Source filtering

Nix considers that a derivation must be rebuilt whenever any of its inputs change, including all source files passed into the build. Unfortunately, this means that changes to any "irrelevant" files (such as the project README) would end up rebuilding the project even if the final outputs don't actually care about their contents!

Source filtering is a technique Nix employs that allows for better caching by programmatically filtering out files which are known to not apply to the build before the inputs are hashed.

A default source cleaner is available via craneLib.cleanCargoSource: it cleans a source tree to omit things like version control directories as well omit any non-Rust/non-cargo related files. It can be used like so:

craneLib.buildPackage {
  # other attributes omitted
  src = craneLib.cleanCargoSource (craneLib.path ./.);

It is possible to customize the filter to use when cleaning the source by leveraging craneLib.filterCargoSources. By default this filter will only keep files whose names end with .rs or .toml. Though it is possible to compose it with other filters, especially if it is necessary to include additional files which it might otherwise omit:

  # Only keeps markdown files
  markdownFilter = path: _type: builtins.match ".*md$" path != null;
  markdownOrCargo = path: type:
    (markdownFilter path type) || (craneLib.filterCargoSources path type);
craneLib.buildPackage {
  # other attributes omitted
  src = lib.cleanSourceWith {
    src = craneLib.path ./.; # The original, unfiltered source
    filter = markdownOrCargo;