`transmute-ptr-to-ptr`'d code can no longer be assigned to

[matthiaskrgr]

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