erlang: Add support for installing elp language server (#14923)

This PR updates the Erlang extension with support for installing the
[Erlang Language
Platform](https://github.com/WhatsApp/erlang-language-platform) (`elp`)
language server from the GitHub Release assets.

Release Notes:

- N/A

assets/settings/default.json

@@ -743,6 +743,9 @@
     "Elixir": {
       "language_servers": ["elixir-ls", "!next-ls", "!lexical", "..."]
     },
+    "Erlang": {
+      "language_servers": ["erlang-ls", "!elp", "..."]
+    },
     "Go": {
       "code_actions_on_format": {
         "source.organizeImports": true

extensions/erlang/src/language_servers/erlang_language_platform.rs

@@ -1,12 +1,18 @@
+use std::fs;
+
 use zed_extension_api::{self as zed, LanguageServerId, Result};
 
-pub struct ErlangLanguagePlatform;
+pub struct ErlangLanguagePlatform {
+    cached_binary_path: Option<String>,
+}
 
 impl ErlangLanguagePlatform {
     pub const LANGUAGE_SERVER_ID: &'static str = "elp";
 
     pub fn new() -> Self {
-        Self
+        Self {
+            cached_binary_path: None,
+        }
     }
 
     pub fn language_server_command(
@@ -23,11 +29,84 @@ impl ErlangLanguagePlatform {
 
     fn language_server_binary_path(
         &mut self,
-        _language_server_id: &LanguageServerId,
+        language_server_id: &LanguageServerId,
         worktree: &zed::Worktree,
     ) -> Result<String> {
-        worktree
-            .which("elp")
-            .ok_or_else(|| "elp must be installed and available on your $PATH".to_string())
+        if let Some(path) = worktree.which("elp") {
+            return Ok(path);
+        }
+
+        if let Some(path) = &self.cached_binary_path {
+            if fs::metadata(path).map_or(false, |stat| stat.is_file()) {
+                return Ok(path.clone());
+            }
+        }
+
+        zed::set_language_server_installation_status(
+            &language_server_id,
+            &zed::LanguageServerInstallationStatus::CheckingForUpdate,
+        );
+        let release = zed::latest_github_release(
+            "WhatsApp/erlang-language-platform",
+            zed::GithubReleaseOptions {
+                require_assets: true,
+                pre_release: false,
+            },
+        )?;
+
+        let (platform, arch) = zed::current_platform();
+        let asset_name = {
+            let otp_version = "26.2";
+            let (os, os_target) = match platform {
+                zed::Os::Mac => ("macos", "apple-darwin"),
+                zed::Os::Linux => ("linux", "unknown-linux-gnu"),
+                zed::Os::Windows => return Err(format!("unsupported platform: {platform:?}")),
+            };
+
+            format!(
+                "elp-{os}-{arch}-{os_target}-otp-{otp_version}.tar.gz",
+                arch = match arch {
+                    zed::Architecture::Aarch64 => "aarch64",
+                    zed::Architecture::X8664 => "x86_64",
+                    zed::Architecture::X86 =>
+                        return Err(format!("unsupported architecture: {arch:?}")),
+                },
+            )
+        };
+
+        let asset = release
+            .assets
+            .iter()
+            .find(|asset| asset.name == asset_name)
+            .ok_or_else(|| format!("no asset found matching {:?}", asset_name))?;
+
+        let version_dir = format!("elp-{}", release.version);
+        let binary_path = format!("{version_dir}/elp");
+
+        if !fs::metadata(&binary_path).map_or(false, |stat| stat.is_file()) {
+            zed::set_language_server_installation_status(
+                &language_server_id,
+                &zed::LanguageServerInstallationStatus::Downloading,
+            );
+
+            zed::download_file(
+                &asset.download_url,
+                &version_dir,
+                zed::DownloadedFileType::GzipTar,
+            )
+            .map_err(|e| format!("failed to download file: {e}"))?;
+
+            let entries =
+                fs::read_dir(".").map_err(|e| format!("failed to list working directory {e}"))?;
+            for entry in entries {
+                let entry = entry.map_err(|e| format!("failed to load directory entry {e}"))?;
+                if entry.file_name().to_str() != Some(&version_dir) {
+                    fs::remove_dir_all(&entry.path()).ok();
+                }
+            }
+        }
+
+        self.cached_binary_path = Some(binary_path.clone());
+        Ok(binary_path)
     }
 }