Unused imports cause non-deterministic incremental compiles

[pauldraper]

Suppose

1. Persistent incremental compilation is used.
2. The compilation has no library dependencies, but imports example.A.
3. example.jar has a symbol example.A.

Step 1: Compile (fails)
Step 2: Add library dependency example.jar.
Step 3: Compile (succeeds).
Step 4: Remove library dependency example.jar.
Step 5: Compile (succeeds).

Steps 1 and 5 have the same source code, but 1 fails, and 5 succeeds.

[jvican]

I want to fix all non-deterministic behaviours in Zinc, and whenever I sit to do so this ticket will prove useful.

The reason for this change, assuming you're using 2.12, is most likely the unfinished classpath caching implementation in the compiler (not Zinc's fault): you can use -YdisableFlatCpCaching to confirm this is the case. We were bitten by this in our codebase too #282. For more information on what's missing, have a look at scala/scala-dev#416. We're trying to find a solution to correctly cache in scalac and propagate it to Zinc.

[pauldraper]

This doesn't have to do with 2.12.

This has to do with the fact that Zinc doesn't record information about imports if they are unused. And because the information is not recorded, the compile is no re-run if anything about the imported symbol is change, e.g. the dep is removed.

(A a good workaround of course is to not have unused imports.)

[jvican]

This issue is a duplication of #226, which describes the same underlying limitation. When I initially registered it, I marked it as wont fix but I have some ideas to correctly teach Zinc to register dependencies in this kind of scenarios, so I'll give updates on the original ticket.

[pauldraper]

Hm...yes. I originally didn't think so, but after a second look, the only way I can reproduce this bug is to use a wildcard import.