Why can't function expecting immutable arg take mutable input?

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On Friday, 16 October 2015 at 10:35:23 UTC, Shriramana Sharma 
wrote:
> Hello. I still haven't wrapped my mind around the 
> const/immutable thing yet and am still stuck in C/C++ mode. :-(
>
> A function that takes mutable arguments cannot be called with 
> immutable input at the call site since it does not promise to 
> *not* mutate the input. That's of course clear.
>
> Why can't a function that takes an immutable argument be called 
> with a mutable input at the call site?
>
> IOW, why isn't mutable implicitly convertible to immutable?
>
> I just finished writing a string processing module which calls 
> multiple subroutines, and all of them carrying arguments with 
> type `string` viz. `immutable(char)[]` IIUC, and I tried to 
> pass it something which came from File.byLine(), then got the 
> error:
>
> function textattr.applyTextAttr (string text) is not callable 
> using argument types (char[])
>
> I understand that const can refer to either mutable or 
> immutable, so does this mean I should replace all occurrences 
> of `string` in arguments and return values of functions by 
> `const(char)[]`?

This actually *is* possible, if the type you're passing is a 
value type with no indirections. Then you can just pass it by 
value and it will be implicitly convertible to immutable. This 
doesn't work for char because it has indirections (a pointer to 
its data).