include! is a smell and should be made obsolete for code-generating cargo packages

[emberian]

In particular, we should somehow allow using OUT_DIR in the #[path] of a mod foo, to allow parsing to proceed normally. Since include! doesn't do textual inclusion, but rather AST splicing, and is rather blind, it doesn't pass in the correct context to allow things like outer attributes to work in include!'d code, even if it's mod foo { include!("foo.rs"); }. It's conservatively correct, but conservatively obnoxious.

[huonw]

This is related to/might be fixed by #18849 (in fact, it is the motivating example in that issue). Also, cc rust-lang/rfcs#752.

@alexcrichton, you may have thoughts about this, especially experience with cargo, and what cargo needs.

[steveklabnik]

Triage: regardless of its smelliness, today it's in stable, so it has to stay.

Given the lack of other discussion, I'm going to close this one.