This commit adds the said relation by keeping a map of type `FxHashMap<SourceRootId,Option<SourceRootId>>`
inside the `GlobalState`. Its primary use case is reading the rust-analyzer.toml files that can be
placed under every local source root. As a config will be found by traversing this "tree" we need the parent information
for every local source root. This commit omits defining this relation for library source roots entirely.
Previous solution for binning paths into disjoint directories was
simple and fast -- just a single binary search.
Unfortunatelly, it wasn't coorrect: if the ditr are
/d
/d/a
/d/c
then partitioning the file /d/b/lib.rs won't pick /d as a correct
directory.
The correct solution here is a trie, but it requires exposing path
components.
So, we use a poor man's substitution -- a *vector* of sorted paths,
such that each bucket is prefix-free
closes#5246
The task of `partition` function is to bin the flat list of paths into
disjoint filesets. Ideally, it should be incremental -- each new file
should be added to a specific fileset.
However, preliminary measurnments show that it is actually fast enough
if we just optimize this to use a binary search instead of a linear
scan.
