I have quite a lot of MMIO in an embedded project, defined and used like this:

const REGISTER: *mut u32 = 0xDEADBEEF as *mut u32; unsafe fn func() { *REGISTER = 1; }

(see a playground link)

When I cargo build, I'm getting a whole host of warnings of the type:

warning: attempting to modify a `const` item --> src/lib.rs:3:5 | 3 | *REGISTER = 1; | ^^^^^^^^^^^^^ | = note: `#[warn(const_item_mutation)]` on by default = note: each usage of a `const` item creates a new temporary - the original `const` item will not be modified note: `const` item defined here --> src/lib.rs:1:1 | 1 | const REGISTER: *mut u32 = 0xDEADBEEF as *mut u32; | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 

I expect no warnings, because I'm not assigning to a const, I'm assigning to a dereferenced *mut u32, aka a mutable u32.

I couldn't figure out a playground example that would show the value actually mutating, but by hitting hardware registers I can prove that the value does get mutated, and it makes sense that it would: The pointer is const, but it's pointing to a mutable u32. Dereferencing shouldn't care whether the pointer variable is const.

Meta

rustc --version --verbose:

rustc 1.46.0 (04488afe3 2020-08-24) binary: rustc commit-hash: 04488afe34512aa4c33566eb16d8c912a3ae04f9 commit-date: 2020-08-24 host: x86_64-unknown-linux-gnu release: 1.46.0 LLVM version: 10.0 

No backtrace for this bug; it's a warning.