memcpy() comparison: C, Rust, and D

[Nordlöw via Digitalmars-d]

On Tuesday, 31 January 2017 at 01:30:48 UTC, Walter Bright wrote:

> Rust:
>   pub unsafe extern fn memcpy(dest: *mut c_void,
>                             src: *const c_void,
>                             n: size_t)
>                             -> *mut c_void

> D:
>   pure void* memcpy(return void* s1, scope const void* s2, 
> size_t n);

> 2. The return value is derived from s1.

How can we be sure that the return value points to the same 
content as `s1`? If that is what you mean by "derived".

> The Rust declaration does not give us 1, 2 or 4 (because it is 
> marked as unsafe). If it was safe, the declaration does not 
> give us 2.

I don't see how Rust doesn't provide information 2 aswell. Is it 
because of differences in the meaning of `const` in Rust compared 
to D?