std.digest toHexString

[Adam D. Ruppe via Digitalmars-d-learn]

On Thursday, 16 March 2017 at 17:20:10 UTC, Carl Sturtivant wrote:
> OK, but if I try to do this,
>
> char[2] u;
> string s = u;

Yeah, that's because `u` is not necessarily unique like a 
function return value:

char[2] u;
char[] mutable = u[];
string s = u[];

char before = s[0];

mutable[0] = 'n';

char after = s[0];

assert(before == after); // fails! because mutable[0] changed it.



But, with a function return value, there's no opportunity to 
insert that sneaky `char[] mutable = u[];` alias, so the compiler 
is free to assume that it is the only reference and do the 
immutable thing.

If it returns a value type, the compiler can cast to immutable at 
any time because it knows there is no chance to insert an alias 
at all - the function return is the only way to access it.

If it returns a reference type, the compiler can cast to 
immutable if the function is *strongly* pure, because the purity 
rules also don't give you an opportunity to sneak a mutable 
reference out (pure cannot write to stuff outside the return 
values).

I said *strongly* because the compiler will correctly complain if 
you try to pass a mutable argument back as the return value. It 
realizes whomever passed the argument might still have an alias 
and will not allow the implicit cast:

pure char[] foo(char[] a) { return a; }
char[] lolStillMutable;
string tricky = foo(lolStillMutable); // illegal