From cfdba897c989d2b8fec6c99be33e16c43ff7109b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jalil=20David=20Salam=C3=A9=20Messina?=
 <jalil.salame@gmail.com>
Date: Sat, 19 Oct 2024 18:27:49 +0200
Subject: [PATCH] feat: add rustc-hash feature

Changes in preparation of [rust-lang/rust#131936][1]:

- Introduce `rustc-hash` dependency and feature.
- Modify the `update.sh` script accordingly.

[1]: https://github.com/rust-lang/rust/pull/131936
---
 Cargo.toml | 10 ++++++++++
 README.md  | 12 ++++++++++++
 update.sh  |  4 +---
 3 files changed, 23 insertions(+), 3 deletions(-)

diff --git a/Cargo.toml b/Cargo.toml
index bfbee52..54a03ab 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -9,6 +9,16 @@ repository = "https://github.com/rust-lang/rustdoc-types"
 
 [dependencies]
 serde = {version="1", features=["derive"]}
+rustc-hash = {version="2", optional=true}
+
+[features]
+default = []
+
+# Switch the hashmaps used in rustdoc-types to the FxHashMap from rustc-hash.
+#
+# This might improve performace if your are reading the rustdoc JSON from large
+# crates like aws_sdk_ec2
+rustc-hash = ["dep:rustc-hash"]
 
 [dev-dependencies]
 bincode = "1.3.3"
diff --git a/README.md b/README.md
index 3578e35..ba6a7fc 100644
--- a/README.md
+++ b/README.md
@@ -13,6 +13,18 @@ let krate: rustdoc_types::Crate = serde_json::from_str(&json_string)?;
 println!("the index has {} items", krate.index.len());
 ```
 
+For performance sensitive crates we suggest turning on the `rustc-hash`
+feature. This switches all data structures from `std::collections::HashMap` to
+`rustc-hash::FxHashMap` which improves performance when reading big JSON files
+(like `aws_sdk_rs`'s).
+
+`cargo-semver-checks` benchmarked this change with `aws_sdk_ec2`'s JSON and
+[observed a -3% improvement to the runtime][csc benchmarks]. The performance
+here depends on how much time you spend querying the `HashMap`s, so as always,
+measure first c:
+
+[csc benchmarks]: https://rust-lang.zulipchat.com/#narrow/channel/266220-t-rustdoc/topic/rustc-hash.20and.20performance.20of.20rustdoc-types/near/474855731
+
 ## Contributing
 
 This repo is a reexport of
diff --git a/update.sh b/update.sh
index 6d10f2c..65612f4 100755
--- a/update.sh
+++ b/update.sh
@@ -9,9 +9,7 @@ repo="rust"
 branch="master"
 
 curl -# https://raw.githubusercontent.com/${user}/${repo}/${branch}/src/rustdoc-json-types/lib.rs \
-    | sed 's/rustc_hash::/std::collections::/g' \
-    | sed 's/FxHashMap/HashMap/g' \
-    | sed 's/^pub use /use /' \
+    | sed '/^pub type FxHashMap.*$/d' \
     > src/lib.rs
 
 curl -# https://raw.githubusercontent.com/${user}/${repo}/${branch}/src/rustdoc-json-types/tests.rs > src/tests.rs