Auto merge of #15956 - ahlinc:label-detail, r=lnicola

Improve completion label details display

This PR improves completion label details display by separating trait and alias info from the `label` LSP field to an additional existing `label_detail` field. Changes look like the following:

### Before

![Screenshot from 2023-11-24 12-03-08](https://github.com/rust-lang/rust-analyzer/assets/14666676/74066f0d-f1ac-4e99-8be5-c5141d513d23)

### After

![Screenshot from 2023-11-24 12-21-57](https://github.com/rust-lang/rust-analyzer/assets/14666676/45fca112-4612-40a3-81b9-07ff12de0962)

_All existing tests are passed without any changes in test themselves logic._

4 files changed, 39 insertions, 9 deletions

diff --git a/crates/ide-completion/src/item.rs b/crates/ide-completion/src/item.rs
index ed74ef7b667..5397a9fc90b 100644
--- a/crates/ide-completion/src/item.rs
+++ b/crates/ide-completion/src/item.rs
@@ -26,6 +26,10 @@ use crate::{
 pub struct CompletionItem {
     /// Label in the completion pop up which identifies completion.
     pub label: SmolStr,
+    /// Addition label details in the completion pop up that are
+    /// displayed and aligned on the right side after the label.
+    pub label_detail: Option<SmolStr>,
+
     /// Range of identifier that is being completed.
     ///
     /// It should be used primarily for UI, but we also use this to convert
@@ -425,13 +429,14 @@ impl Builder {
     pub(crate) fn build(self, db: &RootDatabase) -> CompletionItem {
         let _p = profile::span("item::Builder::build");
 
-        let mut label = self.label;
+        let label = self.label;
+        let mut label_detail = None;
         let mut lookup = self.lookup.unwrap_or_else(|| label.clone());
         let insert_text = self.insert_text.unwrap_or_else(|| label.to_string());
 
         if !self.doc_aliases.is_empty() {
             let doc_aliases = self.doc_aliases.iter().join(", ");
-            label = SmolStr::from(format!("{label} (alias {doc_aliases})"));
+            label_detail.replace(SmolStr::from(format!(" (alias {doc_aliases})")));
             let lookup_doc_aliases = self
                 .doc_aliases
                 .iter()
@@ -454,10 +459,17 @@ impl Builder {
         if let [import_edit] = &*self.imports_to_add {
             // snippets can have multiple imports, but normal completions only have up to one
             if let Some(original_path) = import_edit.original_path.as_ref() {
-                label = SmolStr::from(format!("{label} (use {})", original_path.display(db)));
+                label_detail.replace(SmolStr::from(format!(
+                    "{} (use {})",
+                    label_detail.as_deref().unwrap_or_default(),
+                    original_path.display(db)
+                )));
             }
         } else if let Some(trait_name) = self.trait_name {
-            label = SmolStr::from(format!("{label} (as {trait_name})"));
+            label_detail.replace(SmolStr::from(format!(
+                "{} (as {trait_name})",
+                label_detail.as_deref().unwrap_or_default(),
+            )));
         }
 
         let text_edit = match self.text_edit {
@@ -479,6 +491,7 @@ impl Builder {
         CompletionItem {
             source_range: self.source_range,
             label,
+            label_detail,
             text_edit,
             is_snippet: self.is_snippet,
             detail: self.detail,
diff --git a/crates/ide-completion/src/render.rs b/crates/ide-completion/src/render.rs
index dfe8fe7e2f7..00a9081985b 100644
--- a/crates/ide-completion/src/render.rs
+++ b/crates/ide-completion/src/render.rs
@@ -557,7 +557,11 @@ mod tests {
 
                 let tag = it.kind.tag();
                 let relevance = display_relevance(it.relevance);
-                items.push(format!("{tag} {} {relevance}\n", it.label));
+                items.push(format!(
+                    "{tag} {}{} {relevance}\n",
+                    it.label,
+                    it.label_detail.clone().unwrap_or_default(),
+                ));
 
                 if let Some((label, _indel, relevance)) = it.ref_match() {
                     let relevance = display_relevance(relevance);
diff --git a/crates/ide-completion/src/tests.rs b/crates/ide-completion/src/tests.rs
index 9db8e972dd4..f28afacc586 100644
--- a/crates/ide-completion/src/tests.rs
+++ b/crates/ide-completion/src/tests.rs
@@ -150,16 +150,29 @@ fn render_completion_list(completions: Vec<CompletionItem>) -> String {
     fn monospace_width(s: &str) -> usize {
         s.chars().count()
     }
-    let label_width =
-        completions.iter().map(|it| monospace_width(&it.label)).max().unwrap_or_default().min(22);
+    let label_width = completions
+        .iter()
+        .map(|it| {
+            monospace_width(&it.label)
+                + monospace_width(it.label_detail.as_deref().unwrap_or_default())
+        })
+        .max()
+        .unwrap_or_default()
+        .min(22);
     completions
         .into_iter()
         .map(|it| {
             let tag = it.kind.tag();
             let var_name = format!("{tag} {}", it.label);
             let mut buf = var_name;
+            if let Some(ref label_detail) = it.label_detail {
+                format_to!(buf, "{label_detail}");
+            }
             if let Some(detail) = it.detail {
-                let width = label_width.saturating_sub(monospace_width(&it.label));
+                let width = label_width.saturating_sub(
+                    monospace_width(&it.label)
+                        + monospace_width(&it.label_detail.unwrap_or_default()),
+                );
                 format_to!(buf, "{:width$} {}", "", detail, width = width);
             }
             if it.deprecated {
diff --git a/crates/rust-analyzer/src/lsp/to_proto.rs b/crates/rust-analyzer/src/lsp/to_proto.rs
index 4e3dce35ec3..b4ba5bf4f91 100644
--- a/crates/rust-analyzer/src/lsp/to_proto.rs
+++ b/crates/rust-analyzer/src/lsp/to_proto.rs
@@ -301,7 +301,7 @@ fn completion_item(
 
     if config.completion_label_details_support() {
         lsp_item.label_details = Some(lsp_types::CompletionItemLabelDetails {
-            detail: None,
+            detail: item.label_detail.as_ref().map(ToString::to_string),
             description: lsp_item.detail.clone(),
         });
     }