Auto merge of #31884 - alexcrichton:no-bootstrap-from-existing, r=brson

These commits add support to the rustbuild build system to compile non-build-system compilers. For example this will allow creating an ARM compiler on a x86 system. The high level way this works is:

* The only compiler ever run is the build target compiler. No other compiler is assumed to be runnable.
* As a result, all output artifacts come from the build compiler.
* The libs for the stageN cross-compiled compiler will be linked into place after the build target builds them.

This will break the assumption that a compiler never links to anything it didn't itself produce, but it retains the assumption that a compiler only ever links to anything built in the same stage. I believe this means that the stage1 cross-compiled compilers will still be usable.

I tested this by creating an `arm-unknown-linux-gnueabihf` compiler. So far the linking ended up all working OK (including LLVM being cross compiled), but I haven't been able to run it yet (in QEMU for a raspberry pi). I think that's because my system linker is messed up or something like that (some newer option is assumed when it's not actually there).

Overall, though, this means that rustbuild can compile an arm-unknown-linux-gnueabihf compiler. Ideally we could even start shipping nightlies based on this at some point!

0 files changed, 0 insertions, 0 deletions