Auto merge of #9252 - ehuss:prefer-dynamic-ws, r=alexcrichton

Fix logic for determining prefer-dynamic for a dylib.

The logic for determining if a dylib should use `prefer-dynamic` used to be something like "do not use prefer-dynamic if it is the current package".

The current logic has a strange behavior where it works as intended if there is only one package in the workspace, but a workspace with multiple packages will always use `prefer-dynamic`.

Instead of using `current_opt`, which isn't a good concept to use in a workspace, I switched this to be "primary" (a package selected on the command-line).

**History**
* 9879a0a Initial prefer-dynamic behavior.
* #3221 changed to the faulty logic (see comments at #3221 (comment)). I think there was some confusion there.
* #3478 fixed the logic for one of the changed conditions, but not the one for prefer-dynamic

src/cargo/core/compiler/mod.rs

@@ -792,7 +792,7 @@ fn build_base_args(
     }
 
     let prefer_dynamic = (unit.target.for_host() && !unit.target.is_custom_build())
-        || (crate_types.contains(&CrateType::Dylib) && bcx.ws.members().any(|p| *p != unit.pkg));
+        || (crate_types.contains(&CrateType::Dylib) && !cx.is_primary_package(unit));
     if prefer_dynamic {
         cmd.arg("-C").arg("prefer-dynamic");
     }