Deprecate assigning a slice to a static array?

[Nick Treleaven]

I didn't know initializing a static array from a slice of unknown 
length was allowed until I saw:
https://dlang.org/blog/2022/06/21/dip1000-memory-safety-in-a-modern-system-programming-language-pt-1/

     // allocated in static program memory
     auto hello1 = "Hello world!";
     // allocated on the stack, copied from hello1
     immutable(char)[12] hello2 = hello1;

How does the compiler know `hello1.length` at compile-time for 
the initialisation of the static array? It doesn't - there is a 
runtime check. I think it's better to require slicing of either 
the static array or the slice to make that clear:

     int[] s = ...;
     int[2] a;
     a[] = s; // copy slice to slice, checking lengths match
     a = s[0..2]; // copy 2 elements, no runtime check

(The last line is recognised as a slice of compile-time known 
length - described here:
https://dlang.org/spec/expression.html#slice_to_static_array).

For just `a = s;`, the programmer may have accidentally assumed 
`s` was a static array, or forgot to slice it to match the static 
array size. I think it's bug prone. It's also hard to detect if 
the slice normally has a matching length but in some weird 
runtime condition it doesn't and it throws. It's also a hidden 
runtime check that could hurt performance.

Note this doesn't seem to be documented in the spec yet:
[1] https://dlang.org/spec/arrays.html#assignment
[2] https://dlang.org/spec/arrays.html#array-copying
[3] https://dlang.org/spec/arrays.html#array-initialization

[1] actually has:

     //s = a; // error, as a's length is not known at compile-time

I added that to the docs but it's wrong (if this feature is to 
stay I will make a PR to fix that line).

[2] only talks about slices. [3] doesn't mention initializing 
from an lvalue. *AssignExpression* just links to Array Copying 
[2] and Array Setting (for assigning a single element).

Would deprecating this break much code? Perhaps an exploratory 
dmd PR could be made to see if it breaks any of the tested key 
projects?