feat: completion list suggests methods in the order we most likely want

When typing `MyType::` the completion items' order could be re-ordered based on how likely we want to select those:
* Constructors: `new` like functions to be able to create the type,
* Builder Methods: any builder methods available,
* Constructors that take args: Any other function that creates `Self`,
* Regular methods & associated functions (no change there)

![image](https://github.com/rust-lang/rust-analyzer/assets/1546896/54593b91-07b3-455a-8a71-8d203d4eaf4a)

In this photo, the order is:
* `new` constructor is first
* `new_builder` second is a builder method
* `aaaanew` is a constructor that takes arguments, is third  and is irrespective of its alphabetical order among names.

I've dropped my previous idea of highlighting these functions in the rustdoc (rust-lang/rust#107926)

Cargo.toml

@@ -117,9 +117,9 @@ text-size = "1.1.1"
 tracing = "0.1.40"
 tracing-tree = "0.3.0"
 tracing-subscriber = { version = "0.3.18", default-features = false, features = [
-    "registry",
-    "fmt",
-    "tracing-log",
+  "registry",
+  "fmt",
+  "tracing-log",
 ] }
 triomphe = { version = "0.1.10", default-features = false, features = ["std"] }
 xshell = "0.2.5"

crates/ide-completion/src/item.rs

@@ -164,6 +164,9 @@ pub struct CompletionRelevance {
     pub postfix_match: Option<CompletionRelevancePostfixMatch>,
     /// This is set for type inference results
     pub is_definite: bool,
+    /// Any other bonuses we want to add,
+    /// eg. bonus for good behavior!
+    pub bonus_score: u32,
 }
 
 #[derive(Debug, Clone, Copy, Eq, PartialEq)]
@@ -228,6 +231,7 @@ impl CompletionRelevance {
             is_private_editable,
             postfix_match,
             is_definite,
+            bonus_score,
         } = self;
 
         // lower rank private things
@@ -269,7 +273,8 @@ impl CompletionRelevance {
         if is_definite {
             score += 10;
         }
-        score
+
+        score + bonus_score
     }
 
     /// Returns true when the score for this threshold is above

crates/ide-completion/src/render.rs

@@ -17,7 +17,7 @@ use ide_db::{
     imports::import_assets::LocatedImport,
     RootDatabase, SnippetCap, SymbolKind,
 };
-use syntax::{AstNode, SmolStr, SyntaxKind, TextRange};
+use syntax::{AstNode, SmolStr, SyntaxKind, SyntaxToken, TextRange};
 use text_edit::TextEdit;
 
 use crate::{
@@ -72,6 +72,10 @@ impl<'a> RenderContext<'a> {
         self.completion.db
     }
 
+    fn token(&self) -> &SyntaxToken {
+        &self.completion.token
+    }
+
     fn source_range(&self) -> TextRange {
         self.completion.source_range()
     }
@@ -1176,6 +1180,7 @@ fn main() { let _: m::Spam = S$0 }
                             is_private_editable: false,
                             postfix_match: None,
                             is_definite: false,
+                            bonus_score: 0,
                         },
                         trigger_call_info: true,
                     },
@@ -1202,6 +1207,7 @@ fn main() { let _: m::Spam = S$0 }
                             is_private_editable: false,
                             postfix_match: None,
                             is_definite: false,
+                            bonus_score: 0,
                         },
                         trigger_call_info: true,
                     },
@@ -1280,6 +1286,7 @@ fn foo() { A { the$0 } }
                             is_private_editable: false,
                             postfix_match: None,
                             is_definite: false,
+                            bonus_score: 0,
                         },
                     },
                 ]
@@ -2002,6 +2009,60 @@ fn main() {
         );
     }
 
+    #[test]
+    fn colon_complete_preferred_order_relevances() {
+        check_relevance(
+            r#"
+struct A;
+struct ABuilder;
+impl A {
+    fn foo(&self) {}
+    fn new_1(input: u32) -> A { A }
+    fn new_2() -> Self { A }
+    fn aaaabuilder() -> ABuilder { A }
+}
+
+fn test() {
+    let a = A::$0;
+}
+"#,
+            // preference:
+            // fn with no param that returns itself
+            // builder like fn
+            // fn with param that returns itself
+            expect![[r#"
+                fn new_2() [type_could_unify]
+                fn aaaabuilder() [type_could_unify]
+                fn new_1(…) [type_could_unify]
+                me foo(…) [type_could_unify]
+            "#]],
+        );
+
+        // Generic
+        check_relevance(
+            r#"
+struct A<T>{item: T}
+struct ABuilder;
+impl A {
+    fn foo(&self) {}
+    fn new_1<T>(input: u32, l: T) -> A<T> { A }
+    fn new_2<T>() -> Self { A { item: <_>::default()} }
+    fn aaaabuilder<T>() -> ABuilder<T> { A }
+}
+
+fn test() {
+    let a = A::$0;
+}
+"#,
+            expect![[r#"
+                fn new_2() [type_could_unify]
+                fn aaaabuilder() [type_could_unify]
+                fn new_1(…) [type_could_unify]
+                me foo(…) [type_could_unify]
+            "#]],
+        );
+    }
+
     #[test]
     fn struct_field_method_ref() {
         check_kinds(
@@ -2095,6 +2156,7 @@ fn foo() {
                             is_private_editable: false,
                             postfix_match: None,
                             is_definite: false,
+                            bonus_score: 0,
                         },
                     },
                 ]
@@ -2132,6 +2194,19 @@ fn main() {
                         ),
                         lookup: "foo",
                         detail: "fn() -> S",
+                        relevance: CompletionRelevance {
+                            exact_name_match: false,
+                            type_match: None,
+                            is_local: false,
+                            is_item_from_trait: false,
+                            is_name_already_imported: false,
+                            requires_import: false,
+                            is_op_method: false,
+                            is_private_editable: false,
+                            postfix_match: None,
+                            is_definite: false,
+                            bonus_score: 30,
+                        },
                         ref_match: "&@92",
                     },
                 ]

crates/ide-completion/src/render/function.rs

@@ -105,6 +105,7 @@ fn render(
         },
         exact_name_match: compute_exact_name_match(completion, &call),
         is_op_method,
+        bonus_score: calculate_bonus(&ctx, func, db),
         ..ctx.completion_relevance()
     });
 
@@ -153,6 +154,34 @@ fn render(
     item
 }
 
+/// When typing `::` of a type, the preferred order is:
+/// * Constructors: new like functions to be able to create the type,
+/// * Builder Methods,
+/// * Constructors that take args: Any other function that creates Self
+/// * Regular methods & Associated functions
+///
+fn calculate_bonus(ctx: &RenderContext<'_>, func: hir::Function, db: &dyn HirDatabase) -> u32 {
+    if ctx.token().kind() != syntax::SyntaxKind::COLON2 || func.self_param(db).is_some() {
+        return 0;
+    }
+
+    let mut bonus = 0;
+
+    let has_args = !func.assoc_fn_params(db).is_empty();
+    let ret_type = func.ret_type(db);
+    if !has_args && !ret_type.is_unit() {
+        // fn() -> A
+        bonus += 30;
+    } else if has_args && !ret_type.is_unit() {
+        // fn(..) -> A
+        bonus += 20;
+    } else if !has_args && ret_type.display(db).to_string().ends_with("Builder") {
+        // -> [..]Builder
+        bonus += 10;
+    }
+    bonus
+}
+
 pub(super) fn add_call_parens<'b>(
     builder: &'b mut Builder,
     ctx: &CompletionContext<'_>,