extern(C) void main()
{
    import core.stdc.stdlib : alloca;
    
    enum length = 20;
    auto msg = (cast(char*)alloca(length))[0 .. length];
}

https://run.dlang.io/is/dHAqPo

$ dmd -betterC main.d
error: undefined reference to '__alloca'
collect2: error: ld returned 1 exit status
Error: linker exited with status 1

Works fine with LDC, however: https://run.dlang.io/is/m45jG1

Also, `alloca` should not be in `core.stdc.stdlib`; it's not a standard C function.

It may also be better to allocate static arrays on the stack when their length is supplied at runtime.  See https://issues.dlang.org/show_bug.cgi?id=18788.  That would make `alloca` and `__alloca` just an implementation detail that users wouldn't even need to use directly.