[Issue 19874] New: imported CTFE static initializers are run for no reason

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19874

          Issue ID: 19874
           Summary: imported CTFE static initializers are run for no
                    reason
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Keywords: CTFE, performance
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: schveiguy at yahoo.com

If you import a module that has a static initializer for a mutable variable
that obviously cannot be used at compile time, and that imported module isn't
being compiled, the compiler should NOT run the CTFE function, as long as it
can know the type without running it.

For example:

-----
string buildModData()
{
    string result;
    foreach(i; 0 .. 10000)
        result = result ~ "lots and lots and lots of data";
    return result;
}

string moddata = buildModData(); 
----

buildModData should not be run at compile time if this is simply imported,
because the compiler isn't going to store it anywhere. My system consumes 1
second to import this file, regardless if I use moddata or not.

In addition, even if the variable is type-inferred:

---

static moddata = buildModData();
---

This should still not result in CTFE execution because the type is clear from
the function definition. Essentially, only the TYPE is important here, not what
the value is, since it's not readable at compile time anyway.

I would also wish to have the value left uncalculated even if the value IS
readable at compile time, but is not used. This may be a more difficult problem
to solve, as it requires lazy evaluation of imported items.

See forum thread: https://forum.dlang.org/post/qbckmt$1oks$1@digitalmars.com

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