feat(rustdoc-json-types): introduce rustc-hash feature

[jalil-salame]

This allows the public rustdoc-types crate to expose this feature easily and allows consumers of the crate to get the performance advantages from doing so.

The reasoning for this was discussed on Zulip

Changes:

• Make rustc-hash optional but default to including it
• Rename all occurrences of FxHashMap to HashMap.
• Feature gate the import and rename the imported FxHashMap to HashMap
• Introduce a type alias FxHashMap which resolves to the currently used HashMap (rustc_hash::FxHashMap or std::collections::HashMap) for use in src/librustdoc.

extra context from the zulip thread:

• @obi1kenobi requested benchmarks of the switch to rustc-hash
• I benchmarked switching rustdoc-types to rustc-hash which yielded a ~300ms improvement to cargo-semver-checks's index building step (this step is done twice so the improvements are ~150ms per index).
• The benchmarks were presented in Zulip and people were in favor of introducing rustc-hash to the public rustdoc-types crate.
• There were differing opinions on how to introduce the dependency:
  1. "Hard" dependency: remove use of std::collections::HashMap in favor of FxHashMap.
  2. "Soft" dependency: make optional and introduce a feature then enable/disable it by default (this PR).
  3. Make rustdoc-types generic and expose the RandomState (a lot of work & complexity for little gain over a feature gate).

@obi1kenobi and I prefer the feature gate so that is what I am adding here.

My reasons for the preference are:

• cargo-semver-checks is especially perf sensitive, we don't expect people to care about ~150ms extra time when reading in a 500MB file (the size of the sample we used for benchmarking).
• Keeping rustdoc-types lean by having its only direct dependency be serde is nice for the general consumer of the crate.
• rustc-hash is not HashDOS resistant (but it is questionable whether rustdoc-types would be used on adversarial inputs).

r? @aDotInTheVoid

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @aDotInTheVoid (or someone else) some time within the next two weeks.

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[rustbot]

rustdoc-json-types is a public (although nightly-only) API. If possible, consider changing src/librustdoc/json/conversions.rs; otherwise, make sure you bump the FORMAT_VERSION constant.

cc @CraftSpider, @aDotInTheVoid, @Enselic, @obi1kenobi

[aDotInTheVoid]

Thanks for working on this!

@bors r+ rollup

[bors]

📌 Commit d1fa49b has been approved by aDotInTheVoid

It is now in the queue for this repository.