Brainstorming improvements for new rustc devs

compiler
20

I think a possible fix for everybody would be:

  • make stage3 libxyz.so a copy of stage2 libxyz.so
  • have the --test tests be a part of stage2 (i.e. built by stage1)
  • have a rustbuild lock stage1/rustbuild unlock stage1 commands.

rustbuild lock stage1 makes sure stage1 is up-to-date, and prevents any further calls to rustbuild from recompiling it. rustbuild unlock stage1 reverts.

This means that ABI changes while stage1 is locked will cause stage2 and stage3 to differ - so this is unsafe, and works best if we weaken the metadata guard.

However, now everything works perfectly - the std tests now only depend on stage2 libstd and its dependencies, and if you change a part of the compiler, it only builds the compiler libs (incrementally, when we get that) and promotes the old stage2 to stage3.

If you change the ABI, you just need to rustbuild unlock stage1; rustbuild lock stage1 to recompile stage1 (maybe combine it: rustbuild relock stage1).

I think we will still want a separate stage3 on bors, to assure us that our compiler can compile itself correctly.

Adapting rustbuild to incremental compilation