Declare proguard rules automatically

[juliocbcotta]

Could gson have basic proguard rules automatically declared as retrofit does ?
https://github.com/square/retrofit/blob/master/retrofit/src/main/resources/META-INF/proguard/retrofit2.pro

[eamonnmcmanus]

I think that's exactly what #1929 proposes, but for that PR to proceed someone should write a test for it.

[basurahan]

Any update to this one

[Marcono1234]

Default rules have been added by #2397 and will be available in the next version. However, would be great if you could try building Gson locally already and see if the snapshot version of Gson correctly applies the rules.

Any feedback regarding these rules, such as if something is missing or is too coarse, is highly appreciated!

Also keep in mind that these default library rules cannot cover everything; if you have specific classes or fields which should be preserved and you are not using @SerializedName you might have to add additional rules for your build.