https://treeherder.mozilla.org/#/jobs?repo=try&revision=1f9b3dbde85126a9fb5beb827abf020b38cb72ec is an example of when I modified the mingw32-gcc toolchain. There are two other toolchain jobs that depend on that toolchain (mingw32-fxc and mingw32-nsis), but these were not rebuilt. Instead it went straight to the job requested (win32-mingw32). My guess is that if toolchain B depends on toolchain A, all of toolchain A's resources need to be included in toolchain B's resources as well. Not a big deal in my case, but might matter down the road.
Bug 1383880 might help this. I don't think the existing optimization stuff does a great job of optimizing chained dependencies like this (C depends on B depends on A, where A and B might be optimized).
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