This fixes the double copy problem and improves performance
for expressions that don't force the whole source to be added to the
store.
Rules for fast expressions:
- Use path literals where possible
- import ./foo.nix
- Use + operator with slash in string
- src = fetchTree foo + "/src";
- Use source filtering, lib.fileset
- AVOID toString
- If possible, AVOID interpolations ("${./.}")
- If possible, move slashes into the interpolation to add less to the store
- "${./src}/foo" -> "${./src/foo}"
toString may be improved later as part of lazy-trees, so these
recommendations are a snapshot. Path values are quite nice though.
This requires moving resolveSymlinks() into SourceAccessor. Also, it
requires LocalStoreAccessor::maybeLstat() to work on parents of the
store (to avoid an error like "/nix is not in the store").
Fixes#10375.
