76fd47124b
rustc_metadata: Preprocess search paths for better performance Over in Zed we've noticed that loading crates for a large-ish workspace (~100 members of workspace, over 1000 crates being built for the main binary target) can take almost 200ms. We've pinned it down to how rustc searches for paths to dependency files, as it performs a linear search over the list of candidate paths. In our case the candidate list had about 20k entries which we had to iterate over for each dependency being loaded. Our workspace is also pretty bottom-heavy, e.g. most of the workspace members pull in most of the transitive dependencies one way or another, which means that we spend quite some time loading crates at rustc startup. This commit introduces a simple FilesIndex that's just a BTreeMap under the hood. Since crates are looked up by both prefix and suffix, we perform a range search on said BTree (which constraints the search space based on prefix) and follow up with a linear scan of entries with matching suffixes. Overall, this commit brings down build time for us in dev scenarios by about 6%. 100ms might not seem like much, but this is a constant cost that each of our workspace crates has to pay, even when said crate is miniscule.
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