Real cross-compiling tests instead of `#![no_core]` silliness

[workingjubilee]

We have a zillion tests that use the #![no_core] hack, like this one:

rust/tests/ui/asm/bad-template.rs

Line 10 in 5e3ede2

#![no_core]

A given test involves:

• Host A
• Target B
• Some Rust source (compiled for Target B)
• Test tooling (run on Host A)

Most of our test infra assumes that Host A and Target B are the same thing. This greatly simplifies a lot of assumptions. But this hack we have introduced in so many tests essentially exists so that we can be sure the test runs even when cross-compiling from Host A to Target B. It's possible there's some weird finagling of test flags that can enable this without the hack, but that would be another hack itself. Cross-compiled tests should be relatively easy, like just writing something like:

//@ cross-compile-targets: arch_1-unknown-os_3-env_5 arch_2-unknown-os_4-env_6 arch_1-unknown-os_4-env_5 arch_2-unknown-os_3-env_7

[workingjubilee]

sorry @jieyouxu!

[nikic]

The problem here is that without no_core, you actually have to build core/std for the target. We have some tests that test ~all targets, and having everyone build core/std for 100 targets just to run some codegen or assembly tests would be ... not great.

[workingjubilee]

That would probably be bad, yes. Handling this correctly probably requires some coordination between bootstrap and compiletest about which stdlibs are built (or can be built, or may be desirable to build).