rust requires all 4 toolchains to build properly.  i'm not sure this scenario is worth supporting as no one uses --nousepkg.

Are you implying this is a WontFix, vapier?

It is more of a won't fix but the failing msg could be made more clear.
You can add a pkg_setup function in rust ebuild that verifies that all required toolchains are installed and dies with a more precise message.

i think there's a situation the toolchain team is running into that needs to be figured out, and while i'm not sure exactly what it is, i don't think it'd be a change on the rust ebuild side (at least, as we've written+designed rust today).

so i'd expect to leave this open if there isn't another toolchain bug floating around until the toolchain team is unblocked.

why is this failing only occasionally? 

This problem only happens when Rust's ebuild is updated but SDK builder has not yet uploaded the prebuilts.
Since toolchain builder use --nousepkg, they try to build an updated rust compiler and fail.

The duration of the problem is the time frame between rust's ebuild update and SDK builder prebuilt upload time. Once prebuilts are available, builders are happy since they don't try to upgrade rust.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/overlays/chromiumos-overlay/+/09daff0739261bb4c810746552bf709721b9cb14

commit 09daff0739261bb4c810746552bf709721b9cb14
Author: Zach Reizner <zachr@google.com>
Date: Fri May 18 21:33:17 2018

dev-lang/rust: check that required toolchains are installed in pkg_setup

Instead of failing rather late in the emerge process of the rust
package, this change adds a check very early with a precise error
message if a required toolchain is missing.

TEST=emerge rust
BUG=chromium:842557

Change-Id: Ia24d70734e858aa1f9e7c37bd526fd42183dbc5b
Reviewed-on: https://chromium-review.googlesource.com/1058721
Commit-Ready: Zach Reizner <zachr@chromium.org>
Tested-by: Zach Reizner <zachr@chromium.org>
Reviewed-by: Manoj Gupta <manojgupta@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>

[rename] https://crrev.com/09daff0739261bb4c810746552bf709721b9cb14/dev-lang/rust/rust-1.26.0-r1.ebuild

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/overlays/chromiumos-overlay/+/9dde4c04fd9514a1eadaea2c6c47426ceff235f5

commit 9dde4c04fd9514a1eadaea2c6c47426ceff235f5
Author: Manoj Gupta <manojgupta@google.com>
Date: Sat May 19 16:02:30 2018

Revert "dev-lang/rust: check that required toolchains are installed in pkg_setup"

This reverts commit 09daff0739261bb4c810746552bf709721b9cb14.

pkg_setup is apparently called even for binary installation.
This is causing SDK builder to fail since aarch64
toolchain is not installed.
https://uberchromegw.corp.google.com/i/chromiumos/builders/chromiumos-sdk/builds/8808

Probably need to find another way or leave it as is for now.

 > Instead of failing rather late in the emerge process of the rust
 > package, this change adds a check very early with a precise error
 > message if a required toolchain is missing.

TEST=emerge rust
BUG=chromium:842557

Change-Id: If85df1b62cf30c4103a196c30812bd0285c9787e
Reviewed-on: https://chromium-review.googlesource.com/1067075
Reviewed-by: Manoj Gupta <manojgupta@chromium.org>
Commit-Queue: Manoj Gupta <manojgupta@chromium.org>
Tested-by: Manoj Gupta <manojgupta@chromium.org>

[rename] https://crrev.com/9dde4c04fd9514a1eadaea2c6c47426ceff235f5/dev-lang/rust/rust-1.26.0-r2.ebuild

This rust error is causing all of our nightly build/performance tests to fail in the SetupBoard stage.  We need this fixed asap.

Example builds with this failure:

https://ci.chromium.org/p/chromeos/builds/b8946025775836859424
https://ci.chromium.org/p/chromeos/builds/b8946025775836859424
https://ci.chromium.org/p/chromeos/builds/b8946025148290505472
https://ci.chromium.org/p/chromeos/builds/b8946024832960572512

and we have lots more...

I spoke too soon: all the weekend builds failed, but the ones today look ok...Sorry.

please see Manoj's comment #6 for an explanation

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/overlays/chromiumos-overlay/+/82d4e4d94b67d6136a34500aa44389e0bc5f73ad

commit 82d4e4d94b67d6136a34500aa44389e0bc5f73ad
Author: Zach Reizner <zachr@google.com>
Date: Tue May 22 22:54:08 2018

dev-lang/rust: check toolchains only when building in pkg_setup

Instead of failing rather late in the emerge process of the rust
package, this change adds a check very early with a precise error
message if a required toolchain is missing. This check is skipped if the
package is not being built locally because it is a binpkg.

TEST=emerge rust
BUG=chromium:842557

Change-Id: I203bda9044946b7691bcaf80883db13415802563
Reviewed-on: https://chromium-review.googlesource.com/1067505
Commit-Ready: Zach Reizner <zachr@chromium.org>
Tested-by: Zach Reizner <zachr@chromium.org>
Reviewed-by: Manoj Gupta <manojgupta@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>

[rename] https://crrev.com/82d4e4d94b67d6136a34500aa44389e0bc5f73ad/dev-lang/rust/rust-1.26.0-r3.ebuild

 Issue 851618  has been merged into this issue.

FWIW, I just tried a local build of Rust at ToT and it succeeded.