@system blocks and safer @trusted (ST) functions

[jfondren]

On Monday, 26 July 2021 at 00:37:36 UTC, ag0aep6g wrote:
> What if favoriteNumber originally returns a ubyte, and 
> favoriteElement takes an int[256]?
>
> ```d
> ubyte favoriteNumber() @safe { return 42; }
> int favoriteElement(ref int[256] array) @trusted
> {
>     return array.ptr[favoriteNumber()];
> }
> ```
>
> To your reviewer, there's nothing wrong with favoriteElement, 
> right?
>
> But later someone might change the return type of 
> favoriteNumber to size_t and let it return 300. Badaboom: 
> undefined behavior after touching @safe code.

That's a much more obviously program-affecting change though, 
you're changing a function signature. It wouldn't make as 
compelling an example of someone being surprised that they have 
to review more than just a @safe function when that only that 
function is changed.

If you do name the index type then you can do something like this 
Nim translation of the Ada:

```nim
type
   Array = array[50, int]
   Index = range[0..49]

var myarray: Array
myarray[42] = 5

func favoriteNumber: Index = 42
func favoriteElement(arg: Array): int =
   let i: Index = favoriteNumber()
   return arg[i]

echo favoriteElement(myarray)
```

(But Nim disappoints here: if you change favoriteNumber to return 
an int, and then change the number to 142, then Nim doesn't 
complain at all about this code that assigns an int to a Index 
variable.)