[Issue 7994] New: Impure mixin generator of pure code inside pure functions too

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=7994

           Summary: Impure mixin generator of pure code inside pure
                    functions too
           Product: D
           Version: D2
          Platform: x86
        OS/Version: Windows
            Status: NEW
          Keywords: rejects-valid
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody at puremagic.com
        ReportedBy: bearophile_hugs at eml.cc


--- Comment #0 from bearophile_hugs at eml.cc 2012-04-26 17:33:22 PDT ---
This is yet another small refinement for the implementation of purity in D (I
think this is more than an enhancement request, because I think the second
example is supposed to work in a good purity implementation).

This is wrong D2 code:

import std.stdio;
string foo() pure {
    return `writeln("hello");`;
}
void main() pure {
    mixin(foo());
}


And the error message is correct because mixin(foo) causes main() call an
impure function (DMD 2.060alpha):

test.d(6): Error: pure function 'main' cannot call impure function 'writeln'


But in this case:


string foo() {
    import std.string;
    return xformat("%d", 1);
}
void main() pure {
    int x = mixin(foo());
}


It gives:
test.d(6): Error: pure function 'main' cannot call impure function 'foo'


This time the error message is wrong, because desite foo() itself is not pure,
mixin() calls it at compile-time, and the resulting main is equivalent to this,
that is pure and correct:

void main() pure {
    int x = 1;
}

So mixin() needs to be allowed to call impure functions too, even inside pure
functions, as long as the resulting mixed-in code is pure.

This is quite handy, because the generation of text to mix-in often calls
functions like xformat and text that currently are not pure.

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