Gentoo has the concept of an implicit system dependency. The idea is simply put: there are a number of packages that every system will need to function at any level (to boot, build, etc...), and rather than have every other package in the tree depend on them, we stick it in the @system set instead. Then things in that set can be ignored when declaring a package's dependency (build and run time).
It's a convenient way of:
Consider this: how useful is a system if you don't have programs like sh, cat, grep, ls, mv, sed, test? Or a C library? Or to try to compile code, but have no compiler/assembler/linker? Is it really useful for developers to analyze every single shell script/line of source that goes into a distribution to see which tools are run (including every time a package releases a new version)? Or do we simply state up front: at run time, you may assume that a certain baseline of libraries/programs exist, and at build time, certain libraries/compilers exist?
The question is obviously rhetorical -- developer time is better spent doing real work than keeping track of these things.
Gentoo doesn't explicitly define what packages qualify as part of the implicit system dependency. It alludes to them by talking about the @system set, but then tempers that requirement by stating not all things in it may be ignored. Chromium OS refines the concept by splitting them: there are "the implicit system dependencies" and there is the "@system set". The former concept is what we want to define, and the latter is merely a (partial) consequence of the first.
More specifically, we don't want to say "the sys-apps/coreutils package is part of the implicit system dependencies". Instead, we say "`mv` and `ls` are part of the implicit system dependencies". Then, with that declaration in hand, it is obvious to say the default @system set includes the sys-apps/coreutils package.
The reason for this is that the coreutils package includes a ton of utilities, many of which we would not consider part of the implicit system set. While it's unreasonable to say "if you use `mv` or `touch` or `ls` or `echo` or `cat`, you have to depend on coreutils" due to their high prevalence, you can make a reasonable case that if your program runs `pinky`, then you probably should depend on sys-apps/coreutils. The number of packages that use that tool is low (if there are any at all).
The only library packages you may assume exist in the implicit system set are the C library and runtime libraries provided by the compiler. This isn't just the main C library, but encompasses all the libraries that POSIX documents like:
For internal runtime libraries that the compiler provides (like -lstdc++ and -lgcc and -lgcc_s and -lasan and the rest), you should not depend on these yourself.
For all other third party libraries like -lz or -luuid or -lblkid or -lcrack, you must depend on the relevant package that provides that library.
In order to build from source, you do not have to have build time dependencies on:
Because it wouldn't be a good set of guidelines without exceptions, here they are.
You can find the default implicit system package in the chromiumos-overlay. It documents the various packages it pulls in and gives reasoning as to why each one exists.