Merge #5126

5126: Use more of FxHash* r=matklad a=lnicola

```
-rwxr-xr-x 1 me me 37917528 Jun 29 17:26 /home/me/.cargo/bin/rust-analyzer
-rwxr-xr-x 1 me me 37904056 Jun 29 18:14 /home/me/.cargo/bin/rust-analyzer
```

Saved 13.5 KB there :-).

Co-authored-by: Laurențiu Nicola <[email protected]>

1 files changed, 5 insertions, 0 deletions

diff --git a/docs/dev/README.md b/docs/dev/README.md
index 11dc5261b..6b6824ded 100644
--- a/docs/dev/README.md
+++ b/docs/dev/README.md
@@ -254,6 +254,11 @@ The default name is a lowercased name of the type: `global_state: GlobalState`.
 Avoid ad-hoc acronyms and contractions, but use the ones that exist consistently (`db`, `ctx`, `acc`).
 The default name for "result of the function" local variable is `res`.
 
+## Collection types
+
+We prefer `rustc_hash::FxHashMap` and `rustc_hash::FxHashSet` instead of the ones in `std::collections`.
+They use a hasher that's slightly faster and using them consistently will reduce code size by some small amount.
+
 ## Preconditions
 
 Function preconditions should generally be expressed in types and provided by the caller (rather than checked by callee):