transmute-ptr-to-ptr'd code can no longer be assigned to #15003
Description
Using the following flags
--force-warn clippy::transmute-ptr-to-ptr
this code:
// Stacked Borrows detects that we are casting & to &mut and so it changes why we fail //@compile-flags: -Zmiri-disable-stacked-borrows use std::mem::transmute; #[allow(mutable_transmutes)] fn main() { unsafe { let bs = b"this is a test"; transmute::<&[u8], &mut [u8]>(bs)[4] = 42; //~ ERROR: read-only } }caused the following diagnostics:
Checking _static_memory_modification3 v0.1.0 (/tmp/icemaker_global_tempdir.1WUapHwIh9Kn/icemaker_clippyfix_tempdir.TwLaoJk7SvXJ/_static_memory_modification3) warning: transmute from a reference to a reference --> src/main.rs:10:9 | 10 | transmute::<&[u8], &mut [u8]>(bs)[4] = 42; //~ ERROR: read-only | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `&mut *(bs as *const [u8] as *mut [u8])` | = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#transmute_ptr_to_ptr = note: requested on the command line with `--force-warn clippy::transmute-ptr-to-ptr` warning: `_static_memory_modification3` (bin "_static_memory_modification3") generated 1 warning Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.12s However after applying these diagnostics, the resulting code:
// Stacked Borrows detects that we are casting & to &mut and so it changes why we fail //@compile-flags: -Zmiri-disable-stacked-borrows use std::mem::transmute; #[allow(mutable_transmutes)] fn main() { unsafe { let bs = b"this is a test"; &mut *(bs as *const [u8] as *mut [u8]).wrapping_add(4) = 42; //~ ERROR: read-only } }no longer compiled:
Checking _static_memory_modification3 v0.1.0 (/tmp/icemaker_global_tempdir.1WUapHwIh9Kn/icemaker_clippyfix_tempdir.TwLaoJk7SvXJ/_static_memory_modification3) error[E0277]: the size for values of type `[u8]` cannot be known at compilation time --> src/main.rs:10:48 | 10 | &mut *(bs as *const [u8] as *mut [u8]).wrapping_add(4) = 42; //~ ERROR: read-only | ^^^^^^^^^^^^ doesn't have a size known at compile-time | = help: the trait `std::marker::Sized` is not implemented for `[u8]` note: required by a bound in `std::ptr::mut_ptr::<impl *mut T>::wrapping_add` --> /home/matthias/.rustup/toolchains/master/lib/rustlib/src/rust/library/core/src/ptr/mut_ptr.rs:1213:12 | 1211 | pub const fn wrapping_add(self, count: usize) -> Self | ------------ required by a bound in this associated function 1212 | where 1213 | T: Sized, | ^^^^^ required by this bound in `std::ptr::mut_ptr::<impl *mut T>::wrapping_add` error[E0070]: invalid left-hand side of assignment --> src/main.rs:10:64 | 10 | &mut *(bs as *const [u8] as *mut [u8]).wrapping_add(4) = 42; //~ ERROR: read-only | ------------------------------------------------------ ^ | | | cannot assign to this expression Some errors have detailed explanations: E0070, E0277. For more information about an error, try `rustc --explain E0070`. error: could not compile `_static_memory_modification3` (bin "_static_memory_modification3") due to 2 previous errors warning: build failed, waiting for other jobs to finish... error: could not compile `_static_memory_modification3` (bin "_static_memory_modification3" test) due to 2 previous errors Version:
rustc 1.89.0-nightly (f315e6145 2025-06-06) binary: rustc commit-hash: f315e6145802e091ff9fceab6db627a4b4ec2b86 commit-date: 2025-06-06 host: x86_64-unknown-linux-gnu release: 1.89.0-nightly LLVM version: 20.1.5