File names in string mixins

[Simen Kjærås]

In a thread[0] over on D.learn, bauss suggested adding a second 
argument to MixinExpressions, such that mixin("foo()", "my 
mixin") compiles. The use case for the extra argument is as an 
identifier that will be used in error messages, where the current 
behavior is to create a virtual file name on the form 
__FILE__-mixin-__LINE__. Instead of this behavior:

unittest {
     // .\foo.d-mixin-17(17): Error: undefined identifier bar
     mixin("bar();");
}

You would have this behavior:

unittest {
     // My mixin(1): Error: undefined identifier bar
     mixin("bar();", "My mixin");
}

A toy example like this doesn't really show how useful this would 
be, but in a more complex use case where multiple mixins are 
generated from different template arguments, in a static foreach, 
etc, the relevant context may be added by the programmer as he 
writes the code:

static foreach (i, item; getCodeSources()) {
     mixin(item.generateCode(), "Code source "~i.to!string);
}

When the above code fails on item 15, the error message would 
immediately show "Code source 15(__LINE__): <Error message>", 
making it much easier to figure out where to start fixing.

Destroy!

--
   Simen
[0]: 
https://forum.dlang.org/post/tbyqqefafgzfiltglltd@forum.dlang.org