Issue 4539 - Refuse assignment to string literal

[9rnsr]

This change might not be enhancement, but breaks existing code.

[WalterBright]

Needs more work, fails the following from xtest46.d:

void test69()
{
digestToString69(cast(ubyte[16])x"c3fcd3d76192e4007dfb496cca67e13b");
}

void digestToString69(ref const ubyte[16] digest)
{
assert(digest[0] == 0xc3);
assert(digest[15] == 0x3b);
}

/***************************************************/

void test70()
{
digestToString70("1234567890123456");
}

void digestToString70(ref const char[16] digest)
{
assert(digest[0] == '1');
assert(digest[15] == '6');
}

Reverted for now.

[9rnsr]

I think ref parameter should not receive string literal, because it is rvalue.
Now dmd treats array literal (not string) as rvalue, so following code is invalid:
void f(ref int[] arr){...}
f([1,2]); // rejected

Does D specification treat string against ref parameter as special case?
If not, this changes valid and test suite is invalid.