If you could make any changes to D, what would they look like?

[Dennis]

On Wednesday, 27 October 2021 at 19:53:52 UTC, H. S. Teoh wrote:
> Unfortunately, a quick test with dmd did not show the expected 
> optimization.  Which confirms what I said before that pure's 
> optimization benefit is really rather marginal.

The function needs to be `nothrow` and compiled with `-O 
-release`, because dmd needs to account for exceptions / errors, 
and even then the optimization [shouldn't really be 
done](https://issues.dlang.org/show_bug.cgi?id=22277). LDC 
doesn't do anything with `pure` for optimizations, but since it 
has cross module inlining, it doesn't need it.

> The bigger benefit is the mechanical verification that you 
> didn't accidentally depend on global state when you didn't want 
> to, which can help with improving code quality.

That, but it can also help proving a lack of aliasing. For 
example:
```D
int[] duplicate(scope int[] x) pure;
```
The returned array can safely be converted to `immutable int[]`. 
(N.b. this is not implemented yet)

```D
void foo(int[] x, int[] y) pure nothrow;
```
Parameters `x` and `y` can freely be inferred `scope`, which is 
useful since dmd's `scope` inference from function bodies is not 
good.