Auto merge of #4592 - rust-lang:transmute-collection, r=flip1995

New lint: `unsound_collection_transmute`

changelog: Add `unsound_collection_transmute` lint

This fixes #4515

CHANGELOG.md

@@ -1225,6 +1225,7 @@ Released 2018-09-13
 [`unsafe_removed_from_name`]: https://rust-lang.github.io/rust-clippy/master/index.html#unsafe_removed_from_name
 [`unsafe_vector_initialization`]: https://rust-lang.github.io/rust-clippy/master/index.html#unsafe_vector_initialization
 [`unseparated_literal_suffix`]: https://rust-lang.github.io/rust-clippy/master/index.html#unseparated_literal_suffix
+[`unsound_collection_transmute`]: https://rust-lang.github.io/rust-clippy/master/index.html#unsound_collection_transmute
 [`unstable_as_mut_slice`]: https://rust-lang.github.io/rust-clippy/master/index.html#unstable_as_mut_slice
 [`unstable_as_slice`]: https://rust-lang.github.io/rust-clippy/master/index.html#unstable_as_slice
 [`unused_collect`]: https://rust-lang.github.io/rust-clippy/master/index.html#unused_collect

README.md

@@ -6,7 +6,7 @@
 
 A collection of lints to catch common mistakes and improve your [Rust](https://github.com/rust-lang/rust) code.
 
-[There are 318 lints included in this crate!](https://rust-lang.github.io/rust-clippy/master/index.html)
+[There are 319 lints included in this crate!](https://rust-lang.github.io/rust-clippy/master/index.html)
 
 We have a bunch of lint categories to allow you to choose how much Clippy is supposed to ~~annoy~~ help you:
 

clippy_lints/src/lib.rs

@@ -893,6 +893,7 @@ pub fn register_plugins(reg: &mut rustc_driver::plugin::Registry<'_>, conf: &Con
         transmute::TRANSMUTE_INT_TO_FLOAT,
         transmute::TRANSMUTE_PTR_TO_PTR,
         transmute::TRANSMUTE_PTR_TO_REF,
+        transmute::UNSOUND_COLLECTION_TRANSMUTE,
         transmute::USELESS_TRANSMUTE,
         transmute::WRONG_TRANSMUTE,
         transmuting_null::TRANSMUTING_NULL,
@@ -1148,6 +1149,7 @@ pub fn register_plugins(reg: &mut rustc_driver::plugin::Registry<'_>, conf: &Con
         suspicious_trait_impl::SUSPICIOUS_ARITHMETIC_IMPL,
         suspicious_trait_impl::SUSPICIOUS_OP_ASSIGN_IMPL,
         swap::ALMOST_SWAPPED,
+        transmute::UNSOUND_COLLECTION_TRANSMUTE,
         transmute::WRONG_TRANSMUTE,
         transmuting_null::TRANSMUTING_NULL,
         types::ABSURD_EXTREME_COMPARISONS,

clippy_lints/src/transmute.rs

@@ -216,6 +216,33 @@ declare_clippy_lint! {
     "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,
@@ -226,15 +253,28 @@ declare_lint_pass!(Transmute => [
     TRANSMUTE_BYTES_TO_STR,
     TRANSMUTE_INT_TO_BOOL,
     TRANSMUTE_INT_TO_FLOAT,
+    UNSOUND_COLLECTION_TRANSMUTE,
 ]);
 
+// 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<'a, 'tcx> LateLintPass<'a, 'tcx> for Transmute {
     #[allow(clippy::similar_names, clippy::too_many_lines)]
     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, e: &'tcx Expr) {
-        if let ExprKind::Call(ref path_expr, ref args) = e.kind {
+        if_chain! {
-            if let ExprKind::Path(ref qpath) = path_expr.kind {
+            if let ExprKind::Call(ref path_expr, ref args) = e.kind;
-                if let Some(def_id) = cx.tables.qpath_res(qpath, path_expr.hir_id).opt_def_id() {
+            if let ExprKind::Path(ref qpath) = path_expr.kind;
-                    if match_def_path(cx, def_id, &paths::TRANSMUTE) {
+            if let Some(def_id) = cx.tables.qpath_res(qpath, path_expr.hir_id).opt_def_id();
+            if match_def_path(cx, def_id, &paths::TRANSMUTE);
+            then {
                 let from_ty = cx.tables.expr_ty(&args[0]);
                 let to_ty = cx.tables.expr_ty(e);
 
@@ -479,9 +519,26 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Transmute {
                             );
                         },
                     ),
-                            _ => return,
+                    (&ty::Adt(ref from_adt, ref from_substs), &ty::Adt(ref to_adt, ref 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
+                                )
+                            );
+                        }
+                    },
+                    _ => return,
                 }
             }
         }
@@ -510,3 +567,16 @@ fn get_type_snippet(cx: &LateContext<'_, '_>, path: &QPath, to_ref_ty: Ty<'_>) -
 
     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<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, from: Ty<'tcx>, to: Ty<'tcx>) -> bool {
+    let from_ty_layout = cx.tcx.layout_of(ty::ParamEnv::empty().and(from));
+    let to_ty_layout = cx.tcx.layout_of(ty::ParamEnv::empty().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
+    }
+}

src/lintlist/mod.rs

@@ -6,7 +6,7 @@ pub use lint::Lint;
 pub use lint::LINT_LEVELS;
 
 // begin lint list, do not remove this comment, it’s used in `update_lints`
-pub const ALL_LINTS: [Lint; 318] = [
+pub const ALL_LINTS: [Lint; 319] = [
     Lint {
         name: "absurd_extreme_comparisons",
         group: "correctness",
@@ -2030,6 +2030,13 @@ pub const ALL_LINTS: [Lint; 318] = [
         deprecation: None,
         module: "misc_early",
     },
+    Lint {
+        name: "unsound_collection_transmute",
+        group: "correctness",
+        desc: "transmute between collections of layout-incompatible types",
+        deprecation: None,
+        module: "transmute",
+    },
     Lint {
         name: "unused_io_amount",
         group: "correctness",

tests/ui/transmute.rs

@@ -80,12 +80,6 @@ fn useless() {
 
         let _: Vec<i32> = my_transmute(my_vec());
 
-        let _: Vec<u32> = core::intrinsics::transmute(my_vec());
-        let _: Vec<u32> = core::mem::transmute(my_vec());
-        let _: Vec<u32> = std::intrinsics::transmute(my_vec());
-        let _: Vec<u32> = std::mem::transmute(my_vec());
-        let _: Vec<u32> = my_transmute(my_vec());
-
         let _: *const usize = std::mem::transmute(5_isize);
 
         let _ = 5_isize as *const usize;

tests/ui/transmute.stderr

@@ -117,19 +117,19 @@ LL |         let _: Vec<i32> = my_transmute(my_vec());
    |                           ^^^^^^^^^^^^^^^^^^^^^^
 
 error: transmute from an integer to a pointer
-  --> $DIR/transmute.rs:89:31
+  --> $DIR/transmute.rs:83:31
    |
 LL |         let _: *const usize = std::mem::transmute(5_isize);
    |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `5_isize as *const usize`
 
 error: transmute from an integer to a pointer
-  --> $DIR/transmute.rs:93:31
+  --> $DIR/transmute.rs:87:31
    |
 LL |         let _: *const usize = std::mem::transmute(1 + 1usize);
    |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `(1 + 1usize) as *const usize`
 
 error: transmute from a type (`*const Usize`) to the type that it points to (`Usize`)
-  --> $DIR/transmute.rs:108:24
+  --> $DIR/transmute.rs:102:24
    |
 LL |         let _: Usize = core::intrinsics::transmute(int_const_ptr);
    |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -137,25 +137,25 @@ LL |         let _: Usize = core::intrinsics::transmute(int_const_ptr);
    = note: `-D clippy::crosspointer-transmute` implied by `-D warnings`
 
 error: transmute from a type (`*mut Usize`) to the type that it points to (`Usize`)
-  --> $DIR/transmute.rs:110:24
+  --> $DIR/transmute.rs:104:24
    |
 LL |         let _: Usize = core::intrinsics::transmute(int_mut_ptr);
    |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: transmute from a type (`Usize`) to a pointer to that type (`*const Usize`)
-  --> $DIR/transmute.rs:112:31
+  --> $DIR/transmute.rs:106:31
    |
 LL |         let _: *const Usize = core::intrinsics::transmute(my_int());
    |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: transmute from a type (`Usize`) to a pointer to that type (`*mut Usize`)
-  --> $DIR/transmute.rs:114:29
+  --> $DIR/transmute.rs:108:29
    |
 LL |         let _: *mut Usize = core::intrinsics::transmute(my_int());
    |                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: transmute from a `u32` to a `char`
-  --> $DIR/transmute.rs:120:28
+  --> $DIR/transmute.rs:114:28
    |
 LL |     let _: char = unsafe { std::mem::transmute(0_u32) };
    |                            ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `std::char::from_u32(0_u32).unwrap()`
@@ -163,13 +163,13 @@ LL |     let _: char = unsafe { std::mem::transmute(0_u32) };
    = note: `-D clippy::transmute-int-to-char` implied by `-D warnings`
 
 error: transmute from a `i32` to a `char`
-  --> $DIR/transmute.rs:121:28
+  --> $DIR/transmute.rs:115:28
    |
 LL |     let _: char = unsafe { std::mem::transmute(0_i32) };
    |                            ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `std::char::from_u32(0_i32 as u32).unwrap()`
 
 error: transmute from a `u8` to a `bool`
-  --> $DIR/transmute.rs:126:28
+  --> $DIR/transmute.rs:120:28
    |
 LL |     let _: bool = unsafe { std::mem::transmute(0_u8) };
    |                            ^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `0_u8 != 0`
@@ -177,7 +177,7 @@ LL |     let _: bool = unsafe { std::mem::transmute(0_u8) };
    = note: `-D clippy::transmute-int-to-bool` implied by `-D warnings`
 
 error: transmute from a `u32` to a `f32`
-  --> $DIR/transmute.rs:131:27
+  --> $DIR/transmute.rs:125:27
    |
 LL |     let _: f32 = unsafe { std::mem::transmute(0_u32) };
    |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `f32::from_bits(0_u32)`
@@ -185,13 +185,13 @@ LL |     let _: f32 = unsafe { std::mem::transmute(0_u32) };
    = note: `-D clippy::transmute-int-to-float` implied by `-D warnings`
 
 error: transmute from a `i32` to a `f32`
-  --> $DIR/transmute.rs:132:27
+  --> $DIR/transmute.rs:126:27
    |
 LL |     let _: f32 = unsafe { std::mem::transmute(0_i32) };
    |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `f32::from_bits(0_i32 as u32)`
 
 error: transmute from a `&[u8]` to a `&str`
-  --> $DIR/transmute.rs:136:28
+  --> $DIR/transmute.rs:130:28
    |
 LL |     let _: &str = unsafe { std::mem::transmute(b) };
    |                            ^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `std::str::from_utf8(b).unwrap()`
@@ -199,13 +199,13 @@ LL |     let _: &str = unsafe { std::mem::transmute(b) };
    = note: `-D clippy::transmute-bytes-to-str` implied by `-D warnings`
 
 error: transmute from a `&mut [u8]` to a `&mut str`
-  --> $DIR/transmute.rs:137:32
+  --> $DIR/transmute.rs:131:32
    |
 LL |     let _: &mut str = unsafe { std::mem::transmute(mb) };
    |                                ^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `std::str::from_utf8_mut(mb).unwrap()`
 
 error: transmute from a pointer to a pointer
-  --> $DIR/transmute.rs:169:29
+  --> $DIR/transmute.rs:163:29
    |
 LL |         let _: *const f32 = std::mem::transmute(ptr);
    |                             ^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `ptr as *const f32`
@@ -213,31 +213,31 @@ LL |         let _: *const f32 = std::mem::transmute(ptr);
    = note: `-D clippy::transmute-ptr-to-ptr` implied by `-D warnings`
 
 error: transmute from a pointer to a pointer
-  --> $DIR/transmute.rs:170:27
+  --> $DIR/transmute.rs:164:27
    |
 LL |         let _: *mut f32 = std::mem::transmute(mut_ptr);
    |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `mut_ptr as *mut f32`
 
 error: transmute from a reference to a reference
-  --> $DIR/transmute.rs:172:23
+  --> $DIR/transmute.rs:166:23
    |
 LL |         let _: &f32 = std::mem::transmute(&1u32);
    |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `&*(&1u32 as *const u32 as *const f32)`
 
 error: transmute from a reference to a reference
-  --> $DIR/transmute.rs:173:23
+  --> $DIR/transmute.rs:167:23
    |
 LL |         let _: &f64 = std::mem::transmute(&1f32);
    |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `&*(&1f32 as *const f32 as *const f64)`
 
 error: transmute from a reference to a reference
-  --> $DIR/transmute.rs:176:27
+  --> $DIR/transmute.rs:170:27
    |
 LL |         let _: &mut f32 = std::mem::transmute(&mut 1u32);
    |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `&mut *(&mut 1u32 as *mut u32 as *mut f32)`
 
 error: transmute from a reference to a reference
-  --> $DIR/transmute.rs:177:37
+  --> $DIR/transmute.rs:171:37
    |
 LL |         let _: &GenericParam<f32> = std::mem::transmute(&GenericParam { t: 1u32 });
    |                                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `&*(&GenericParam { t: 1u32 } as *const GenericParam<u32> as *const GenericParam<f32>)`

tests/ui/transmute_collection.rs

@@ -0,0 +1,47 @@
+#![warn(clippy::unsound_collection_transmute)]
+
+use std::collections::{BTreeMap, BTreeSet, BinaryHeap, HashMap, HashSet, VecDeque};
+use std::mem::transmute;
+
+fn main() {
+    unsafe {
+        // wrong size
+        let _ = transmute::<_, Vec<u32>>(vec![0u8]);
+        // wrong layout
+        let _ = transmute::<_, Vec<[u8; 4]>>(vec![1234u32]);
+
+        // wrong size
+        let _ = transmute::<_, VecDeque<u32>>(VecDeque::<u8>::new());
+        // wrong layout
+        let _ = transmute::<_, VecDeque<u32>>(VecDeque::<[u8; 4]>::new());
+
+        // wrong size
+        let _ = transmute::<_, BinaryHeap<u32>>(BinaryHeap::<u8>::new());
+        // wrong layout
+        let _ = transmute::<_, BinaryHeap<u32>>(BinaryHeap::<[u8; 4]>::new());
+
+        // wrong size
+        let _ = transmute::<_, BTreeSet<u32>>(BTreeSet::<u8>::new());
+        // wrong layout
+        let _ = transmute::<_, BTreeSet<u32>>(BTreeSet::<[u8; 4]>::new());
+
+        // wrong size
+        let _ = transmute::<_, HashSet<u32>>(HashSet::<u8>::new());
+        // wrong layout
+        let _ = transmute::<_, HashSet<u32>>(HashSet::<[u8; 4]>::new());
+
+        // wrong size
+        let _ = transmute::<_, BTreeMap<u8, u32>>(BTreeMap::<u8, u8>::new());
+        let _ = transmute::<_, BTreeMap<u8, u32>>(BTreeMap::<u32, u32>::new());
+        // wrong layout
+        let _ = transmute::<_, BTreeMap<u8, u32>>(BTreeMap::<u8, [u8; 4]>::new());
+        let _ = transmute::<_, BTreeMap<u32, u32>>(BTreeMap::<[u8; 4], u32>::new());
+
+        // wrong size
+        let _ = transmute::<_, HashMap<u8, u32>>(HashMap::<u8, u8>::new());
+        let _ = transmute::<_, HashMap<u8, u32>>(HashMap::<u32, u32>::new());
+        // wrong layout
+        let _ = transmute::<_, HashMap<u8, u32>>(HashMap::<u8, [u8; 4]>::new());
+        let _ = transmute::<_, HashMap<u32, u32>>(HashMap::<[u8; 4], u32>::new());
+    }
+}

tests/ui/transmute_collection.stderr

@@ -0,0 +1,112 @@
+error: transmute from `std::vec::Vec<u8>` to `std::vec::Vec<u32>` with mismatched layout is unsound
+  --> $DIR/transmute_collection.rs:9:17
+   |
+LL |         let _ = transmute::<_, Vec<u32>>(vec![0u8]);
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `-D clippy::unsound-collection-transmute` implied by `-D warnings`
+
+error: transmute from `std::vec::Vec<u32>` to `std::vec::Vec<[u8; 4]>` with mismatched layout is unsound
+  --> $DIR/transmute_collection.rs:11:17
+   |
+LL |         let _ = transmute::<_, Vec<[u8; 4]>>(vec![1234u32]);
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: transmute from `std::collections::VecDeque<u8>` to `std::collections::VecDeque<u32>` with mismatched layout is unsound
+  --> $DIR/transmute_collection.rs:14:17
+   |
+LL |         let _ = transmute::<_, VecDeque<u32>>(VecDeque::<u8>::new());
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: transmute from `std::collections::VecDeque<[u8; 4]>` to `std::collections::VecDeque<u32>` with mismatched layout is unsound
+  --> $DIR/transmute_collection.rs:16:17
+   |
+LL |         let _ = transmute::<_, VecDeque<u32>>(VecDeque::<[u8; 4]>::new());
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: transmute from `std::collections::BinaryHeap<u8>` to `std::collections::BinaryHeap<u32>` with mismatched layout is unsound
+  --> $DIR/transmute_collection.rs:19:17
+   |
+LL |         let _ = transmute::<_, BinaryHeap<u32>>(BinaryHeap::<u8>::new());
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: transmute from `std::collections::BinaryHeap<[u8; 4]>` to `std::collections::BinaryHeap<u32>` with mismatched layout is unsound
+  --> $DIR/transmute_collection.rs:21:17
+   |
+LL |         let _ = transmute::<_, BinaryHeap<u32>>(BinaryHeap::<[u8; 4]>::new());
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: transmute from `std::collections::BTreeSet<u8>` to `std::collections::BTreeSet<u32>` with mismatched layout is unsound
+  --> $DIR/transmute_collection.rs:24:17
+   |
+LL |         let _ = transmute::<_, BTreeSet<u32>>(BTreeSet::<u8>::new());
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: transmute from `std::collections::BTreeSet<[u8; 4]>` to `std::collections::BTreeSet<u32>` with mismatched layout is unsound
+  --> $DIR/transmute_collection.rs:26:17
+   |
+LL |         let _ = transmute::<_, BTreeSet<u32>>(BTreeSet::<[u8; 4]>::new());
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: transmute from `std::collections::HashSet<u8>` to `std::collections::HashSet<u32>` with mismatched layout is unsound
+  --> $DIR/transmute_collection.rs:29:17
+   |
+LL |         let _ = transmute::<_, HashSet<u32>>(HashSet::<u8>::new());
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: transmute from `std::collections::HashSet<[u8; 4]>` to `std::collections::HashSet<u32>` with mismatched layout is unsound
+  --> $DIR/transmute_collection.rs:31:17
+   |
+LL |         let _ = transmute::<_, HashSet<u32>>(HashSet::<[u8; 4]>::new());
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: transmute from `std::collections::BTreeMap<u8, u8>` to `std::collections::BTreeMap<u8, u32>` with mismatched layout is unsound
+  --> $DIR/transmute_collection.rs:34:17
+   |
+LL |         let _ = transmute::<_, BTreeMap<u8, u32>>(BTreeMap::<u8, u8>::new());
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: transmute from `std::collections::BTreeMap<u32, u32>` to `std::collections::BTreeMap<u8, u32>` with mismatched layout is unsound
+  --> $DIR/transmute_collection.rs:35:17
+   |
+LL |         let _ = transmute::<_, BTreeMap<u8, u32>>(BTreeMap::<u32, u32>::new());
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: transmute from `std::collections::BTreeMap<u8, [u8; 4]>` to `std::collections::BTreeMap<u8, u32>` with mismatched layout is unsound
+  --> $DIR/transmute_collection.rs:37:17
+   |
+LL |         let _ = transmute::<_, BTreeMap<u8, u32>>(BTreeMap::<u8, [u8; 4]>::new());
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: transmute from `std::collections::BTreeMap<[u8; 4], u32>` to `std::collections::BTreeMap<u32, u32>` with mismatched layout is unsound
+  --> $DIR/transmute_collection.rs:38:17
+   |
+LL |         let _ = transmute::<_, BTreeMap<u32, u32>>(BTreeMap::<[u8; 4], u32>::new());
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: transmute from `std::collections::HashMap<u8, u8>` to `std::collections::HashMap<u8, u32>` with mismatched layout is unsound
+  --> $DIR/transmute_collection.rs:41:17
+   |
+LL |         let _ = transmute::<_, HashMap<u8, u32>>(HashMap::<u8, u8>::new());
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: transmute from `std::collections::HashMap<u32, u32>` to `std::collections::HashMap<u8, u32>` with mismatched layout is unsound
+  --> $DIR/transmute_collection.rs:42:17
+   |
+LL |         let _ = transmute::<_, HashMap<u8, u32>>(HashMap::<u32, u32>::new());
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: transmute from `std::collections::HashMap<u8, [u8; 4]>` to `std::collections::HashMap<u8, u32>` with mismatched layout is unsound
+  --> $DIR/transmute_collection.rs:44:17
+   |
+LL |         let _ = transmute::<_, HashMap<u8, u32>>(HashMap::<u8, [u8; 4]>::new());
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: transmute from `std::collections::HashMap<[u8; 4], u32>` to `std::collections::HashMap<u32, u32>` with mismatched layout is unsound
+  --> $DIR/transmute_collection.rs:45:17
+   |
+LL |         let _ = transmute::<_, HashMap<u32, u32>>(HashMap::<[u8; 4], u32>::new());
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 18 previous errors
+