The instructions for updating webrender in-tree have changes slightly, because of changes in bug 1324462. In particular, doing |mach vendor rust| no longer updates the root Cargo.lock files, and so if you update webrender you need to manually make sure those Cargo.lock files are up to date. You can do this by running |cargo update --aggressive -p webrender| on them. The instructions should be updated to reflect that.
Pushed by firstname.lastname@example.org: https://hg.mozilla.org/projects/graphics/rev/c242e1a93e89 Update README.webrender to fix WR update process. r=gfx? and DONTBUILD