[Issue 10258] New: Add hasAccess trait

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=10258

           Summary: Add hasAccess trait
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody at puremagic.com
        ReportedBy: andrej.mitrovich at gmail.com


--- Comment #0 from Andrej Mitrovic <andrej.mitrovich at gmail.com> 2013-06-03 15:53:20 PDT ---
One of the most common implementation problems in my template code is that I
rarely properly check for access issues. Take the following for example:

b.d:
-----
module b;
import std.conv;
import std.stdio;

string[] getValues(T)(T var)
{
    string[] values;

    foreach (member; __traits(allMembers, T))
        values ~= to!string(__traits(getMember, var, member));

    return values;
}

struct S
{
    int x;
    private int y;
}

void main()
{
    S s;
    writeln(getValues(s));
}
-----

All is well and good. But now try moving the S struct and main function into
another module:

-----
module a;
import b;

import std.stdio;

struct S
{
    int x;
    private int y;
}

void main()
{
    S s;
    writeln(getValues(s));
}
-----

And now we get: Error: struct test.S member y is not accessible

This is a commonly overlooked problem because unittests for templates are
usually written right alongside them, and "private" extends to the entire
module.

We currently have a "getProtection" attribute, but it's awkward to use it
because if a symbol has a package or protected attribute we still won't know
whether we actually have access to it *from the call point*.

Using is(typeof()) to check accessibility is a hacky workaround, so instead I
propose that we implement a new trait, called hasAccess. It would be used like
so:

-----
string[] getValues(T)(T var)
{
    string[] values;

    foreach (member; __traits(allMembers, T))
    {
        // new
        static if (__traits(hasAccess, __traits(getMember, var, member))
        {
            values ~= to!string(__traits(getMember, var, member));
        }
    }

    return values;
}
-----

The compiler would then check if the current scope has access to a symbol and
return true or false.

The true benefit of the 'hasAccess' trait is that it will automatically work
for templates, mixin templates, and string mixin code.

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