Description
Rust 1.51 was released yesterday and included the ptr::addr_of! macro. Its documentation talks about its usage in creating pointers to unaligned fields without the need for an intermediate reference:
However, &expr as *const _ creates a reference before casting it to a raw pointer, and that reference is subject to the same rules as all other references. This macro can create a raw pointer without creating a reference first.
Despite that, the documentation of ptr::write_unaligned states the following:
It is currently impossible to create raw pointers to unaligned fields of a packed struct.
Attempting to create a raw pointer to an unaligned struct field with an expression such as &packed.unaligned as *const FieldType creates an intermediate unaligned reference before converting that to a raw pointer. That this reference is temporary and immediately cast is inconsequential as the compiler always expects references to be properly aligned. As a result, using &packed.unaligned as *const FieldType causes immediate undefined behavior in your program.
The documentation of ptr::write_unaligned should be updated to reflect the new changes in the language.
Meta
rustc --version --verbose:
rustc 1.51.0 (2fd73fabe 2021-03-23)
binary: rustc
commit-hash: 2fd73fabe469357a12c2c974c140f67e7cdd76d0
commit-date: 2021-03-23
host: x86_64-unknown-linux-gnu
release: 1.51.0
LLVM version: 11.0.1
and
rustc 1.53.0-nightly (07e0e2ec2 2021-03-24)
binary: rustc
commit-hash: 07e0e2ec268c140e607e1ac7f49f145612d0f597
commit-date: 2021-03-24
host: x86_64-unknown-linux-gnu
release: 1.53.0-nightly
LLVM version: 12.0.0