diff rust/hg-core/Cargo.toml @ 48950:11c0411bf4e2

dirstate-tree: optimize HashMap lookups with raw_entry_mut

This switches to using `HashMap` from the hashbrown crate,
in order to use its `raw_entry_mut` method.
The standard library’s `HashMap` is also based on this same crate,
but `raw_entry_mut` is not yet stable there:
https://github.com/rust-lang/rust/issues/56167

Using version 0.9 because 0.10 is yanked and 0.11 requires Rust 1.49

This replaces in `DirstateMap::get_or_insert_node` a call to
`HashMap<K, V>::entry` with `K = WithBasename<Cow<'on_disk, HgPath>>`.

`entry` takes and consumes an "owned" `key: K` parameter, in case a new entry
ends up inserted. This key is converted by `to_cow` from a value that borrows
the `'path` lifetime.

When this function is called by `Dirstate::new_v1`, `'path` is in fact
the same as `'on_disk` so `to_cow` can return an owned key that contains
`Cow::Borrowed`.

For other callers, `to_cow` needs to create a `Cow::Owned` and thus make
a costly heap memory allocation. This is wasteful if this key was already
present in the map. Even when inserting a new node this is typically the case
for its ancestor nodes (assuming most directories have numerous descendants).

Differential Revision: https://phab.mercurial-scm.org/D12317

--- a/rust/hg-core/Cargo.toml	Fri Mar 04 13:33:55 2022 +0100
+++ b/rust/hg-core/Cargo.toml	Tue Feb 08 15:51:52 2022 +0100
@@ -13,6 +13,7 @@
 bytes-cast = "0.2"
 byteorder = "1.3.4"
 derive_more = "0.99"
+hashbrown = {version = "0.9.1", features = ["rayon"]}
 home = "0.5"
 im-rc = "15.0.*"
 itertools = "0.9"