Document why 'mix xref' can give false positives

[nathanl]

"False positives" might not be the best way to describe this case; if not, please advise.

[josevalim]

Thank you. I have incorporated your changes into the existing description, so we provide an example. Let me know what you think.

[michallepicki]

I also have been using the term "unnecessary recompilations" at work when explaining this topic like @nathanl originally did here. "False positives" does not sound right, these are real dependencies caused by (understandable) limitations in compiler's dependency tracking model/algorithm

[josevalim]

💚 💙 💜 💛 ❤️

[josevalim]

I have merged this as is. If you want to provide more context, another PR is definitely welcome!

[nathanl]

B does not need to be recompiled, because it depends on C at runtime, but anything that depends on B at compile-time has to recompile, and that includes A.

I still find this confusing. I would picture the compiler saying:

• C changed, but B depends on it at runtime so B doesn't need to recompile
• A depends on B at compile time, so since B doesn't need to be recompiled, A doesn't need to be recompiled

... except it does. I was trying to have the docs explain why that's the case, but I don't really understand.

[josevalim]

Anything that is called at compile-time becomes compile-time behaviour. If you only call things at runtime, then it is all runtime. But as soon as you call something at compile-time, anything in that module and what that module calls becomes compile-time, because all of them can now change the behaviour of A.

[michallepicki]

@nathanl Here are some resources, I'm sure there are even more out there. I hope at least one of them is helpful!

• ElixirConf 2018 - Understanding Elixir's Re compilation - Renan Ranelli (conference talk)
• Wojtek Mach - Speeding up re-compilation of Elixir projects (blog post)
• Thinking Elixir - Compile Faster with Marc-André Lafortune (podcast)
• Tim Gent - How to speed up your Elixir compile times (blog post series)
• Exploring Elixir Project Re compilation | Anton Satin | ElixirConf EU 2022 (conference talk)
• Understanding Elixir Compilation by Tim Gent | ElixirConf EU 2023 (conference talk)