Fix Rust compiler download error on Android

[MortimerGoro]

After 6b52330 landed Android builds fail when downloading Rustc Compiler:

Downloading Host rust library for target armv7-linux-androideabi...
Download failed (404): Not Found - https://s3.amazonaws.com/rust-lang-ci/rustc-builds-alt/3bfc18a9619a5151ff4f11618db9cd882996ba6f/rust-std-nightly-armv7-linux-androideabi.tar.gz

I asked in rust-infra and they said that alt builds aren't compiled yet for all platforms (e.g. Android)

---

• ./mach build -d does not report any errors
• ./mach test-tidy does not report any errors
• These changes fix #__ (github issue number if applicable).

• There are tests for these changes OR
• These changes do not require tests because _____

---

This change is 

[highfive]

Heads up! This PR modifies the following files:

• @wafflespeanut: python/servo/bootstrap_commands.py

[nox]

I would rather have us error out if llvm-assertions is set on a platform that doesn't support that. What do you think @jdm?

[SimonSapin]

This isn’t specific to Android. We have an explicit list of platforms that have these builds:

servo/python/servo/command_base.py

Lines 263 to 266 in 42e551f

	# https://github.com/rust-lang/rust/pull/39754
	platforms_with_rustc_alt_builds = ["unknown-linux-gnu", "apple-darwin", "pc-windows-msvc"]
	llvm_assertions_default = ("SERVO_RUSTC_LLVM_ASSERTIONS" in os.environ
	or host_platform() not in platforms_with_rustc_alt_builds)

It looks like the problem is that that code only checks the host platform, not the target when cross-compiling. I wonder how #17561 landed though, since we do have CI for Android. Anyway, this should use the same list of platforms (maybe by making it a Python global variable).

[MortimerGoro]

Ok, I updated the PR to also check target platform in order to set llvm-assertions default configuration value

[SimonSapin]

Does this really fix the issue? The code added in bootstrap_rustc is executed well after CommandBase.__init__, modifying self.config this late often doesn’t work (since whatever code reads the config has already executed).

Specifically here, I think the rust_path variable would be wrong.

[SimonSapin]

Taking a step back, do we need to use a "non-alt" compiler when using a non-alt standard library (because the alt one is not available for a given cross-compilation target)? If so, I think we have a problem with with how mach’s configuration is currently structured.

[MortimerGoro]

Does this really fix the issue? The code added in bootstrap_rustc is executed well after CommandBase.init, modifying self.config this late often doesn’t work (since whatever code reads the config has already executed).

Specifically here, I think the rust_path variable would be wrong.

I moved the code before using rust_path var. I have tested to compile, link and run on a clean directory and it worked.

Is there a better place for that code? We need to use the target argument.

[SimonSapin]

By on a clean directory, do you mean you also removed .servo/rust? If you do that and then compile for Android, is rustc not downloaded into .servo/rust/*-alt despite being a non-alt build? If you clean everything and then build twice (changing a source file in-between to force some re-compilation), does the second build (which should not go through bootstap_rust again) still work?

[SimonSapin]

We need to use the target argument.

Yeah, that’s the problem. Command-line arguments are not easily available in Command.__init__.

[MortimerGoro]

I reproduced the error. On a clean checkout:

1. ./mach build --release --android builds OK. New non-alt rust compiler in .servo/rust/
2. code change && ./mach build --release --android builds OK (shows Rust compiler already downloaded message.). The same non-alt rust compiler in .servo/rust/
3. ./mach build --release builds OK. New alt rustc compiler in .servo/rust/
4. ./mach build --release --android fails when the alt rustc compiler exists:

error[E0463]: can't find crate for `std`
  |
  = note: the `armv7-linux-androideabi` target may not be installed

[SimonSapin]

@MortimerGoro Until we figure something out, you can copy servobuild.example to .servobuild and in the [build] section replace #llvm-assertions = false with llvm-assertions = true. This will force alternate compilers to never be used, so you shouldn’t have any bootstrapping problem.