx.mk_ptr(ty_to_and_mut));
-                                            let sugg = if *to_mutbl == Mutability::Mut {
-                                                sugg_paren.mut_addr_deref()
-                                            } else {
-                                                sugg_paren.addr_deref()
-                                            };
-                                            diag.span_suggestion(
-                                                e.span,
-                                                "try",
-                                                sugg.to_string(),
-                                                Applicability::Unspecified,
-                                            );
-                                        },
-                                    )
-                                }
-                            }
-                        }
-                    },
-                    (ty::RawPtr(_), ty::RawPtr(to_ty)) => span_lint_and_then(
-                        cx,
-                        TRANSMUTE_PTR_TO_PTR,
-                        e.span,
-                        "transmute from a pointer to a pointer",
-                        |diag| {
-                            if let Some(arg) = sugg::Sugg::hir_opt(cx, &args[0]) {
-                                let sugg = arg.as_ty(cx.tcx.mk_ptr(*to_ty));
-                                diag.span_suggestion(e.span, "try", sugg.to_string(), Applicability::Unspecified);
-                            }
-                        },
-                    ),
-                    (ty::Int(ty::IntTy::I8) | ty::Uint(ty::UintTy::U8), ty::Bool) => {
-                        span_lint_and_then(
-                            cx,
-                            TRANSMUTE_INT_TO_BOOL,
-                            e.span,
-                            &format!("transmute from a `{}` to a `bool`", from_ty),
-                            |diag| {
-                                let arg = sugg::Sugg::hir(cx, &args[0], "..");
-                                let zero = sugg::Sugg::NonParen(Cow::from("0"));
-                                diag.span_suggestion(
-                                    e.span,
-                                    "consider using",
-                                    sugg::make_binop(ast::BinOpKind::Ne, &arg, &zero).to_string(),
-                                    Applicability::Unspecified,
-                                );
-                            },
-                        )
-                    },
-                    (ty::Int(_) | ty::Uint(_), ty::Float(_)) if !const_context => span_lint_and_then(
-                        cx,
-                        TRANSMUTE_INT_TO_FLOAT,
-                        e.span,
-                        &format!("transmute from a `{}` to a `{}`", from_ty, to_ty),
-                        |diag| {
-                            let arg = sugg::Sugg::hir(cx, &args[0], "..");
-                            let arg = if let ty::Int(int_ty) = from_ty.kind() {
-                                arg.as_ty(format!(
-                                    "u{}",
-                                    int_ty.bit_width().map_or_else(|| "size".to_string(), |v| v.to_string())
-                                ))
-                            } else {
-                                arg
-                            };
-                            diag.span_suggestion(
-                                e.span,
-                                "consider using",
-                                format!("{}::from_bits({})", to_ty, arg.to_string()),
-                                Applicability::Unspecified,
-                            );
-                        },
-                    ),
-                    (ty::Float(float_ty), ty::Int(_) | ty::Uint(_)) if !const_context => span_lint_and_then(
-                        cx,
-                        TRANSMUTE_FLOAT_TO_INT,
-                        e.span,
-                        &format!("transmute from a `{}` to a `{}`", from_ty, to_ty),
-                        |diag| {
-                            let mut expr = &args[0];
-                            let mut arg = sugg::Sugg::hir(cx, expr, "..");
-
-                            if let ExprKind::Unary(UnOp::Neg, inner_expr) = &expr.kind {
-                                expr = &inner_expr;
-                            }
-
-                            if_chain! {
-                                // if the expression is a float literal and it is unsuffixed then
-                                // add a suffix so the suggestion is valid and unambiguous
-                                let op = format!("{}{}", arg, float_ty.name_str()).into();
-                                if let ExprKind::Lit(lit) = &expr.kind;
-                                if let ast::LitKind::Float(_, ast::LitFloatType::Unsuffixed) = lit.node;
-                                then {
-                                    match arg {
-                                        sugg::Sugg::MaybeParen(_) => arg = sugg::Sugg::MaybeParen(op),
-                                        _ => arg = sugg::Sugg::NonParen(op)
-                                    }
-                                }
-                            }
-
-                            arg = sugg::Sugg::NonParen(format!("{}.to_bits()", arg.maybe_par()).into());
-
-                            // cast the result of `to_bits` if `to_ty` is signed
-                            arg = if let ty::Int(int_ty) = to_ty.kind() {
-                                arg.as_ty(int_ty.name_str().to_string())
-                            } else {
-                                arg
-                            };
-
-                            diag.span_suggestion(
-                                e.span,
-                                "consider using",
-                                arg.to_string(),
-                                Applicability::Unspecified,
-                            );
-                        },
-                    ),
-                    (ty::Adt(from_adt, from_substs), ty::Adt(to_adt, to_substs)) => {
-                        if from_adt.did != to_adt.did ||
-                                !COLLECTIONS.iter().any(|path| match_def_path(cx, to_adt.did, path)) {
-                            return;
-                        }
-                        if from_substs.types().zip(to_substs.types())
-                                              .any(|(from_ty, to_ty)| is_layout_incompatible(cx, from_ty, to_ty)) {
-                            span_lint(
-                                cx,
-                                UNSOUND_COLLECTION_TRANSMUTE,
-                                e.span,
-                                &format!(
-                                    "transmute from `{}` to `{}` with mismatched layout is unsound",
-                                    from_ty,
-                                    to_ty
-                                )
-                            );
-                        }
-                    },
-                    (_, _) if can_be_expressed_as_pointer_cast(cx, e, from_ty, to_ty) => span_lint_and_then(
-                        cx,
-                        TRANSMUTES_EXPRESSIBLE_AS_PTR_CASTS,
-                        e.span,
-                        &format!(
-                            "transmute from `{}` to `{}` which could be expressed as a pointer cast instead",
-                            from_ty,
-                            to_ty
-                        ),
-                        |diag| {
-                            if let Some(arg) = sugg::Sugg::hir_opt(cx, &args[0]) {
-                                let sugg = arg.as_ty(&to_ty.to_string()).to_string();
-                                diag.span_suggestion(e.span, "try", sugg, Applicability::MachineApplicable);
-                            }
-                        }
-                    ),
-                    _ => {
-                        return;
-                    },
-                }
-            }
-        }
-    }
-}
-
-/// Gets the snippet of `Bar` in `…::transmute<Foo, &Bar>`. If that snippet is
-/// not available , use
-/// the type's `ToString` implementation. In weird cases it could lead to types
-/// with invalid `'_`
-/// lifetime, but it should be rare.
-fn get_type_snippet(cx: &LateContext<'_>, path: &QPath<'_>, to_ref_ty: Ty<'_>) -> String {
-    let seg = last_path_segment(path);
-    if_chain! {
-        if let Some(ref params) = seg.args;
-        if !params.parenthesized;
-        if let Some(to_ty) = params.args.iter().filter_map(|arg| match arg {
-            GenericArg::Type(ty) => Some(ty),
-            _ => None,
-        }).nth(1);
-        if let TyKind::Rptr(_, ref to_ty) = to_ty.kind;
-        then {
-            return snippet(cx, to_ty.ty.span, &to_ref_ty.to_string()).to_string();
-        }
-    }
-
-    to_ref_ty.to_string()
-}
-
-// check if the component types of the transmuted collection and the result have different ABI,
-// size or alignment
-fn is_layout_incompatible<'tcx>(cx: &LateContext<'tcx>, from: Ty<'tcx>, to: Ty<'tcx>) -> bool {
-    let empty_param_env = ty::ParamEnv::empty();
-    // check if `from` and `to` are normalizable to avoid ICE (#4968)
-    if !(is_normalizable(cx, empty_param_env, from) && is_normalizable(cx, empty_param_env, to)) {
-        return false;
-    }
-    let from_ty_layout = cx.tcx.layout_of(empty_param_env.and(from));
-    let to_ty_layout = cx.tcx.layout_of(empty_param_env.and(to));
-    if let (Ok(from_layout), Ok(to_layout)) = (from_ty_layout, to_ty_layout) {
-        from_layout.size != to_layout.size || from_layout.align != to_layout.align || from_layout.abi != to_layout.abi
-    } else {
-        // no idea about layout, so don't lint
-        false
-    }
-}
-
-/// Check if the type conversion can be expressed as a pointer cast, instead of
-/// a transmute. In certain cases, including some invalid casts from array
-/// references to pointers, this may cause additional errors to be emitted and/or
-/// ICE error messages. This function will panic if that occurs.
-fn can_be_expressed_as_pointer_cast<'tcx>(
-    cx: &LateContext<'tcx>,
-    e: &'tcx Expr<'_>,
-    from_ty: Ty<'tcx>,
-    to_ty: Ty<'tcx>,
-) -> bool {
-    use CastKind::{AddrPtrCast, ArrayPtrCast, FnPtrAddrCast, FnPtrPtrCast, PtrAddrCast, PtrPtrCast};
-    matches!(
-        check_cast(cx, e, from_ty, to_ty),
-        Some(PtrPtrCast | PtrAddrCast | AddrPtrCast | ArrayPtrCast | FnPtrPtrCast | FnPtrAddrCast)
-    )
-}
-
-/// If a cast from `from_ty` to `to_ty` is valid, returns an Ok containing the kind of
-/// the cast. In certain cases, including some invalid casts from array references
-/// to pointers, this may cause additional errors to be emitted and/or ICE error
-/// messages. This function will panic if that occurs.
-fn check_cast<'tcx>(cx: &LateContext<'tcx>, e: &'tcx Expr<'_>, from_ty: Ty<'tcx>, to_ty: Ty<'tcx>) -> Option<CastKind> {
-    let hir_id = e.hir_id;
-    let local_def_id = hir_id.owner;
-
-    Inherited::build(cx.tcx, local_def_id).enter(|inherited| {
-        let fn_ctxt = FnCtxt::new(&inherited, cx.param_env, hir_id);
-
-        // If we already have errors, we can't be sure we can pointer cast.
-        assert!(
-            !fn_ctxt.errors_reported_since_creation(),
-            "Newly created FnCtxt contained errors"
-        );
-
-        if let Ok(check) = CastCheck::new(
-            &fn_ctxt, e, from_ty, to_ty,
-            // We won't show any error to the user, so we don't care what the span is here.
-            DUMMY_SP, DUMMY_SP,
-        ) {
-            let res = check.do_check(&fn_ctxt);
-
-            // do_check's documentation says that it might return Ok and create
-            // errors in the fcx instead of returing Err in some cases. Those cases
-            // should be filtered out before getting here.
-            assert!(
-                !fn_ctxt.errors_reported_since_creation(),
-                "`fn_ctxt` contained errors after cast check!"
-            );
-
-            res.ok()
-        } else {
-            None
-        }
-    })
-}
diff --git a/clippy_lints/src/transmute/crosspointer_transmute.rs b/clippy_lints/src/transmute/crosspointer_transmute.rs
new file mode 100644
index 00000000000..ce87defaa94
--- /dev/null
+++ b/clippy_lints/src/transmute/crosspointer_transmute.rs
@@ -0,0 +1,37 @@
+use super::CROSSPOINTER_TRANSMUTE;
+use crate::utils::span_lint;
+use rustc_hir::Expr;
+use rustc_lint::LateContext;
+use rustc_middle::ty::{self, Ty};
+
+/// Checks for `crosspointer_transmute` lint.
+/// Returns `true` if it's triggered, otherwise returns `false`.
+pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, e: &'tcx Expr<'_>, from_ty: Ty<'tcx>, to_ty: Ty<'tcx>) -> bool {
+    match (&from_ty.kind(), &to_ty.kind()) {
+        (ty::RawPtr(from_ptr), _) if from_ptr.ty == to_ty => {
+            span_lint(
+                cx,
+                CROSSPOINTER_TRANSMUTE,
+                e.span,
+                &format!(
+                    "transmute from a type (`{}`) to the type that it points to (`{}`)",
+                    from_ty, to_ty
+                ),
+            );
+            true
+        },
+        (_, ty::RawPtr(to_ptr)) if to_ptr.ty == from_ty => {
+            span_lint(
+                cx,
+                CROSSPOINTER_TRANSMUTE,
+                e.span,
+                &format!(
+                    "transmute from a type (`{}`) to a pointer to that type (`{}`)",
+                    from_ty, to_ty
+                ),
+            );
+            true
+        },
+        _ => false,
+    }
+}
diff --git a/clippy_lints/src/transmute/mod.rs b/clippy_lints/src/transmute/mod.rs
new file mode 100644
index 00000000000..c1870f5208b
--- /dev/null
+++ b/clippy_lints/src/transmute/mod.rs
@@ -0,0 +1,363 @@
+mod crosspointer_transmute;
+mod transmute_float_to_int;
+mod transmute_int_to_bool;
+mod transmute_int_to_char;
+mod transmute_int_to_float;
+mod transmute_ptr_to_ptr;
+mod transmute_ptr_to_ref;
+mod transmute_ref_to_ref;
+mod transmutes_expressible_as_ptr_casts;
+mod unsound_collection_transmute;
+mod useless_transmute;
+mod utils;
+mod wrong_transmute;
+
+use crate::utils::{in_constant, match_def_path, paths};
+use if_chain::if_chain;
+use rustc_hir::{Expr, ExprKind};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for transmutes that can't ever be correct on any
+    /// architecture.
+    ///
+    /// **Why is this bad?** It's basically guaranteed to be undefined behaviour.
+    ///
+    /// **Known problems:** When accessing C, users might want to store pointer
+    /// sized objects in `extradata` arguments to save an allocation.
+    ///
+    /// **Example:**
+    /// ```ignore
+    /// let ptr: *const T = core::intrinsics::transmute('x')
+    /// ```
+    pub WRONG_TRANSMUTE,
+    correctness,
+    "transmutes that are confusing at best, undefined behaviour at worst and always useless"
+}
+
+// FIXME: Move this to `complexity` again, after #5343 is fixed
+declare_clippy_lint! {
+    /// **What it does:** Checks for transmutes to the original type of the object
+    /// and transmutes that could be a cast.
+    ///
+    /// **Why is this bad?** Readability. The code tricks people into thinking that
+    /// something complex is going on.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    /// ```rust,ignore
+    /// core::intrinsics::transmute(t); // where the result type is the same as `t`'s
+    /// ```
+    pub USELESS_TRANSMUTE,
+    nursery,
+    "transmutes that have the same to and from types or could be a cast/coercion"
+}
+
+// FIXME: Merge this lint with USELESS_TRANSMUTE once that is out of the nursery.
+declare_clippy_lint! {
+    /// **What it does:**Checks for transmutes that could be a pointer cast.
+    ///
+    /// **Why is this bad?** Readability. The code tricks people into thinking that
+    /// something complex is going on.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    ///
+    /// ```rust
+    /// # let p: *const [i32] = &[];
+    /// unsafe { std::mem::transmute::<*const [i32], *const [u16]>(p) };
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// # let p: *const [i32] = &[];
+    /// p as *const [u16];
+    /// ```
+    pub TRANSMUTES_EXPRESSIBLE_AS_PTR_CASTS,
+    complexity,
+    "transmutes that could be a pointer cast"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for transmutes between a type `T` and `*T`.
+    ///
+    /// **Why is this bad?** It's easy to mistakenly transmute between a type and a
+    /// pointer to that type.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    /// ```rust,ignore
+    /// core::intrinsics::transmute(t) // where the result type is the same as
+    ///                                // `*t` or `&t`'s
+    /// ```
+    pub CROSSPOINTER_TRANSMUTE,
+    complexity,
+    "transmutes that have to or from types that are a pointer to the other"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for transmutes from a pointer to a reference.
+    ///
+    /// **Why is this bad?** This can always be rewritten with `&` and `*`.
+    ///
+    /// **Known problems:**
+    /// - `mem::transmute` in statics and constants is stable from Rust 1.46.0,
+    /// while dereferencing raw pointer is not stable yet.
+    /// If you need to do this in those places,
+    /// you would have to use `transmute` instead.
+    ///
+    /// **Example:**
+    /// ```rust,ignore
+    /// unsafe {
+    ///     let _: &T = std::mem::transmute(p); // where p: *const T
+    /// }
+    ///
+    /// // can be written:
+    /// let _: &T = &*p;
+    /// ```
+    pub TRANSMUTE_PTR_TO_REF,
+    complexity,
+    "transmutes from a pointer to a reference type"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for transmutes from an integer to a `char`.
+    ///
+    /// **Why is this bad?** Not every integer is a Unicode scalar value.
+    ///
+    /// **Known problems:**
+    /// - [`from_u32`] which this lint suggests using is slower than `transmute`
+    /// as it needs to validate the input.
+    /// If you are certain that the input is always a valid Unicode scalar value,
+    /// use [`from_u32_unchecked`] which is as fast as `transmute`
+    /// but has a semantically meaningful name.
+    /// - You might want to handle `None` returned from [`from_u32`] instead of calling `unwrap`.
+    ///
+    /// [`from_u32`]: https://doc.rust-lang.org/std/char/fn.from_u32.html
+    /// [`from_u32_unchecked`]: https://doc.rust-lang.org/std/char/fn.from_u32_unchecked.html
+    ///
+    /// **Example:**
+    /// ```rust
+    /// let x = 1_u32;
+    /// unsafe {
+    ///     let _: char = std::mem::transmute(x); // where x: u32
+    /// }
+    ///
+    /// // should be:
+    /// let _ = std::char::from_u32(x).unwrap();
+    /// ```
+    pub TRANSMUTE_INT_TO_CHAR,
+    complexity,
+    "transmutes from an integer to a `char`"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for transmutes from a `&[u8]` to a `&str`.
+    ///
+    /// **Why is this bad?** Not every byte slice is a valid UTF-8 string.
+    ///
+    /// **Known problems:**
+    /// - [`from_utf8`] which this lint suggests using is slower than `transmute`
+    /// as it needs to validate the input.
+    /// If you are certain that the input is always a valid UTF-8,
+    /// use [`from_utf8_unchecked`] which is as fast as `transmute`
+    /// but has a semantically meaningful name.
+    /// - You might want to handle errors returned from [`from_utf8`] instead of calling `unwrap`.
+    ///
+    /// [`from_utf8`]: https://doc.rust-lang.org/std/str/fn.from_utf8.html
+    /// [`from_utf8_unchecked`]: https://doc.rust-lang.org/std/str/fn.from_utf8_unchecked.html
+    ///
+    /// **Example:**
+    /// ```rust
+    /// let b: &[u8] = &[1_u8, 2_u8];
+    /// unsafe {
+    ///     let _: &str = std::mem::transmute(b); // where b: &[u8]
+    /// }
+    ///
+    /// // should be:
+    /// let _ = std::str::from_utf8(b).unwrap();
+    /// ```
+    pub TRANSMUTE_BYTES_TO_STR,
+    complexity,
+    "transmutes from a `&[u8]` to a `&str`"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for transmutes from an integer to a `bool`.
+    ///
+    /// **Why is this bad?** This might result in an invalid in-memory representation of a `bool`.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    /// ```rust
+    /// let x = 1_u8;
+    /// unsafe {
+    ///     let _: bool = std::mem::transmute(x); // where x: u8
+    /// }
+    ///
+    /// // should be:
+    /// let _: bool = x != 0;
+    /// ```
+    pub TRANSMUTE_INT_TO_BOOL,
+    complexity,
+    "transmutes from an integer to a `bool`"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for transmutes from an integer to a float.
+    ///
+    /// **Why is this bad?** Transmutes are dangerous and error-prone, whereas `from_bits` is intuitive
+    /// and safe.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    /// ```rust
+    /// unsafe {
+    ///     let _: f32 = std::mem::transmute(1_u32); // where x: u32
+    /// }
+    ///
+    /// // should be:
+    /// let _: f32 = f32::from_bits(1_u32);
+    /// ```
+    pub TRANSMUTE_INT_TO_FLOAT,
+    complexity,
+    "transmutes from an integer to a float"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for transmutes from a float to an integer.
+    ///
+    /// **Why is this bad?** Transmutes are dangerous and error-prone, whereas `to_bits` is intuitive
+    /// and safe.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    /// ```rust
+    /// unsafe {
+    ///     let _: u32 = std::mem::transmute(1f32);
+    /// }
+    ///
+    /// // should be:
+    /// let _: u32 = 1f32.to_bits();
+    /// ```
+    pub TRANSMUTE_FLOAT_TO_INT,
+    complexity,
+    "transmutes from a float to an integer"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for transmutes from a pointer to a pointer, or
+    /// from a reference to a reference.
+    ///
+    /// **Why is this bad?** Transmutes are dangerous, and these can instead be
+    /// written as casts.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    /// ```rust
+    /// let ptr = &1u32 as *const u32;
+    /// unsafe {
+    ///     // pointer-to-pointer transmute
+    ///     let _: *const f32 = std::mem::transmute(ptr);
+    ///     // ref-ref transmute
+    ///     let _: &f32 = std::mem::transmute(&1u32);
+    /// }
+    /// // These can be respectively written:
+    /// let _ = ptr as *const f32;
+    /// let _ = unsafe{ &*(&1u32 as *const u32 as *const f32) };
+    /// ```
+    pub TRANSMUTE_PTR_TO_PTR,
+    complexity,
+    "transmutes from a pointer to a pointer / a reference to a reference"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for transmutes between collections whose
+    /// types have different ABI, size or alignment.
+    ///
+    /// **Why is this bad?** This is undefined behavior.
+    ///
+    /// **Known problems:** Currently, we cannot know whether a type is a
+    /// collection, so we just lint the ones that come with `std`.
+    ///
+    /// **Example:**
+    /// ```rust
+    /// // different size, therefore likely out-of-bounds memory access
+    /// // You absolutely do not want this in your code!
+    /// unsafe {
+    ///     std::mem::transmute::<_, Vec<u32>>(vec![2_u16])
+    /// };
+    /// ```
+    ///
+    /// You must always iterate, map and collect the values:
+    ///
+    /// ```rust
+    /// vec![2_u16].into_iter().map(u32::from).collect::<Vec<_>>();
+    /// ```
+    pub UNSOUND_COLLECTION_TRANSMUTE,
+    correctness,
+    "transmute between collections of layout-incompatible types"
+}
+
+declare_lint_pass!(Transmute => [
+    CROSSPOINTER_TRANSMUTE,
+    TRANSMUTE_PTR_TO_REF,
+    TRANSMUTE_PTR_TO_PTR,
+    USELESS_TRANSMUTE,
+    WRONG_TRANSMUTE,
+    TRANSMUTE_INT_TO_CHAR,
+    TRANSMUTE_BYTES_TO_STR,
+    TRANSMUTE_INT_TO_BOOL,
+    TRANSMUTE_INT_TO_FLOAT,
+    TRANSMUTE_FLOAT_TO_INT,
+    UNSOUND_COLLECTION_TRANSMUTE,
+    TRANSMUTES_EXPRESSIBLE_AS_PTR_CASTS,
+]);
+
+impl<'tcx> LateLintPass<'tcx> for Transmute {
+    #[allow(clippy::similar_names, clippy::too_many_lines)]
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) {
+        if_chain! {
+            if let ExprKind::Call(ref path_expr, ref args) = e.kind;
+            if let ExprKind::Path(ref qpath) = path_expr.kind;
+            if let Some(def_id) = cx.qpath_res(qpath, path_expr.hir_id).opt_def_id();
+            if match_def_path(cx, def_id, &paths::TRANSMUTE);
+            then {
+                // Avoid suggesting from/to bits and dereferencing raw pointers in const contexts.
+                // See https://github.com/rust-lang/rust/issues/73736 for progress on making them `const fn`.
+                // And see https://github.com/rust-lang/rust/issues/51911 for dereferencing raw pointers.
+                let const_context = in_constant(cx, e.hir_id);
+
+                let from_ty = cx.typeck_results().expr_ty(&args[0]);
+                let to_ty = cx.typeck_results().expr_ty(e);
+
+                // If useless_transmute is triggered, the other lints can be skipped.
+                if useless_transmute::check(cx, e, from_ty, to_ty, args) {
+                    return;
+                }
+
+                let mut linted = wrong_transmute::check(cx, e, from_ty, to_ty);
+                linted |= crosspointer_transmute::check(cx, e, from_ty, to_ty);
+                linted |= transmute_ptr_to_ref::check(cx, e, from_ty, to_ty, args, qpath);
+                linted |= transmute_int_to_char::check(cx, e, from_ty, to_ty, args);
+                linted |= transmute_ref_to_ref::check(cx, e, from_ty, to_ty, args, const_context);
+                linted |= transmute_ptr_to_ptr::check(cx, e, from_ty, to_ty, args);
+                linted |= transmute_int_to_bool::check(cx, e, from_ty, to_ty, args);
+                linted |= transmute_int_to_float::check(cx, e, from_ty, to_ty, args, const_context);
+                linted |= transmute_float_to_int::check(cx, e, from_ty, to_ty, args, const_context);
+                linted |= unsound_collection_transmute::check(cx, e, from_ty, to_ty);
+
+                if !linted {
+                    transmutes_expressible_as_ptr_casts::check(cx, e, from_ty, to_ty, args);
+                }
+            }
+        }
+    }
+}
diff --git a/clippy_lints/src/transmute/transmute_float_to_int.rs b/clippy_lints/src/transmute/transmute_float_to_int.rs
new file mode 100644
index 00000000000..562d880e39a
--- /dev/null
+++ b/clippy_lints/src/transmute/transmute_float_to_int.rs
@@ -0,0 +1,65 @@
+use super::TRANSMUTE_FLOAT_TO_INT;
+use crate::utils::{span_lint_and_then, sugg};
+use if_chain::if_chain;
+use rustc_ast as ast;
+use rustc_errors::Applicability;
+use rustc_hir::{Expr, ExprKind, UnOp};
+use rustc_lint::LateContext;
+use rustc_middle::ty::{self, Ty};
+
+/// Checks for `transmute_float_to_int` lint.
+/// Returns `true` if it's triggered, otherwise returns `false`.
+pub(super) fn check<'tcx>(
+    cx: &LateContext<'tcx>,
+    e: &'tcx Expr<'_>,
+    from_ty: Ty<'tcx>,
+    to_ty: Ty<'tcx>,
+    args: &'tcx [Expr<'_>],
+    const_context: bool,
+) -> bool {
+    match (&from_ty.kind(), &to_ty.kind()) {
+        (ty::Float(float_ty), ty::Int(_) | ty::Uint(_)) if !const_context => {
+            span_lint_and_then(
+                cx,
+                TRANSMUTE_FLOAT_TO_INT,
+                e.span,
+                &format!("transmute from a `{}` to a `{}`", from_ty, to_ty),
+                |diag| {
+                    let mut expr = &args[0];
+                    let mut arg = sugg::Sugg::hir(cx, expr, "..");
+
+                    if let ExprKind::Unary(UnOp::Neg, inner_expr) = &expr.kind {
+                        expr = &inner_expr;
+                    }
+
+                    if_chain! {
+                        // if the expression is a float literal and it is unsuffixed then
+                        // add a suffix so the suggestion is valid and unambiguous
+                        let op = format!("{}{}", arg, float_ty.name_str()).into();
+                        if let ExprKind::Lit(lit) = &expr.kind;
+                        if let ast::LitKind::Float(_, ast::LitFloatType::Unsuffixed) = lit.node;
+                        then {
+                            match arg {
+                                sugg::Sugg::MaybeParen(_) => arg = sugg::Sugg::MaybeParen(op),
+                                _ => arg = sugg::Sugg::NonParen(op)
+                            }
+                        }
+                    }
+
+                    arg = sugg::Sugg::NonParen(format!("{}.to_bits()", arg.maybe_par()).into());
+
+                    // cast the result of `to_bits` if `to_ty` is signed
+                    arg = if let ty::Int(int_ty) = to_ty.kind() {
+                        arg.as_ty(int_ty.name_str().to_string())
+                    } else {
+                        arg
+                    };
+
+                    diag.span_suggestion(e.span, "consider using", arg.to_string(), Applicability::Unspecified);
+                },
+            );
+            true
+        },
+        _ => false,
+    }
+}
diff --git a/clippy_lints/src/transmute/transmute_int_to_bool.rs b/clippy_lints/src/transmute/transmute_int_to_bool.rs
new file mode 100644
index 00000000000..5b609f906a3
--- /dev/null
+++ b/clippy_lints/src/transmute/transmute_int_to_bool.rs
@@ -0,0 +1,41 @@
+use super::TRANSMUTE_INT_TO_BOOL;
+use crate::utils::{span_lint_and_then, sugg};
+use rustc_ast as ast;
+use rustc_errors::Applicability;
+use rustc_hir::Expr;
+use rustc_lint::LateContext;
+use rustc_middle::ty::{self, Ty};
+use std::borrow::Cow;
+
+/// Checks for `transmute_int_to_bool` lint.
+/// Returns `true` if it's triggered, otherwise returns `false`.
+pub(super) fn check<'tcx>(
+    cx: &LateContext<'tcx>,
+    e: &'tcx Expr<'_>,
+    from_ty: Ty<'tcx>,
+    to_ty: Ty<'tcx>,
+    args: &'tcx [Expr<'_>],
+) -> bool {
+    match (&from_ty.kind(), &to_ty.kind()) {
+        (ty::Int(ty::IntTy::I8) | ty::Uint(ty::UintTy::U8), ty::Bool) => {
+            span_lint_and_then(
+                cx,
+                TRANSMUTE_INT_TO_BOOL,
+                e.span,
+                &format!("transmute from a `{}` to a `bool`", from_ty),
+                |diag| {
+                    let arg = sugg::Sugg::hir(cx, &args[0], "..");
+                    let zero = sugg::Sugg::NonParen(Cow::from("0"));
+                    diag.span_suggestion(
+                        e.span,
+                        "consider using",
+                        sugg::make_binop(ast::BinOpKind::Ne, &arg, &zero).to_string(),
+                        Applicability::Unspecified,
+                    );
+                },
+            );
+            true
+        },
+        _ => false,
+    }
+}
diff --git a/clippy_lints/src/transmute/transmute_int_to_char.rs b/clippy_lints/src/transmute/transmute_int_to_char.rs
new file mode 100644
index 00000000000..48473e0d799
--- /dev/null
+++ b/clippy_lints/src/transmute/transmute_int_to_char.rs
@@ -0,0 +1,46 @@
+use super::TRANSMUTE_INT_TO_CHAR;
+use crate::utils::{span_lint_and_then, sugg};
+use rustc_ast as ast;
+use rustc_errors::Applicability;
+use rustc_hir::Expr;
+use rustc_lint::LateContext;
+use rustc_middle::ty::{self, Ty};
+
+/// Checks for `transmute_int_to_char` lint.
+/// Returns `true` if it's triggered, otherwise returns `false`.
+pub(super) fn check<'tcx>(
+    cx: &LateContext<'tcx>,
+    e: &'tcx Expr<'_>,
+    from_ty: Ty<'tcx>,
+    to_ty: Ty<'tcx>,
+    args: &'tcx [Expr<'_>],
+) -> bool {
+    match (&from_ty.kind(), &to_ty.kind()) {
+        (ty::Int(ty::IntTy::I32) | ty::Uint(ty::UintTy::U32), &ty::Char) => {
+            {
+                span_lint_and_then(
+                    cx,
+                    TRANSMUTE_INT_TO_CHAR,
+                    e.span,
+                    &format!("transmute from a `{}` to a `char`", from_ty),
+                    |diag| {
+                        let arg = sugg::Sugg::hir(cx, &args[0], "..");
+                        let arg = if let ty::Int(_) = from_ty.kind() {
+                            arg.as_ty(ast::UintTy::U32.name_str())
+                        } else {
+                            arg
+                        };
+                        diag.span_suggestion(
+                            e.span,
+                            "consider using",
+                            format!("std::char::from_u32({}).unwrap()", arg.to_string()),
+                            Applicability::Unspecified,
+                        );
+                    },
+                )
+            };
+            true
+        },
+        _ => false,
+    }
+}
diff --git a/clippy_lints/src/transmute/transmute_int_to_float.rs b/clippy_lints/src/transmute/transmute_int_to_float.rs
new file mode 100644
index 00000000000..f83fba8966a
--- /dev/null
+++ b/clippy_lints/src/transmute/transmute_int_to_float.rs
@@ -0,0 +1,47 @@
+use super::TRANSMUTE_INT_TO_FLOAT;
+use crate::utils::{span_lint_and_then, sugg};
+use rustc_errors::Applicability;
+use rustc_hir::Expr;
+use rustc_lint::LateContext;
+use rustc_middle::ty::{self, Ty};
+
+/// Checks for `transmute_int_to_float` lint.
+/// Returns `true` if it's triggered, otherwise returns `false`.
+pub(super) fn check<'tcx>(
+    cx: &LateContext<'tcx>,
+    e: &'tcx Expr<'_>,
+    from_ty: Ty<'tcx>,
+    to_ty: Ty<'tcx>,
+    args: &'tcx [Expr<'_>],
+    const_context: bool,
+) -> bool {
+    match (&from_ty.kind(), &to_ty.kind()) {
+        (ty::Int(_) | ty::Uint(_), ty::Float(_)) if !const_context => {
+            span_lint_and_then(
+                cx,
+                TRANSMUTE_INT_TO_FLOAT,
+                e.span,
+                &format!("transmute from a `{}` to a `{}`", from_ty, to_ty),
+                |diag| {
+                    let arg = sugg::Sugg::hir(cx, &args[0], "..");
+                    let arg = if let ty::Int(int_ty) = from_ty.kind() {
+                        arg.as_ty(format!(
+                            "u{}",
+                            int_ty.bit_width().map_or_else(|| "size".to_string(), |v| v.to_string())
+                        ))
+                    } else {
+                        arg
+                    };
+                    diag.span_suggestion(
+                        e.span,
+                        "consider using",
+                        format!("{}::from_bits({})", to_ty, arg.to_string()),
+                        Applicability::Unspecified,
+                    );
+                },
+            );
+            true
+        },
+        _ => false,
+    }
+}
diff --git a/clippy_lints/src/transmute/transmute_ptr_to_ptr.rs b/clippy_lints/src/transmute/transmute_ptr_to_ptr.rs
new file mode 100644
index 00000000000..f4e60a3020c
--- /dev/null
+++ b/clippy_lints/src/transmute/transmute_ptr_to_ptr.rs
@@ -0,0 +1,35 @@
+use super::TRANSMUTE_PTR_TO_PTR;
+use crate::utils::{span_lint_and_then, sugg};
+use rustc_errors::Applicability;
+use rustc_hir::Expr;
+use rustc_lint::LateContext;
+use rustc_middle::ty::{self, Ty};
+
+/// Checks for `transmute_ptr_to_ptr` lint.
+/// Returns `true` if it's triggered, otherwise returns `false`.
+pub(super) fn check<'tcx>(
+    cx: &LateContext<'tcx>,
+    e: &'tcx Expr<'_>,
+    from_ty: Ty<'tcx>,
+    to_ty: Ty<'tcx>,
+    args: &'tcx [Expr<'_>],
+) -> bool {
+    match (&from_ty.kind(), &to_ty.kind()) {
+        (ty::RawPtr(_), ty::RawPtr(to_ty)) => {
+            span_lint_and_then(
+                cx,
+                TRANSMUTE_PTR_TO_PTR,
+                e.span,
+                "transmute from a pointer to a pointer",
+                |diag| {
+                    if let Some(arg) = sugg::Sugg::hir_opt(cx, &args[0]) {
+                        let sugg = arg.as_ty(cx.tcx.mk_ptr(*to_ty));
+                        diag.span_suggestion(e.span, "try", sugg.to_string(), Applicability::Unspecified);
+                    }
+                },
+            );
+            true
+        },
+        _ => false,
+    }
+}
diff --git a/clippy_lints/src/transmute/transmute_ptr_to_ref.rs b/clippy_lints/src/transmute/transmute_ptr_to_ref.rs
new file mode 100644
index 00000000000..a6719b68098
--- /dev/null
+++ b/clippy_lints/src/transmute/transmute_ptr_to_ref.rs
@@ -0,0 +1,55 @@
+use super::utils::get_type_snippet;
+use super::TRANSMUTE_PTR_TO_REF;
+use crate::utils::{span_lint_and_then, sugg};
+use rustc_errors::Applicability;
+use rustc_hir::{Expr, Mutability, QPath};
+use rustc_lint::LateContext;
+use rustc_middle::ty::{self, Ty};
+
+/// Checks for `transmute_ptr_to_ref` lint.
+/// Returns `true` if it's triggered, otherwise returns `false`.
+pub(super) fn check<'tcx>(
+    cx: &LateContext<'tcx>,
+    e: &'tcx Expr<'_>,
+    from_ty: Ty<'tcx>,
+    to_ty: Ty<'tcx>,
+    args: &'tcx [Expr<'_>],
+    qpath: &'tcx QPath<'_>,
+) -> bool {
+    match (&from_ty.kind(), &to_ty.kind()) {
+        (ty::RawPtr(from_pty), ty::Ref(_, to_ref_ty, mutbl)) => {
+            span_lint_and_then(
+                cx,
+                TRANSMUTE_PTR_TO_REF,
+                e.span,
+                &format!(
+                    "transmute from a pointer type (`{}`) to a reference type (`{}`)",
+                    from_ty, to_ty
+                ),
+                |diag| {
+                    let arg = sugg::Sugg::hir(cx, &args[0], "..");
+                    let (deref, cast) = if *mutbl == Mutability::Mut {
+                        ("&mut *", "*mut")
+                    } else {
+                        ("&*", "*const")
+                    };
+
+                    let arg = if from_pty.ty == *to_ref_ty {
+                        arg
+                    } else {
+                        arg.as_ty(&format!("{} {}", cast, get_type_snippet(cx, qpath, to_ref_ty)))
+                    };
+
+                    diag.span_suggestion(
+                        e.span,
+                        "try",
+                        sugg::make_unop(deref, arg).to_string(),
+                        Applicability::Unspecified,
+                    );
+                },
+            );
+            true
+        },
+        _ => false,
+    }
+}
diff --git a/clippy_lints/src/transmute/transmute_ref_to_ref.rs b/clippy_lints/src/transmute/transmute_ref_to_ref.rs
new file mode 100644
index 00000000000..ccfcb03e87a
--- /dev/null
+++ b/clippy_lints/src/transmute/transmute_ref_to_ref.rs
@@ -0,0 +1,88 @@
+use super::{TRANSMUTE_BYTES_TO_STR, TRANSMUTE_PTR_TO_PTR};
+use crate::utils::{snippet, span_lint_and_sugg, span_lint_and_then, sugg};
+use if_chain::if_chain;
+use rustc_errors::Applicability;
+use rustc_hir::{Expr, Mutability};
+use rustc_lint::LateContext;
+use rustc_middle::ty::{self, Ty};
+
+/// Checks for `transmute_bytes_to_str` and `transmute_ptr_to_ptr` lints.
+/// Returns `true` if either one triggered, otherwise returns `false`.
+pub(super) fn check<'tcx>(
+    cx: &LateContext<'tcx>,
+    e: &'tcx Expr<'_>,
+    from_ty: Ty<'tcx>,
+    to_ty: Ty<'tcx>,
+    args: &'tcx [Expr<'_>],
+    const_context: bool,
+) -> bool {
+    let mut triggered = false;
+
+    match (&from_ty.kind(), &to_ty.kind()) {
+        (ty::Ref(_, ty_from, from_mutbl), ty::Ref(_, ty_to, to_mutbl)) => {
+            if_chain! {
+                if let (&ty::Slice(slice_ty), &ty::Str) = (&ty_from.kind(), &ty_to.kind());
+                if let ty::Uint(ty::UintTy::U8) = slice_ty.kind();
+                if from_mutbl == to_mutbl;
+                then {
+                    let postfix = if *from_mutbl == Mutability::Mut {
+                        "_mut"
+                    } else {
+                        ""
+                    };
+
+                    span_lint_and_sugg(
+                        cx,
+                        TRANSMUTE_BYTES_TO_STR,
+                        e.span,
+                        &format!("transmute from a `{}` to a `{}`", from_ty, to_ty),
+                        "consider using",
+                        format!(
+                            "std::str::from_utf8{}({}).unwrap()",
+                            postfix,
+                            snippet(cx, args[0].span, ".."),
+                        ),
+                        Applicability::Unspecified,
+                    );
+                    triggered = true;
+                } else {
+                    if (cx.tcx.erase_regions(from_ty) != cx.tcx.erase_regions(to_ty))
+                        && !const_context {
+                        span_lint_and_then(
+                            cx,
+                            TRANSMUTE_PTR_TO_PTR,
+                            e.span,
+                            "transmute from a reference to a reference",
+                            |diag| if let Some(arg) = sugg::Sugg::hir_opt(cx, &args[0]) {
+                                let ty_from_and_mut = ty::TypeAndMut {
+                                    ty: ty_from,
+                                    mutbl: *from_mutbl
+                                };
+                                let ty_to_and_mut = ty::TypeAndMut { ty: ty_to, mutbl: *to_mutbl };
+                                let sugg_paren = arg
+                                    .as_ty(cx.tcx.mk_ptr(ty_from_and_mut))
+                                    .as_ty(cx.tcx.mk_ptr(ty_to_and_mut));
+                                let sugg = if *to_mutbl == Mutability::Mut {
+                                    sugg_paren.mut_addr_deref()
+                                } else {
+                                    sugg_paren.addr_deref()
+                                };
+                                diag.span_suggestion(
+                                    e.span,
+                                    "try",
+                                    sugg.to_string(),
+                                    Applicability::Unspecified,
+                                );
+                            },
+                        );
+
+                        triggered = true;
+                    }
+                }
+            }
+        },
+        _ => {},
+    }
+
+    triggered
+}
diff --git a/clippy_lints/src/transmute/transmutes_expressible_as_ptr_casts.rs b/clippy_lints/src/transmute/transmutes_expressible_as_ptr_casts.rs
new file mode 100644
index 00000000000..dea896622f1
--- /dev/null
+++ b/clippy_lints/src/transmute/transmutes_expressible_as_ptr_casts.rs
@@ -0,0 +1,38 @@
+use super::utils::can_be_expressed_as_pointer_cast;
+use super::TRANSMUTES_EXPRESSIBLE_AS_PTR_CASTS;
+use crate::utils::{span_lint_and_then, sugg};
+use rustc_errors::Applicability;
+use rustc_hir::Expr;
+use rustc_lint::LateContext;
+use rustc_middle::ty::Ty;
+
+/// Checks for `transmutes_expressible_as_ptr_casts` lint.
+/// Returns `true` if it's triggered, otherwise returns `false`.
+pub(super) fn check<'tcx>(
+    cx: &LateContext<'tcx>,
+    e: &'tcx Expr<'_>,
+    from_ty: Ty<'tcx>,
+    to_ty: Ty<'tcx>,
+    args: &'tcx [Expr<'_>],
+) -> bool {
+    if can_be_expressed_as_pointer_cast(cx, e, from_ty, to_ty) {
+        span_lint_and_then(
+            cx,
+            TRANSMUTES_EXPRESSIBLE_AS_PTR_CASTS,
+            e.span,
+            &format!(
+                "transmute from `{}` to `{}` which could be expressed as a pointer cast instead",
+                from_ty, to_ty
+            ),
+            |diag| {
+                if let Some(arg) = sugg::Sugg::hir_opt(cx, &args[0]) {
+                    let sugg = arg.as_ty(&to_ty.to_string()).to_string();
+                    diag.span_suggestion(e.span, "try", sugg, Applicability::MachineApplicable);
+                }
+            },
+        );
+        true
+    } else {
+        false
+    }
+}
diff --git a/clippy_lints/src/transmute/unsound_collection_transmute.rs b/clippy_lints/src/transmute/unsound_collection_transmute.rs
new file mode 100644
index 00000000000..503c5e0ff38
--- /dev/null
+++ b/clippy_lints/src/transmute/unsound_collection_transmute.rs
@@ -0,0 +1,48 @@
+use super::utils::is_layout_incompatible;
+use super::UNSOUND_COLLECTION_TRANSMUTE;
+use crate::utils::{match_def_path, paths, span_lint};
+use rustc_hir::Expr;
+use rustc_lint::LateContext;
+use rustc_middle::ty::{self, Ty};
+
+// used to check for UNSOUND_COLLECTION_TRANSMUTE
+static COLLECTIONS: &[&[&str]] = &[
+    &paths::VEC,
+    &paths::VEC_DEQUE,
+    &paths::BINARY_HEAP,
+    &paths::BTREESET,
+    &paths::BTREEMAP,
+    &paths::HASHSET,
+    &paths::HASHMAP,
+];
+
+/// Checks for `unsound_collection_transmute` lint.
+/// Returns `true` if it's triggered, otherwise returns `false`.
+pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, e: &'tcx Expr<'_>, from_ty: Ty<'tcx>, to_ty: Ty<'tcx>) -> bool {
+    match (&from_ty.kind(), &to_ty.kind()) {
+        (ty::Adt(from_adt, from_substs), ty::Adt(to_adt, to_substs)) => {
+            if from_adt.did != to_adt.did || !COLLECTIONS.iter().any(|path| match_def_path(cx, to_adt.did, path)) {
+                return false;
+            }
+            if from_substs
+                .types()
+                .zip(to_substs.types())
+                .any(|(from_ty, to_ty)| is_layout_incompatible(cx, from_ty, to_ty))
+            {
+                span_lint(
+                    cx,
+                    UNSOUND_COLLECTION_TRANSMUTE,
+                    e.span,
+                    &format!(
+                        "transmute from `{}` to `{}` with mismatched layout is unsound",
+                        from_ty, to_ty
+                    ),
+                );
+                true
+            } else {
+                false
+            }
+        },
+        _ => false,
+    }
+}
diff --git a/clippy_lints/src/transmute/useless_transmute.rs b/clippy_lints/src/transmute/useless_transmute.rs
new file mode 100644
index 00000000000..83441514af0
--- /dev/null
+++ b/clippy_lints/src/transmute/useless_transmute.rs
@@ -0,0 +1,73 @@
+use super::USELESS_TRANSMUTE;
+use crate::utils::{span_lint, span_lint_and_then, sugg};
+use rustc_errors::Applicability;
+use rustc_hir::Expr;
+use rustc_lint::LateContext;
+use rustc_middle::ty::{self, Ty};
+
+/// Checks for `useless_transmute` lint.
+/// Returns `true` if it's triggered, otherwise returns `false`.
+pub(super) fn check<'tcx>(
+    cx: &LateContext<'tcx>,
+    e: &'tcx Expr<'_>,
+    from_ty: Ty<'tcx>,
+    to_ty: Ty<'tcx>,
+    args: &'tcx [Expr<'_>],
+) -> bool {
+    match (&from_ty.kind(), &to_ty.kind()) {
+        _ if from_ty == to_ty => {
+            span_lint(
+                cx,
+                USELESS_TRANSMUTE,
+                e.span,
+                &format!("transmute from a type (`{}`) to itself", from_ty),
+            );
+            true
+        },
+        (ty::Ref(_, rty, rty_mutbl), ty::RawPtr(ptr_ty)) => {
+            span_lint_and_then(
+                cx,
+                USELESS_TRANSMUTE,
+                e.span,
+                "transmute from a reference to a pointer",
+                |diag| {
+                    if let Some(arg) = sugg::Sugg::hir_opt(cx, &args[0]) {
+                        let rty_and_mut = ty::TypeAndMut {
+                            ty: rty,
+                            mutbl: *rty_mutbl,
+                        };
+
+                        let sugg = if *ptr_ty == rty_and_mut {
+                            arg.as_ty(to_ty)
+                        } else {
+                            arg.as_ty(cx.tcx.mk_ptr(rty_and_mut)).as_ty(to_ty)
+                        };
+
+                        diag.span_suggestion(e.span, "try", sugg.to_string(), Applicability::Unspecified);
+                    }
+                },
+            );
+            true
+        },
+        (ty::Int(_) | ty::Uint(_), ty::RawPtr(_)) => {
+            span_lint_and_then(
+                cx,
+                USELESS_TRANSMUTE,
+                e.span,
+                "transmute from an integer to a pointer",
+                |diag| {
+                    if let Some(arg) = sugg::Sugg::hir_opt(cx, &args[0]) {
+                        diag.span_suggestion(
+                            e.span,
+                            "try",
+                            arg.as_ty(&to_ty.to_string()).to_string(),
+                            Applicability::Unspecified,
+                        );
+                    }
+                },
+            );
+            true
+        },
+        _ => false,
+    }
+}
diff --git a/clippy_lints/src/transmute/utils.rs b/clippy_lints/src/transmute/utils.rs
new file mode 100644
index 00000000000..55008d8ec3f
--- /dev/null
+++ b/clippy_lints/src/transmute/utils.rs
@@ -0,0 +1,104 @@
+use crate::utils::{is_normalizable, last_path_segment, snippet};
+use if_chain::if_chain;
+use rustc_hir::{Expr, GenericArg, QPath, TyKind};
+use rustc_lint::LateContext;
+use rustc_middle::ty::{self, cast::CastKind, Ty};
+use rustc_span::DUMMY_SP;
+use rustc_typeck::check::{cast::CastCheck, FnCtxt, Inherited};
+
+/// Gets the snippet of `Bar` in `…::transmute<Foo, &Bar>`. If that snippet is
+/// not available , use
+/// the type's `ToString` implementation. In weird cases it could lead to types
+/// with invalid `'_`
+/// lifetime, but it should be rare.
+pub(super) fn get_type_snippet(cx: &LateContext<'_>, path: &QPath<'_>, to_ref_ty: Ty<'_>) -> String {
+    let seg = last_path_segment(path);
+    if_chain! {
+        if let Some(ref params) = seg.args;
+        if !params.parenthesized;
+        if let Some(to_ty) = params.args.iter().filter_map(|arg| match arg {
+            GenericArg::Type(ty) => Some(ty),
+            _ => None,
+        }).nth(1);
+        if let TyKind::Rptr(_, ref to_ty) = to_ty.kind;
+        then {
+            return snippet(cx, to_ty.ty.span, &to_ref_ty.to_string()).to_string();
+        }
+    }
+
+    to_ref_ty.to_string()
+}
+
+// check if the component types of the transmuted collection and the result have different ABI,
+// size or alignment
+pub(super) fn is_layout_incompatible<'tcx>(cx: &LateContext<'tcx>, from: Ty<'tcx>, to: Ty<'tcx>) -> bool {
+    let empty_param_env = ty::ParamEnv::empty();
+    // check if `from` and `to` are normalizable to avoid ICE (#4968)
+    if !(is_normalizable(cx, empty_param_env, from) && is_normalizable(cx, empty_param_env, to)) {
+        return false;
+    }
+    let from_ty_layout = cx.tcx.layout_of(empty_param_env.and(from));
+    let to_ty_layout = cx.tcx.layout_of(empty_param_env.and(to));
+    if let (Ok(from_layout), Ok(to_layout)) = (from_ty_layout, to_ty_layout) {
+        from_layout.size != to_layout.size || from_layout.align != to_layout.align || from_layout.abi != to_layout.abi
+    } else {
+        // no idea about layout, so don't lint
+        false
+    }
+}
+
+/// Check if the type conversion can be expressed as a pointer cast, instead of
+/// a transmute. In certain cases, including some invalid casts from array
+/// references to pointers, this may cause additional errors to be emitted and/or
+/// ICE error messages. This function will panic if that occurs.
+pub(super) fn can_be_expressed_as_pointer_cast<'tcx>(
+    cx: &LateContext<'tcx>,
+    e: &'tcx Expr<'_>,
+    from_ty: Ty<'tcx>,
+    to_ty: Ty<'tcx>,
+) -> bool {
+    use CastKind::{AddrPtrCast, ArrayPtrCast, FnPtrAddrCast, FnPtrPtrCast, PtrAddrCast, PtrPtrCast};
+    matches!(
+        check_cast(cx, e, from_ty, to_ty),
+        Some(PtrPtrCast | PtrAddrCast | AddrPtrCast | ArrayPtrCast | FnPtrPtrCast | FnPtrAddrCast)
+    )
+}
+
+/// If a cast from `from_ty` to `to_ty` is valid, returns an Ok containing the kind of
+/// the cast. In certain cases, including some invalid casts from array references
+/// to pointers, this may cause additional errors to be emitted and/or ICE error
+/// messages. This function will panic if that occurs.
+fn check_cast<'tcx>(cx: &LateContext<'tcx>, e: &'tcx Expr<'_>, from_ty: Ty<'tcx>, to_ty: Ty<'tcx>) -> Option<CastKind> {
+    let hir_id = e.hir_id;
+    let local_def_id = hir_id.owner;
+
+    Inherited::build(cx.tcx, local_def_id).enter(|inherited| {
+        let fn_ctxt = FnCtxt::new(&inherited, cx.param_env, hir_id);
+
+        // If we already have errors, we can't be sure we can pointer cast.
+        assert!(
+            !fn_ctxt.errors_reported_since_creation(),
+            "Newly created FnCtxt contained errors"
+        );
+
+        if let Ok(check) = CastCheck::new(
+            &fn_ctxt, e, from_ty, to_ty,
+            // We won't show any error to the user, so we don't care what the span is here.
+            DUMMY_SP, DUMMY_SP,
+        ) {
+            let res = check.do_check(&fn_ctxt);
+
+            // do_check's documentation says that it might return Ok and create
+            // errors in the fcx instead of returing Err in some cases. Those cases
+            // should be filtered out before getting here.
+            assert!(
+                !fn_ctxt.errors_reported_since_creation(),
+                "`fn_ctxt` contained errors after cast check!"
+            );
+
+            res.ok()
+        } else {
+            None
+        }
+    })
+}
diff --git a/clippy_lints/src/transmute/wrong_transmute.rs b/clippy_lints/src/transmute/wrong_transmute.rs
new file mode 100644
index 00000000000..d6d77f2c834
--- /dev/null
+++ b/clippy_lints/src/transmute/wrong_transmute.rs
@@ -0,0 +1,22 @@
+use super::WRONG_TRANSMUTE;
+use crate::utils::span_lint;
+use rustc_hir::Expr;
+use rustc_lint::LateContext;
+use rustc_middle::ty::{self, Ty};
+
+/// Checks for `wrong_transmute` lint.
+/// Returns `true` if it's triggered, otherwise returns `false`.
+pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, e: &'tcx Expr<'_>, from_ty: Ty<'tcx>, to_ty: Ty<'tcx>) -> bool {
+    match (&from_ty.kind(), &to_ty.kind()) {
+        (ty::Float(_) | ty::Char, ty::Ref(..) | ty::RawPtr(_)) => {
+            span_lint(
+                cx,
+                WRONG_TRANSMUTE,
+                e.span,
+                &format!("transmute from a `{}` to a pointer", from_ty),
+            );
+            true
+        },
+        _ => false,
+    }
+}