Fix confusing message on array cast failure.

[schveiguy]

Still needs a bug report. First I want to see it pass CI, as I'm not sure if there's a test somewhere for this message.

[dlang-bot]

Thanks for your pull request, @schveiguy!

Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	22964	enhancement	array cast message is awkwardly worded

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + druntime#3792"

[schveiguy]

OK, this is ready for review, bug report is written.

[schveiguy]

An example:

    auto arr = [1,2,3,4,5];
    auto x = cast(long[])arr;

With current library:

An array of size 20 does not align on an array of size 16, so `int` cannot be cast to `long`

With change:

`int[]` of length 5 cannot be cast to `long[]` as its length in bytes (20) is not a multiple of `long.sizeof` (8).

[adamdruppe]

i like it, nice small improvement

[MoonlightSentinel]

@schveiguy I've added a unittest for this change.

[schveiguy]

@MoonlightSentinel thanks I didn't know how to make a unittest with this. Obvious in hindsight!

[PetarKirov]

How about:

`int[]` cannot be cast to `long[]` as its length in bytes (20) is not a multiple of `long.sizeof` (8).

[schveiguy]

@PetarKirov nice! I will update when at home.

[schveiguy]

@PetarKirov I left in the array length, as I still don't want the user to have to do too much math. But should be good now.