Clippy lint · Correctness · checked with Clippy 0.1.99

clippy::unsound_collection_transmute: transmute from `std::vec::Vec<u16>` to `std::vec::Vec<u32>` with mismatched layout is unsound

What it meansChecks for transmutes between collections whose types have different ABI, size or alignment. It is in the correctness group and denied by default (the build fails).

What you see

error: transmute from `std::vec::Vec<u16>` to `std::vec::Vec<u32>` with mismatched layout is unsound
 --> main.rs:6:5
  |
6 |     std::mem::transmute::<_, Vec<u32>>(vec![2_u16])
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#unsound_collection_transmute
  = note: `#[deny(clippy::unsound_collection_transmute)]` on by default

Real output of Clippy 0.1.99 (Rust 1.99) on the example below. Names and line numbers in yours will differ.

Does it matter?

This is undefined behavior.

Code that is almost certainly wrong. Denied by default: the build fails until you fix it or allow it.

How to fix it

Clippy does not fix this one automatically. Change the code by hand:

Flagged

// 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])
};

Write this instead

vec![2_u16].into_iter().map(u32::from).collect::<Vec<_>>();

Turn it off, or on

Allow it where it does not fit your code, as narrowly as you can:

On one item

#[allow(clippy::unsound_collection_transmute)]
fn my_function() { /* ... */ }

Whole crate (Cargo.toml)

[lints.clippy]
unsound_collection_transmute = "allow"

One run

cargo clippy -- -A clippy::unsound-collection-transmute

The [lints] table needs Rust 1.74 or newer. In a workspace, put it under [workspace.lints.clippy] and add [lints] workspace = true to each member.

Correctnessgroup

denydefault level

nofixed by cargo clippy --fix

1.40.0added in Rust

Clippy's own documentation

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

// 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:

vec![2_u16].into_iter().map(u32::from).collect::<Vec<_>>();

From the Clippy lint list (0.1.99), part of the Rust project, MIT or Apache-2.0. Official entry.

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