I floated this idea in bug 1497477 but the idea of making rusttests Tier 1 came up today and part of that discussion is that it's kind of a pain to run them locally. We should implement `mach rusttests` to make it easy to run these tests. For an MVP we could put a simple mach command alongside other test commands here: https://dxr.mozilla.org/mozilla-central/source/testing/mach_commands.py We don't currently have a good way to invoke just rust tests though, they get run as part of `make check`. I think maybe we can reuse the machinery chmanchester added in bug 1474028 to make this work, though. We'd need to make the emitter run `RUST_TESTS` through the same logic we use for compile targets right now, but with a non-default target like we do with gtests: https://dxr.mozilla.org/mozilla-central/rev/c291143e24019097d087f9307e59b49facaf90cb/python/mozbuild/mozbuild/backend/recursivemake.py#796-845 If we plumb that all through then that would give us a `recurse_rusttest` make target in the root Makefile which we could easily invoke from the mach command.

Chris: is this something you could find time for?

Tentatively yes. We should be able to make the automation tasks way less wasteful along the way.

The rusttests no longer require a special build flag, however we still need mozconfigs specific to these jobs for 1.) Turning off the clang plugin, which will not get built during these builds and will cause failures when C/C++ needed by the rust build tries to find it. and 2.) Adding --output-sync=line to our make invocation: at some point the "export" phase started relying on this, and it's missing because the non-default build targets invoked by the rusttests builds don't go through client.mk.

Pushed by cmanchester@mozilla.com: https://hg.mozilla.org/integration/autoland/rev/03e11ea4c03a Implement rusttests as a non-default sub-graph of the build. r=froydnj https://hg.mozilla.org/integration/autoland/rev/2920480fd7b7 Allow mozharness build jobs to specify non-default build targets. r=froydnj https://hg.mozilla.org/integration/autoland/rev/9e381bd1d350 Automation and mozconfig changes to convert rusttests builds to use non-default build target. r=froydnj

The ergonomics around running these aren't ideal still, I kept the --enable-rust-tests flag around so code coverage builds can still use it. Filed bug 1505859 to handle this.

chmanchester: nice work here!

https://hg.mozilla.org/mozilla-central/rev/03e11ea4c03a https://hg.mozilla.org/mozilla-central/rev/2920480fd7b7 https://hg.mozilla.org/mozilla-central/rev/9e381bd1d350

We gain some build boost on Linux: == Change summary for alert #17440 (as of Thu, 08 Nov 2018 16:39:26 GMT) == Improvements: 65% build times linux32 debug rusttests taskcluster-c4.4xlarge 566.13 -> 200.04 For up to date results, see: https://treeherder.mozilla.org/perf.html#/alerts?id=17440