Tell us your DIP1000 woes

[Paul Backus]

On Sunday, 25 August 2024 at 13:10:22 UTC, Mike Parker wrote:
> First, it's pretty clear that inference by default is necessary 
> to make DIP1000 easier to use. It's not clear at the moment how 
> this can be achieved, but it's something that we need to work 
> out.

In addition to inference, it is vitally important to have 
detailed error messages showing exactly where and why inference 
failed.

Here's an illustrative example:

     int* global;

     void foo()(int* p)
     {
         global = p;
     }

     void bar()()
     {
         int n;
         foo(&n);
     }

     @safe void main()
     {
         bar();
     }

Currently, compiling this program with `-preview=dip1000` gives 
the following error message:

     example.d(16): Error: `@safe` function `D main` cannot call 
`@system` function `example.bar!().bar`
     example.d(11):        which wasn't inferred `@safe` because 
of:
     example.d(11):        reference to local variable `n` 
assigned to non-scope parameter `p` calling `foo`

The problem with this message is that it does not explain *why* 
`p` is a non-scope parameter. In order to get that information, 
we have to modify the definition of `foo` and add `scope` 
manually:

     -void foo()(int* p)
     +void foo()(scope int* p)

When we do, we get an error message that points to the *real* 
source of the problem:

     example2.d(16): Error: `@safe` function `D main` cannot call 
`@system` function `example2.bar!().bar`
     example2.d(3):        which calls `example2.foo!().foo`
     example2.d(5):        which wasn't inferred `@safe` because 
of:
     example2.d(5):        scope variable `p` assigned to global 
variable `global`

In order for normal D programmers to have any hope of using DIP 
1000 successfully, all of the relevant information *must* be 
present in the original error message. Users must not be required 
to modify their code (or, worse, the code of libraries they 
depend on) in order to reveal this information.