Why do "const inout" and "const inout shared" exist?

[Walter Bright via Digitalmars-d]

On 7/2/2017 5:46 AM, Shachar Shemesh wrote:
> Second, there are optimizations that can take place over "const" that cannot 
> over "shared const" even assuming aliasing (such as if the compiler knows no 
> other pointer was changed between two accesses).
> 
> The last point is that assuming no pointer aliasing is a fairly common 
> optimization to take in C and C++, simply because of the huge performance gains 
> it provides. It is so huge that it is sometimes turned on by default despite the 
> fact it changes language semantics. It would be a pity to block any potential to 
> have it in D.

Pointer aliasing is indeed a big deal for optimization.

But that really has nothing to do with const in C++. The trouble with C++ const 
is you can legally cast it away in C++. Chandler Carruth (LLVM optimizer) told 
me that const was basically ignored by the optimizer because of that.

D const is different in that the compiler is allowed to generate code as if 
const was never cast to immutable. Such casts are also not allowed in @safe code.