master ← lazy-trees-v2
opened 09:33PM - 17 May 25 UTC
## Motivation
This adds a setting `lazy-trees` that causes flake inputs to be… "mounted" as virtual filesystems on top of /nix/store as random "virtual" store paths. Only when the store path is actually used as a dependency of a store derivation do we materialize ("devirtualize") the input by copying it to its content-addressed location in the store.
String contexts determine when devirtualization happens. One wrinkle is that there are cases where we had store paths without proper contexts, in particular when the user does `toString <path>` (where `<path>` is a source tree in the Nix store) and passes the result to a derivation. This usage was always broken, since it can result in derivations that lack correct references. But to ensure that we don't change evaluation results, we introduce a new type of context that results in devirtualization but not in store references. We also now print a warning about this.
Supersedes/incorporates #12432 and #12915. It replaces #6530. The big difference with #6530 is that the latter treated each source tree as its own root filesystem, which caused a lot of backward compatibility issues that required ugly hacks. With the new approach, lazy source trees appear under `/nix/store`, just using a virtual store path that doesn't exist in the real filesystem.
TODO: run the test suite with `lazy-trees = true`.
## Context
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