www.digitalmars.com         C & C++   DMDScript  

digitalmars.D.bugs - [Issue 20101] New: BetterC: Template instantiation in CTFE only

https://issues.dlang.org/show_bug.cgi?id=20101

          Issue ID: 20101
           Summary: BetterC: Template instantiation in CTFE only context
                    should skip codegen / nogc / ... Phases
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody puremagic.com
          Reporter: johannespfau gmail.com

In D, we often uses helpers to generate code for string mixins and similar
purposes:

--------------------------------------
public string ctfeHelper()(string a)
{
    return "int " ~ a ~ " = 42;";
}

extern(C) int main()  nogc
{
    mixin(ctfeHelper("a"));
    return a;
}
--------------------------------------

With -betterC, this code currently fails with this message:
test.d(3): Error: array concatenation of expression "int " ~ a ~ " = 42;"
requires the GC which is not available with -betterC

However, the code is exclusively used in a CTFE-only context where the GC is
conceptionally always available.

Note that if ctfeHelper was a regular function, we would have to emit runtime 
code, as it is public and could be called from external code. So in that case,
the error would be correct and there's no way to make this work (except for
making ctfeHelper private, proofing that it's never used at runtime and that
it's unreachable from other modules (which is never the case, if you consider
explicitly linking to the mangled name to be valid)).

But for this example, where `ctfeHelper` is a template, we could argue that
CTFE-only instatiations should be separated from runtime instantiations: If we
do not generate code for the template in CTFE contexts, that's still perfectly
fine: Anyone calling ctfeHelper at runtime will instantiate the template again,
so the codegen can be executed there. And the user should get the errors there,
when instantiating the template in a runtime context.

Fixing this would probably involve marking an TemplateInstance as ctfeOnly.
However, it seems the implementation would be 99% identical to isSpeculative
templates. Consider this code, which already works with -betterC:


--------------------------------------
string ctfeHelper()(string a)
{
    return "int " ~ a ~ " = 42;";
}

extern(C) int main()  nogc
{
    return  __traits(compiles, ctfeHelper("a"));
}

--------------------------------------

--
Aug 04 2019