11 files changed, 280 insertions, 207 deletions

diff --git a/editors/code/package.json b/editors/code/package.json
index a607c2148..ed1cae2ab 100644
--- a/editors/code/package.json
+++ b/editors/code/package.json
@@ -6,7 +6,7 @@
     "private": true,
     "icon": "icon.png",
     "//": "The real version is in release.yaml, this one just needs to be bigger",
-    "version": "0.2.0-dev",
+    "version": "0.2.20200211-dev",
     "publisher": "matklad",
     "repository": {
         "url": "https://github.com/rust-analyzer/rust-analyzer.git",
@@ -233,11 +233,10 @@
                     "description": "Trace requests to the ra_lsp_server"
                 },
                 "rust-analyzer.lruCapacity": {
-                    "type": [
-                        "number",
-                        "null"
-                    ],
+                    "type": [ "null", "integer" ],
                     "default": null,
+                    "minimum": 0,
+                    "exclusiveMinimum": true,
                     "description": "Number of syntax trees rust-analyzer keeps in memory"
                 },
                 "rust-analyzer.displayInlayHints": {
@@ -246,9 +245,10 @@
                     "description": "Display additional type and parameter information in the editor"
                 },
                 "rust-analyzer.maxInlayHintLength": {
-                    "type": "number",
+                    "type": [ "null", "integer" ],
                     "default": 20,
-                    "exclusiveMinimum": 0,
+                    "minimum": 0,
+                    "exclusiveMinimum": true,
                     "description": "Maximum length for inlay hints"
                 },
                 "rust-analyzer.cargoFeatures.noDefaultFeatures": {
diff --git a/editors/code/rollup.config.js b/editors/code/rollup.config.js
index f8d320f46..337385a24 100644
--- a/editors/code/rollup.config.js
+++ b/editors/code/rollup.config.js
@@ -18,6 +18,7 @@ export default {
     external: [...nodeBuiltins, 'vscode'],
     output: {
         file: './out/main.js',
-        format: 'cjs'
+        format: 'cjs',
+        exports: 'named'
     }
 };
diff --git a/editors/code/src/client.ts b/editors/code/src/client.ts
index dcf9d0c06..efef820ab 100644
--- a/editors/code/src/client.ts
+++ b/editors/code/src/client.ts
@@ -2,7 +2,7 @@ import * as lc from 'vscode-languageclient';
 import * as vscode from 'vscode';
 
 import { Config } from './config';
-import { ensureLanguageServerBinary } from './installation/language_server';
+import { ensureServerBinary } from './installation/server';
 import { CallHierarchyFeature } from 'vscode-languageclient/lib/callHierarchy.proposed';
 
 export async function createClient(config: Config): Promise<null | lc.LanguageClient> {
@@ -11,11 +11,11 @@ export async function createClient(config: Config): Promise<null | lc.LanguageCl
     // It might be a good idea to test if the uri points to a file.
     const workspaceFolderPath = vscode.workspace.workspaceFolders?.[0]?.uri.fsPath ?? '.';
 
-    const langServerPath = await ensureLanguageServerBinary(config.langServerBinarySource);
-    if (!langServerPath) return null;
+    const serverPath = await ensureServerBinary(config.serverSource);
+    if (!serverPath) return null;
 
     const run: lc.Executable = {
-        command: langServerPath,
+        command: serverPath,
         options: { cwd: workspaceFolderPath },
     };
     const serverOptions: lc.ServerOptions = {
diff --git a/editors/code/src/config.ts b/editors/code/src/config.ts
index 8cd89e119..53e2a414b 100644
--- a/editors/code/src/config.ts
+++ b/editors/code/src/config.ts
@@ -24,6 +24,19 @@ export class Config {
     ]
     .map(opt => `${Config.rootSection}.${opt}`);
 
+    private static readonly extensionVersion: string = (() => {
+        const packageJsonVersion = vscode
+            .extensions
+            .getExtension("matklad.rust-analyzer")!
+            .packageJSON
+            .version as string; // n.n.YYYYMMDD
+
+        const realVersionRegexp = /^\d+\.\d+\.(\d{4})(\d{2})(\d{2})/;
+        const [, yyyy, mm, dd] = packageJsonVersion.match(realVersionRegexp)!;
+
+        return `${yyyy}-${mm}-${dd}`;
+    })();
+
     private cfg!: vscode.WorkspaceConfiguration;
 
     constructor(private readonly ctx: vscode.ExtensionContext) {
@@ -31,7 +44,6 @@ export class Config {
         this.refreshConfig();
     }
 
-
     private refreshConfig() {
         this.cfg = vscode.workspace.getConfiguration(Config.rootSection);
         console.log("Using configuration:", this.cfg);
@@ -68,7 +80,7 @@ export class Config {
      * `platform` on GitHub releases. (It is also stored under the same name when
      * downloaded by the extension).
      */
-    get prebuiltLangServerFileName(): null | string {
+    get prebuiltServerFileName(): null | string {
         // See possible `arch` values here:
         // https://nodejs.org/api/process.html#process_process_arch
 
@@ -98,17 +110,17 @@ export class Config {
         }
     }
 
-    get langServerBinarySource(): null | BinarySource {
-        const langServerPath = RA_LSP_DEBUG ?? this.cfg.get<null | string>("raLspServerPath");
+    get serverSource(): null | BinarySource {
+        const serverPath = RA_LSP_DEBUG ?? this.cfg.get<null | string>("raLspServerPath");
 
-        if (langServerPath) {
+        if (serverPath) {
             return {
                 type: BinarySource.Type.ExplicitPath,
-                path: Config.replaceTildeWithHomeDir(langServerPath)
+                path: Config.replaceTildeWithHomeDir(serverPath)
             };
         }
 
-        const prebuiltBinaryName = this.prebuiltLangServerFileName;
+        const prebuiltBinaryName = this.prebuiltServerFileName;
 
         if (!prebuiltBinaryName) return null;
 
@@ -116,6 +128,8 @@ export class Config {
             type: BinarySource.Type.GithubRelease,
             dir:  this.ctx.globalStoragePath,
             file: prebuiltBinaryName,
+            storage: this.ctx.globalState,
+            version: Config.extensionVersion,
             repo: {
                 name: "rust-analyzer",
                 owner: "rust-analyzer",
@@ -153,5 +167,5 @@ export class Config {
     }
 
     // for internal use
-    get withSysroot() { return this.cfg.get("withSysroot", false); }
+    get withSysroot() { return this.cfg.get("withSysroot", true) as boolean; }
 }
diff --git a/editors/code/src/ctx.ts b/editors/code/src/ctx.ts
index 70042a479..ff6245f78 100644
--- a/editors/code/src/ctx.ts
+++ b/editors/code/src/ctx.ts
@@ -60,6 +60,10 @@ export class Ctx {
         this.pushCleanup(d);
     }
 
+    get globalState(): vscode.Memento {
+        return this.extCtx.globalState;
+    }
+
     get subscriptions(): Disposable[] {
         return this.extCtx.subscriptions;
     }
@@ -87,15 +91,11 @@ export async function sendRequestWithRetry<R>(
     for (const delay of [2, 4, 6, 8, 10, null]) {
         try {
             return await (token ? client.sendRequest(method, param, token) : client.sendRequest(method, param));
-        } catch (e) {
-            if (
-                e.code === lc.ErrorCodes.ContentModified &&
-                delay !== null
-            ) {
-                await sleep(10 * (1 << delay));
-                continue;
+        } catch (err) {
+            if (delay === null || err.code !== lc.ErrorCodes.ContentModified) {
+                throw err;
             }
-            throw e;
+            await sleep(10 * (1 << delay));
         }
     }
     throw 'unreachable';
diff --git a/editors/code/src/inlay_hints.ts b/editors/code/src/inlay_hints.ts
index 1c019a51b..3896878cd 100644
--- a/editors/code/src/inlay_hints.ts
+++ b/editors/code/src/inlay_hints.ts
@@ -13,7 +13,7 @@ export function activateInlayHints(ctx: Ctx) {
 
     vscode.workspace.onDidChangeTextDocument(
         async event => {
-            if (event.contentChanges.length !== 0) return;
+            if (event.contentChanges.length === 0) return;
             if (event.document.languageId !== 'rust') return;
             await hintsUpdater.refresh();
         },
@@ -27,7 +27,9 @@ export function activateInlayHints(ctx: Ctx) {
         ctx.subscriptions
     );
 
-    ctx.onDidRestart(_ => hintsUpdater.setEnabled(ctx.config.displayInlayHints));
+    // We pass async function though it will not be awaited when called,
+    // thus Promise rejections won't be handled, but this should never throw in fact...
+    ctx.onDidRestart(async _ => hintsUpdater.setEnabled(ctx.config.displayInlayHints));
 }
 
 interface InlayHintsParams {
@@ -36,7 +38,7 @@ interface InlayHintsParams {
 
 interface InlayHint {
     range: vscode.Range;
-    kind: string;
+    kind: "TypeHint" | "ParameterHint";
     label: string;
 }
 
@@ -53,7 +55,7 @@ const parameterHintDecorationType = vscode.window.createTextEditorDecorationType
 });
 
 class HintsUpdater {
-    private pending: Map<string, vscode.CancellationTokenSource> = new Map();
+    private pending = new Map<string, vscode.CancellationTokenSource>();
     private ctx: Ctx;
     private enabled: boolean;
 
@@ -62,30 +64,36 @@ class HintsUpdater {
         this.enabled = ctx.config.displayInlayHints;
     }
 
-    async setEnabled(enabled: boolean) {
+    async setEnabled(enabled: boolean): Promise<void> {
         if (this.enabled == enabled) return;
         this.enabled = enabled;
 
         if (this.enabled) {
-            await this.refresh();
-        } else {
-            this.allEditors.forEach(it => {
-                this.setTypeDecorations(it, []);
-                this.setParameterDecorations(it, []);
-            });
+            return await this.refresh();
         }
+        this.allEditors.forEach(it => {
+            this.setTypeDecorations(it, []);
+            this.setParameterDecorations(it, []);
+        });
     }
 
     async refresh() {
         if (!this.enabled) return;
-        const promises = this.allEditors.map(it => this.refreshEditor(it));
-        await Promise.all(promises);
+        await Promise.all(this.allEditors.map(it => this.refreshEditor(it)));
+    }
+
+    private get allEditors(): vscode.TextEditor[] {
+        return vscode.window.visibleTextEditors.filter(
+            editor => editor.document.languageId === 'rust',
+        );
     }
 
     private async refreshEditor(editor: vscode.TextEditor): Promise<void> {
         const newHints = await this.queryHints(editor.document.uri.toString());
         if (newHints == null) return;
-        const newTypeDecorations = newHints.filter(hint => hint.kind === 'TypeHint')
+
+        const newTypeDecorations = newHints
+            .filter(hint => hint.kind === 'TypeHint')
             .map(hint => ({
                 range: hint.range,
                 renderOptions: {
@@ -96,7 +104,8 @@ class HintsUpdater {
             }));
         this.setTypeDecorations(editor, newTypeDecorations);
 
-        const newParameterDecorations = newHints.filter(hint => hint.kind === 'ParameterHint')
+        const newParameterDecorations = newHints
+            .filter(hint => hint.kind === 'ParameterHint')
             .map(hint => ({
                 range: hint.range,
                 renderOptions: {
@@ -108,12 +117,6 @@ class HintsUpdater {
         this.setParameterDecorations(editor, newParameterDecorations);
     }
 
-    private get allEditors(): vscode.TextEditor[] {
-        return vscode.window.visibleTextEditors.filter(
-            editor => editor.document.languageId === 'rust',
-        );
-    }
-
     private setTypeDecorations(
         editor: vscode.TextEditor,
         decorations: vscode.DecorationOptions[],
@@ -137,12 +140,14 @@ class HintsUpdater {
     private async queryHints(documentUri: string): Promise<InlayHint[] | null> {
         const client = this.ctx.client;
         if (!client) return null;
+
         const request: InlayHintsParams = {
             textDocument: { uri: documentUri },
         };
         const tokenSource = new vscode.CancellationTokenSource();
-        const prev = this.pending.get(documentUri);
-        if (prev) prev.cancel();
+        const prevHintsRequest = this.pending.get(documentUri);
+        prevHintsRequest?.cancel();
+
         this.pending.set(documentUri, tokenSource);
         try {
             return await sendRequestWithRetry<InlayHint[] | null>(
diff --git a/editors/code/src/installation/download_artifact.ts b/editors/code/src/installation/download_artifact.ts
new file mode 100644
index 000000000..de655f8f4
--- /dev/null
+++ b/editors/code/src/installation/download_artifact.ts
@@ -0,0 +1,58 @@
+import * as vscode from "vscode";
+import * as path from "path";
+import { promises as fs } from "fs";
+import { strict as assert } from "assert";
+
+import { ArtifactReleaseInfo } from "./interfaces";
+import { downloadFile } from "./download_file";
+import { throttle } from "throttle-debounce";
+
+/**
+ * Downloads artifact from given `downloadUrl`.
+ * Creates `installationDir` if it is not yet created and put the artifact under
+ * `artifactFileName`.
+ * Displays info about the download progress in an info message printing the name
+ * of the artifact as `displayName`.
+ */
+export async function downloadArtifact(
+    {downloadUrl, releaseName}: ArtifactReleaseInfo,
+    artifactFileName: string,
+    installationDir: string,
+    displayName: string,
+) {
+    await fs.mkdir(installationDir).catch(err => assert.strictEqual(
+        err?.code,
+        "EEXIST",
+        `Couldn't create directory "${installationDir}" to download `+
+        `${artifactFileName} artifact: ${err.message}`
+    ));
+
+    const installationPath = path.join(installationDir, artifactFileName);
+
+    console.time(`Downloading ${artifactFileName}`);
+    await vscode.window.withProgress(
+        {
+            location: vscode.ProgressLocation.Notification,
+            cancellable: false, // FIXME: add support for canceling download?
+            title: `Downloading ${displayName} (${releaseName})`
+        },
+        async (progress, _cancellationToken) => {
+            let lastPrecentage = 0;
+            const filePermissions = 0o755; // (rwx, r_x, r_x)
+            await downloadFile(downloadUrl, installationPath, filePermissions, throttle(
+                200,
+                /* noTrailing: */ true,
+                (readBytes, totalBytes) => {
+                    const newPercentage = (readBytes / totalBytes) * 100;
+                    progress.report({
+                        message: newPercentage.toFixed(0) + "%",
+                        increment: newPercentage - lastPrecentage
+                    });
+
+                    lastPrecentage = newPercentage;
+                })
+            );
+        }
+    );
+    console.timeEnd(`Downloading ${artifactFileName}`);
+}
diff --git a/editors/code/src/installation/fetch_latest_artifact_metadata.ts b/editors/code/src/installation/fetch_artifact_release_info.ts
index 7e3700603..7d497057a 100644
--- a/editors/code/src/installation/fetch_latest_artifact_metadata.ts
+++ b/editors/code/src/installation/fetch_artifact_release_info.ts
@@ -1,26 +1,32 @@
 import fetch from "node-fetch";
-import { GithubRepo, ArtifactMetadata } from "./interfaces";
+import { GithubRepo, ArtifactReleaseInfo } from "./interfaces";
 
 const GITHUB_API_ENDPOINT_URL = "https://api.github.com";
 
+
 /**
- * Fetches the latest release from GitHub `repo` and returns metadata about
- * `artifactFileName` shipped with this release or `null` if no such artifact was published.
+ * Fetches the release with `releaseTag` (or just latest release when not specified)
+ * from GitHub `repo` and returns metadata about `artifactFileName` shipped with
+ * this release or `null` if no such artifact was published.
  */
-export async function fetchLatestArtifactMetadata(
-    repo: GithubRepo, artifactFileName: string
-): Promise<null | ArtifactMetadata> {
+export async function fetchArtifactReleaseInfo(
+    repo: GithubRepo, artifactFileName: string, releaseTag?: string
+): Promise<null | ArtifactReleaseInfo> {
 
     const repoOwner = encodeURIComponent(repo.owner);
     const repoName  = encodeURIComponent(repo.name);
 
-    const apiEndpointPath = `/repos/${repoOwner}/${repoName}/releases/latest`;
+    const apiEndpointPath = releaseTag
+        ? `/repos/${repoOwner}/${repoName}/releases/tags/${releaseTag}`
+        : `/repos/${repoOwner}/${repoName}/releases/latest`;
+
     const requestUrl = GITHUB_API_ENDPOINT_URL + apiEndpointPath;
 
     // We skip runtime type checks for simplicity (here we cast from `any` to `GithubRelease`)
 
     console.log("Issuing request for released artifacts metadata to", requestUrl);
 
+    // FIXME: handle non-ok response
     const response: GithubRelease = await fetch(requestUrl, {
             headers: { Accept: "application/vnd.github.v3+json" }
         })
diff --git a/editors/code/src/installation/interfaces.ts b/editors/code/src/installation/interfaces.ts
index 8039d0b90..e40839e4b 100644
--- a/editors/code/src/installation/interfaces.ts
+++ b/editors/code/src/installation/interfaces.ts
@@ -1,3 +1,5 @@
+import * as vscode from "vscode";
+
 export interface GithubRepo {
     name: string;
     owner: string;
@@ -6,7 +8,7 @@ export interface GithubRepo {
 /**
  * Metadata about particular artifact retrieved from GitHub releases.
  */
-export interface ArtifactMetadata {
+export interface ArtifactReleaseInfo {
     releaseName: string;
     downloadUrl: string;
 }
@@ -50,6 +52,17 @@ export namespace BinarySource {
          * and in local `.dir`.
          */
         file: string;
+
+        /**
+         * Tag of github release that denotes a version required by this extension.
+         */
+        version: string;
+
+        /**
+         * Object that provides `get()/update()` operations to store metadata
+         * about the actual binary, e.g. its actual version.
+         */
+        storage: vscode.Memento;
     }
 
 }
diff --git a/editors/code/src/installation/language_server.ts b/editors/code/src/installation/language_server.ts
deleted file mode 100644
index 4797c3f01..000000000
--- a/editors/code/src/installation/language_server.ts
+++ /dev/null
@@ -1,148 +0,0 @@
-import * as vscode from "vscode";
-import * as path from "path";
-import { strict as assert } from "assert";
-import { promises as fs } from "fs";
-import { promises as dns } from "dns";
-import { spawnSync } from "child_process";
-import { throttle } from "throttle-debounce";
-
-import { BinarySource } from "./interfaces";
-import { fetchLatestArtifactMetadata } from "./fetch_latest_artifact_metadata";
-import { downloadFile } from "./download_file";
-
-export async function downloadLatestLanguageServer(
-    {file: artifactFileName, dir: installationDir, repo}: BinarySource.GithubRelease
-) {
-    const { releaseName, downloadUrl } = (await fetchLatestArtifactMetadata(
-        repo, artifactFileName
-    ))!;
-
-    await fs.mkdir(installationDir).catch(err => assert.strictEqual(
-        err?.code,
-        "EEXIST",
-        `Couldn't create directory "${installationDir}" to download `+
-        `language server binary: ${err.message}`
-    ));
-
-    const installationPath = path.join(installationDir, artifactFileName);
-
-    console.time("Downloading ra_lsp_server");
-    await vscode.window.withProgress(
-        {
-            location: vscode.ProgressLocation.Notification,
-            cancellable: false, // FIXME: add support for canceling download?
-            title: `Downloading language server (${releaseName})`
-        },
-        async (progress, _cancellationToken) => {
-            let lastPrecentage = 0;
-            const filePermissions = 0o755; // (rwx, r_x, r_x)
-            await downloadFile(downloadUrl, installationPath, filePermissions, throttle(
-                200,
-                /* noTrailing: */ true,
-                (readBytes, totalBytes) => {
-                    const newPercentage = (readBytes / totalBytes) * 100;
-                    progress.report({
-                        message: newPercentage.toFixed(0) + "%",
-                        increment: newPercentage - lastPrecentage
-                    });
-
-                    lastPrecentage = newPercentage;
-                })
-            );
-        }
-    );
-    console.timeEnd("Downloading ra_lsp_server");
-}
-export async function ensureLanguageServerBinary(
-    langServerSource: null | BinarySource
-): Promise<null | string> {
-
-    if (!langServerSource) {
-        vscode.window.showErrorMessage(
-            "Unfortunately we don't ship binaries for your platform yet. " +
-            "You need to manually clone rust-analyzer repository and " +
-            "run `cargo xtask install --server` to build the language server from sources. " +
-            "If you feel that your platform should be supported, please create an issue " +
-            "about that [here](https://github.com/rust-analyzer/rust-analyzer/issues) and we " +
-            "will consider it."
-        );
-        return null;
-    }
-
-    switch (langServerSource.type) {
-        case BinarySource.Type.ExplicitPath: {
-            if (isBinaryAvailable(langServerSource.path)) {
-                return langServerSource.path;
-            }
-
-            vscode.window.showErrorMessage(
-                `Unable to run ${langServerSource.path} binary. ` +
-                `To use the pre-built language server, set "rust-analyzer.raLspServerPath" ` +
-                "value to `null` or remove it from the settings to use it by default."
-            );
-            return null;
-        }
-        case BinarySource.Type.GithubRelease: {
-            const prebuiltBinaryPath = path.join(langServerSource.dir, langServerSource.file);
-
-            if (isBinaryAvailable(prebuiltBinaryPath)) {
-                return prebuiltBinaryPath;
-            }
-
-            const userResponse = await vscode.window.showInformationMessage(
-                "Language server binary for rust-analyzer was not found. " +
-                "Do you want to download it now?",
-                "Download now", "Cancel"
-            );
-            if (userResponse !== "Download now") return null;
-
-            try {
-                await downloadLatestLanguageServer(langServerSource);
-            } catch (err) {
-                vscode.window.showErrorMessage(
-                    `Failed to download language server from ${langServerSource.repo.name} ` +
-                    `GitHub repository: ${err.message}`
-                );
-
-                console.error(err);
-
-                dns.resolve('example.com').then(
-                    addrs => console.log("DNS resolution for example.com was successful", addrs),
-                    err => {
-                        console.error(
-                            "DNS resolution for example.com failed, " +
-                            "there might be an issue with Internet availability"
-                        );
-                        console.error(err);
-                    }
-                );
-
-                return null;
-            }
-
-            if (!isBinaryAvailable(prebuiltBinaryPath)) assert(false,
-                `Downloaded language server binary is not functional.` +
-                `Downloaded from: ${JSON.stringify(langServerSource)}`
-            );
-
-
-            vscode.window.showInformationMessage(
-                "Rust analyzer language server was successfully installed 🦀"
-            );
-
-            return prebuiltBinaryPath;
-        }
-    }
-
-    function isBinaryAvailable(binaryPath: string) {
-        const res = spawnSync(binaryPath, ["--version"]);
-
-        // ACHTUNG! `res` type declaration is inherently wrong, see
-        // https://github.com/DefinitelyTyped/DefinitelyTyped/issues/42221
-
-        console.log("Checked binary availablity via --version", res);
-        console.log(binaryPath, "--version output:", res.output?.map(String));
-
-        return res.status === 0;
-    }
-}
diff --git a/editors/code/src/installation/server.ts b/editors/code/src/installation/server.ts
new file mode 100644
index 000000000..80cb719e3
--- /dev/null
+++ b/editors/code/src/installation/server.ts
@@ -0,0 +1,124 @@
+import * as vscode from "vscode";
+import * as path from "path";
+import { strict as assert } from "assert";
+import { promises as dns } from "dns";
+import { spawnSync } from "child_process";
+
+import { BinarySource } from "./interfaces";
+import { fetchArtifactReleaseInfo } from "./fetch_artifact_release_info";
+import { downloadArtifact } from "./download_artifact";
+
+export async function ensureServerBinary(source: null | BinarySource): Promise<null | string> {
+    if (!source) {
+        vscode.window.showErrorMessage(
+            "Unfortunately we don't ship binaries for your platform yet. " +
+            "You need to manually clone rust-analyzer repository and " +
+            "run `cargo xtask install --server` to build the language server from sources. " +
+            "If you feel that your platform should be supported, please create an issue " +
+            "about that [here](https://github.com/rust-analyzer/rust-analyzer/issues) and we " +
+            "will consider it."
+        );
+        return null;
+    }
+
+    switch (source.type) {
+        case BinarySource.Type.ExplicitPath: {
+            if (isBinaryAvailable(source.path)) {
+                return source.path;
+            }
+
+            vscode.window.showErrorMessage(
+                `Unable to run ${source.path} binary. ` +
+                `To use the pre-built language server, set "rust-analyzer.raLspServerPath" ` +
+                "value to `null` or remove it from the settings to use it by default."
+            );
+            return null;
+        }
+        case BinarySource.Type.GithubRelease: {
+            const prebuiltBinaryPath = path.join(source.dir, source.file);
+
+            const installedVersion: null | string = getServerVersion(source.storage);
+            const requiredVersion: string = source.version;
+
+            console.log("Installed version:", installedVersion, "required:", requiredVersion);
+
+            if (isBinaryAvailable(prebuiltBinaryPath) && installedVersion == requiredVersion) {
+                // FIXME: check for new releases and notify the user to update if possible
+                return prebuiltBinaryPath;
+            }
+
+            const userResponse = await vscode.window.showInformationMessage(
+                `Language server version ${source.version} for rust-analyzer is not installed. ` +
+                "Do you want to download it now?",
+                "Download now", "Cancel"
+            );
+            if (userResponse !== "Download now") return null;
+
+            if (!await downloadServer(source)) return null;
+
+            return prebuiltBinaryPath;
+        }
+    }
+}
+
+async function downloadServer(source: BinarySource.GithubRelease): Promise<boolean> {
+    try {
+        const releaseInfo = (await fetchArtifactReleaseInfo(source.repo, source.file, source.version))!;
+
+        await downloadArtifact(releaseInfo, source.file, source.dir, "language server");
+        await setServerVersion(source.storage, releaseInfo.releaseName);
+    } catch (err) {
+        vscode.window.showErrorMessage(
+            `Failed to download language server from ${source.repo.name} ` +
+            `GitHub repository: ${err.message}`
+        );
+
+        console.error(err);
+
+        dns.resolve('example.com').then(
+            addrs => console.log("DNS resolution for example.com was successful", addrs),
+            err => {
+                console.error(
+                    "DNS resolution for example.com failed, " +
+                    "there might be an issue with Internet availability"
+                );
+                console.error(err);
+            }
+        );
+        return false;
+    }
+
+    if (!isBinaryAvailable(path.join(source.dir, source.file))) assert(false,
+        `Downloaded language server binary is not functional.` +
+        `Downloaded from: ${JSON.stringify(source, null, 4)}`
+    );
+
+    vscode.window.showInformationMessage(
+        "Rust analyzer language server was successfully installed 🦀"
+    );
+
+    return true;
+}
+
+function isBinaryAvailable(binaryPath: string): boolean {
+    const res = spawnSync(binaryPath, ["--version"]);
+
+    // ACHTUNG! `res` type declaration is inherently wrong, see
+    // https://github.com/DefinitelyTyped/DefinitelyTyped/issues/42221
+
+    console.log("Checked binary availablity via --version", res);
+    console.log(binaryPath, "--version output:", res.output?.map(String));
+
+    return res.status === 0;
+}
+
+function getServerVersion(storage: vscode.Memento): null | string {
+    const version = storage.get<null | string>("server-version", null);
+    console.log("Get server-version:", version);
+    return version;
+}
+
+async function setServerVersion(storage: vscode.Memento, version: string): Promise<void> {
+    console.log("Set server-version:", version);
+    await storage.update("server-version", version.toString());
+}