[Issue 22944] New: Hard-to-read error message when a delegate isn't @safe

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=22944

          Issue ID: 22944
           Summary: Hard-to-read error message when a delegate isn't @safe
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: andy.pj.hanson at gmail.com

The error message for this code is verbose:

```
@safe @nogc pure nothrow:

void a(int* i) {
        b((int x, int y, string s) {
                return x + y + *(i + 1);
        });
}

void b(scope int delegate(int x, int y, string s) @safe @nogc pure nothrow x)
{}
```

The error is:
```
a.d(4): Error: function `a.b(scope int delegate(int x, int y, string s) pure
nothrow @nogc @safe x)` is not callable using argument types `(int delegate(int
x, int y, string s) pure nothrow @nogc @system)`
a.d(4):        cannot pass argument `__lambda2` of type `int delegate(int x,
int y, string s) pure nothrow @nogc @system` to parameter `scope int
delegate(int x, int y, string s) pure nothrow @nogc @safe x`
a.d(3): Error: function `a.a` is `@nogc` yet allocates closures with the GC
a.d(4):        a.a.__lambda2 closes over variable i at a.d(3)
```

The string representation of a delegate type can take up a lot of space. When
dealing with a delegate with several parameters, it's easy to miss that the
provided delegate is `@system` and was expected to be `@safe`.
At least, It would be nice if the diagnostic would point that out: "cannot pass
argument `__lambda2` which is `@system` to parameter `x` which is `@safe`."

But ideally the error message would tell you *why* the delegate isn't @safe. An
ideal error would be just:
```
a.d(5): Error: pointer arithmetic not allowed in @safe functions
```

which is what you get if you explicitly mark the delegate as @safe. Since there
is only one overload of `b`, it seems like that the compiler could just make
the delegate @safe automatically and issue errors on anything it finds that
isn't @safe.

Also, the @nogc error is unrelated and goes away when fixing the @safe issue.

--