Add label_for_completion to extension API (#10175)

This PR adds the ability for extensions to implement
`label_for_completion` to customize completions coming back from the
language server.

We've used the Gleam extension as a motivating example, adding
`label_for_completion` support to it.

Release Notes:

- N/A

---------

Co-authored-by: Max <max@zed.dev>
Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>

extensions/gleam/Cargo.toml

@@ -13,4 +13,5 @@ path = "src/gleam.rs"
 crate-type = ["cdylib"]
 
 [dependencies]
-zed_extension_api = "0.0.4"
+# zed_extension_api = "0.0.4"
+zed_extension_api = { path = "../../crates/extension_api" }

extensions/gleam/languages/gleam/highlights.scm

@@ -7,6 +7,10 @@
 (constant
   name: (identifier) @constant)
 
+; Variables
+(identifier) @variable
+(discard) @comment.unused
+
 ; Modules
 (module) @module
 (import alias: (identifier) @module)
@@ -75,10 +79,6 @@
 ((identifier) @warning
  (#match? @warning "^(auto|delegate|derive|else|implement|macro|test|echo)$"))
 
-; Variables
-(identifier) @variable
-(discard) @comment.unused
-
 ; Keywords
 [
   (visibility_modifier) ; "pub"

extensions/gleam/src/gleam.rs

@@ -1,4 +1,6 @@
 use std::fs;
+use zed::lsp::CompletionKind;
+use zed::{CodeLabel, CodeLabelSpan, LanguageServerId};
 use zed_extension_api::{self as zed, Result};
 
 struct GleamExtension {
@@ -8,7 +10,7 @@ struct GleamExtension {
 impl GleamExtension {
     fn language_server_binary_path(
         &mut self,
-        config: zed::LanguageServerConfig,
+        language_server_id: &LanguageServerId,
         worktree: &zed::Worktree,
     ) -> Result<String> {
         if let Some(path) = &self.cached_binary_path {
@@ -23,7 +25,7 @@ impl GleamExtension {
         }
 
         zed::set_language_server_installation_status(
-            &config.name,
+            &language_server_id,
             &zed::LanguageServerInstallationStatus::CheckingForUpdate,
         );
         let release = zed::latest_github_release(
@@ -61,7 +63,7 @@ impl GleamExtension {
 
         if !fs::metadata(&binary_path).map_or(false, |stat| stat.is_file()) {
             zed::set_language_server_installation_status(
-                &config.name,
+                &language_server_id,
                 &zed::LanguageServerInstallationStatus::Downloading,
             );
 
@@ -96,15 +98,51 @@ impl zed::Extension for GleamExtension {
 
     fn language_server_command(
         &mut self,
-        config: zed::LanguageServerConfig,
+        language_server_id: &LanguageServerId,
         worktree: &zed::Worktree,
     ) -> Result<zed::Command> {
         Ok(zed::Command {
-            command: self.language_server_binary_path(config, worktree)?,
+            command: self.language_server_binary_path(language_server_id, worktree)?,
             args: vec!["lsp".to_string()],
             env: Default::default(),
         })
     }
+
+    fn label_for_completion(
+        &self,
+        _language_server_id: &LanguageServerId,
+        completion: zed::lsp::Completion,
+    ) -> Option<zed::CodeLabel> {
+        let name = &completion.label;
+        let ty = completion.detail?;
+        let let_binding = "let a";
+        let colon = ": ";
+        let assignment = " = ";
+        let call = match completion.kind? {
+            CompletionKind::Function | CompletionKind::Constructor => "()",
+            _ => "",
+        };
+        let code = format!("{let_binding}{colon}{ty}{assignment}{name}{call}");
+
+        Some(CodeLabel {
+            spans: vec![
+                CodeLabelSpan::code_range({
+                    let start = let_binding.len() + colon.len() + ty.len() + assignment.len();
+                    start..start + name.len()
+                }),
+                CodeLabelSpan::code_range({
+                    let start = let_binding.len();
+                    start..start + colon.len()
+                }),
+                CodeLabelSpan::code_range({
+                    let start = let_binding.len() + colon.len();
+                    start..start + ty.len()
+                }),
+            ],
+            filter_range: (0..name.len()).into(),
+            code,
+        })
+    }
 }
 
 zed::register_extension!(GleamExtension);