Use natural trait ordering in derive completion

derive(Clone, Copy) reads better than derive(Copy, Clone).

However, we preserve the reverse ordering in the lookup text for
sorting purposes. That way, it's convenient to type just `Ord` to
derive everything.

Author: matklad

crates/completion/src/completions/attribute.rs

@@ -3,6 +3,7 @@
 //! This module uses a bit of static metadata to provide completions
 //! for built-in attributes.
 
+use itertools::Itertools;
 use rustc_hash::FxHashSet;
 use syntax::{ast, AstNode, SyntaxKind};
 
@@ -162,19 +163,20 @@ const ATTRIBUTES: &[AttrCompletion] = &[
 fn complete_derive(acc: &mut Completions, ctx: &CompletionContext, derive_input: ast::TokenTree) {
     if let Ok(existing_derives) = parse_comma_sep_input(derive_input) {
         for derive_completion in DEFAULT_DERIVE_COMPLETIONS
-            .into_iter()
+            .iter()
             .filter(|completion| !existing_derives.contains(completion.label))
         {
-            let mut label = derive_completion.label.to_owned();
-            for dependency in derive_completion
-                .dependencies
-                .into_iter()
-                .filter(|&&dependency| !existing_derives.contains(dependency))
-            {
-                label.push_str(", ");
-                label.push_str(dependency);
-            }
+            let mut components = vec![derive_completion.label];
+            components.extend(
+                derive_completion
+                    .dependencies
+                    .iter()
+                    .filter(|&&dependency| !existing_derives.contains(dependency)),
+            );
+            let lookup = components.join(", ");
+            let label = components.iter().rev().join(", ");
             CompletionItem::new(CompletionKind::Attribute, ctx.source_range(), label)
+                .lookup_by(lookup)
                 .kind(CompletionItemKind::Attribute)
                 .add_to(acc)
         }
@@ -264,7 +266,6 @@ struct DeriveCompletion {
 
 /// Standard Rust derives and the information about their dependencies
 /// (the dependencies are needed so that the main derive don't break the compilation when added)
-#[rustfmt::skip]
 const DEFAULT_DERIVE_COMPLETIONS: &[DeriveCompletion] = &[
     DeriveCompletion { label: "Clone", dependencies: &[] },
     DeriveCompletion { label: "Copy", dependencies: &["Clone"] },
@@ -421,14 +422,14 @@ struct Test {}
         "#,
             expect![[r#"
                 at Clone
-                at Copy, Clone
+                at Clone, Copy
                 at Debug
                 at Default
-                at Eq, PartialEq
                 at Hash
-                at Ord, PartialOrd, Eq, PartialEq
                 at PartialEq
-                at PartialOrd, PartialEq
+                at PartialEq, Eq
+                at PartialEq, Eq, PartialOrd, Ord
+                at PartialEq, PartialOrd
             "#]],
         );
     }
@@ -453,12 +454,12 @@ struct Test {}
 "#,
             expect![[r#"
                 at Clone
-                at Copy, Clone
+                at Clone, Copy
                 at Debug
                 at Default
                 at Eq
+                at Eq, PartialOrd, Ord
                 at Hash
-                at Ord, PartialOrd, Eq
                 at PartialOrd
             "#]],
         )