User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:85.0) Gecko/20100101 Firefox/85.0

Steps to reproduce:

I am trying to build firefox-85.0.2 or 78.7.1 on Gentoo Linux which updated Rust to v1.50.0. This will fail very early already with

0:02.06 /var/tmp/portage/www-client/firefox-85.0.2-r1/work/firefox_build/_virtualenvs/init_py3/bin/python -m mozbuild.action.check_binary --host nsinstall_real
0:02.12 error: failed to parse manifest at `/var/tmp/portage/www-client/firefox-85.0.2-r1/work/firefox-85.0.2/Cargo.toml`
0:02.12 Caused by:
0:02.12   dependency (nix) specification is ambiguous. Only one of `branch`, `tag` or `rev` is allowed.
0:02.14 gmake[3]: *** [backend.mk:107: config/.deps/cbindgen-metadata.json.stub] Error 101
0:02.14 gmake[3]: *** Waiting for unfinished jobs....
0:02.27 cp nsinstall_real nsinstall.tmp
0:02.27 mv nsinstall.tmp nsinstall
0:02.27 ../config/nsinstall -R -m 755 'nsinstall' '../dist/bin'
0:02.27 gmake[4]: Leaving directory '/var/tmp/portage/www-client/firefox-85.0.2-r1/work/firefox_build/config'
0:05.02 gmake[3]: Leaving directory '/var/tmp/portage/www-client/firefox-85.0.2-r1/work/firefox_build'
0:05.02 gmake[2]: *** [/var/tmp/portage/www-client/firefox-85.0.2-r1/work/firefox-85.0.2/config/recurse.mk:34: export] Error 2
0:05.02 gmake[2]: Leaving directory '/var/tmp/portage/www-client/firefox-85.0.2-r1/work/firefox_build'
0:05.02 gmake[1]: *** [/var/tmp/portage/www-client/firefox-85.0.2-r1/work/firefox-85.0.2/config/rules.mk:355: default] Error 2
0:05.02 gmake[1]: Leaving directory '/var/tmp/portage/www-client/firefox-85.0.2-r1/work/firefox_build'
0:05.02 gmake: *** [client.mk:89: build] Error 2
0:05.02 0 compiler warnings present.

For 78.7.1 ESR the error is

 0:09.57 b''
 0:09.57 b'ERROR: Couldn\'t execute `cargo metadata` with manifest "/var/tmp/portage/www-client/firefox-78.7.1-r1/work/firefox-78.7.1/toolkit/library/rust/Cargo.toml": Metadata(Output { status: ExitStatus(ExitStatus(25856)), stdout: "", stderr: "error: failed to parse manifest at `/var/tmp/portage/www-client/firefox-78.7.1-r1/work/firefox-78.7.1/Cargo.toml`\\n\\nCaused by:\\n  dependency (nix) specification is ambiguous. Only one of `branch`, `tag` or `rev` is allowed.\\n" })\nERROR: Couldn\'t generate bindings for /var/tmp/portage/www-client/firefox-78.7.1-r1/work/firefox-78.7.1/toolkit/library/rust.\n'
 0:09.59 gmake[4]: *** [backend.mk:83: .deps/ServoStyleConsts.h.stub] Error 1
 0:09.59 gmake[4]: Leaving directory '/var/tmp/portage/www-client/firefox-78.7.1-r1/work/firefox_build/layout/style'
 0:09.59 gmake[3]: *** [/var/tmp/portage/www-client/firefox-78.7.1-r1/work/firefox-78.7.1/config/recurse.mk:101: layout/style/export] Error 2

Considering mozilla-central works, I'd say this is fixed. I know something was uplifted to esr78 recently. Emilio, is the error in comment 0 what that was for?

(Yes)