What are the "host and target platforms" of a dependency?

[waffle8946]

I wrote about it here:

Frustrations about splicing

A lot of this is actually in the nixpkgs manual, but I consider it really overwhelming due to too much prose and too many analogies. Let me try to summarise. The three platforms are relevant when building a compiler: buildPlatform = platform where the compiler is built hostPlatform = platform where the compiler is run targetPlatform = platform for which the compiler emits code, if the compiler has the limitation of only being able to emit code for a single platform The word “compiler” her…