Merge pull request #3026 from dwijnand/fxhash

Add an internal lint for FxHashMap/FxHashSet

tests/ui/fxhash.rs

@@ -0,0 +1,16 @@
+#![warn(default_hash_types)]
+#![feature(rustc_private)]
+
+extern crate rustc_data_structures;
+
+use std::collections::{HashMap, HashSet};
+use rustc_data_structures::fx::{FxHashMap, FxHashSet};
+
+fn main() {
+    let _map: HashMap<String, String> = HashMap::default();
+    let _set: HashSet<String> = HashSet::default();
+
+    // test that the lint doesn't also match the Fx variants themselves 😂
+    let _fx_map: FxHashMap<String, String> = FxHashMap::default();
+    let _fx_set: FxHashSet<String> = FxHashSet::default();
+}

tests/ui/fxhash.stderr

@@ -0,0 +1,40 @@
+error: Prefer FxHashMap over HashMap, it has better performance and we don't need any collision prevention in clippy
+ --> $DIR/fxhash.rs:6:24
+  |
+6 | use std::collections::{HashMap, HashSet};
+  |                        ^^^^^^^ help: use: `FxHashMap`
+  |
+  = note: `-D default-hash-types` implied by `-D warnings`
+
+error: Prefer FxHashSet over HashSet, it has better performance and we don't need any collision prevention in clippy
+ --> $DIR/fxhash.rs:6:33
+  |
+6 | use std::collections::{HashMap, HashSet};
+  |                                 ^^^^^^^ help: use: `FxHashSet`
+
+error: Prefer FxHashMap over HashMap, it has better performance and we don't need any collision prevention in clippy
+  --> $DIR/fxhash.rs:10:15
+   |
+10 |     let _map: HashMap<String, String> = HashMap::default();
+   |               ^^^^^^^ help: use: `FxHashMap`
+
+error: Prefer FxHashMap over HashMap, it has better performance and we don't need any collision prevention in clippy
+  --> $DIR/fxhash.rs:10:41
+   |
+10 |     let _map: HashMap<String, String> = HashMap::default();
+   |                                         ^^^^^^^ help: use: `FxHashMap`
+
+error: Prefer FxHashSet over HashSet, it has better performance and we don't need any collision prevention in clippy
+  --> $DIR/fxhash.rs:11:15
+   |
+11 |     let _set: HashSet<String> = HashSet::default();
+   |               ^^^^^^^ help: use: `FxHashSet`
+
+error: Prefer FxHashSet over HashSet, it has better performance and we don't need any collision prevention in clippy
+  --> $DIR/fxhash.rs:11:33
+   |
+11 |     let _set: HashSet<String> = HashSet::default();
+   |                                 ^^^^^^^ help: use: `FxHashSet`
+
+error: aborting due to 6 previous errors
+