Question: I'm developing a Java project using Eclipse, and Ant as a build tool. When I run "ant all" from the command line, my project builds without any errors, but on Eclipse I get many compilation errors.

So I thought I'd copy Ant's Classpath onto my Eclipse Project's Build Path. 

Is there an Ant task/command to show that? Like "ant just show me your assembled classpath" or something?

Answer: 
If you run Ant with the -verbose and -debug flags, you'll see all gory details of what javac is doing, including the classpath.
I would introduce a task for printing the classpath, and call that task with antcall. The classpath would be given as a parameter to that task.