dip1000 and preview in combine to cause extra safety errors

[Nick Treleaven]

On Monday, 13 June 2022 at 11:14:36 UTC, deadalnix wrote:
> If the value stays on stack - which is all that DIP1000 can 
> check for anyways, then modern backend can track it. LLVM for 
> instance, will annotate function parameter to indicate if they 
> escape or not and do so recursively through the callgraph.

LLVM might not have the library source code.

> LDC is already able to do stack promotion when escape analysis 
> proves something doesn't escape.
>
> This is WAY preferable because:
>  - It works regardless of annotations from the dev.

So does scope inference (when it's done).

>  - It is always correct, it will not fubar existing @system 
> code.

I'm not sure why someone would have written scope in @system code 
unless they didn't understand it's only checked in @safe code. 
The stack promotion optimization is the only reason to use it in 
@system code AIUI.

>  - Inlining is likely to uncover more opportunity to do this, 
> there is no point doing it before.

Then the LLVM optimization and the scope optimisation are both 
needed for the best code in all cases.

> Doing this type of optimization to explicitly free elements on 
> heap is worth it. But DIP1000 doesn't allow to track this 
> reliably.

Please can you provide an example? (There's one Timon came up 
with in your scope thread recently, Walter's working on that).