feat: enable localDocs (#1261)

Opening docs (rust-analyzer.openDocs) was broken by commit
1410329. This commit contains changes
preparing for the rust-analyzer "localDocs" feature
https://github.com/rust-lang/rust-analyzer/blob/master/docs/dev/lsp-extensions.md#local-documentation
However, this feature is only activated by specifying the "localDocs"
capability. This commits does so in src/client.ts.

To actually make use of the local documentation, the
src/commands.ts:openDocs function also needs adjustment.
I wrote it such that if a local URI is provided and a corresponding file
exists, it is opened.
Otherwise, if a web link is provided, this link is opened.

Note that "vscode.open" would open the local URI in the editor instead
of the browser.

Whether local documentation exists depends on whether 'cargo doc' has
been executed. If it has been executed but not recently, the
documentation might be outdated, so it might be reasonable to generate
local documentation before trying to open it.
This is not done so far, but could be added.
It might make sense to run 'cargo doc --no-deps'.

Co-authored-by: Daniel Rainer <daniel.rainer@localhost>

src/client.ts

@@ -24,6 +24,7 @@ class ExperimentalFeatures implements StaticFeature {
     const caps: any = capabilities.experimental ?? {};
     caps.snippetTextEdit = true;
     caps.serverStatusNotification = true;
+    caps.localDocs = true;
     caps.commands = {
       commands: [
         'rust-analyzer.runSingle',

src/commands.ts

@@ -14,9 +14,10 @@ import {
   window,
   workspace,
   type WorkspaceEdit,
+  nvim,
 } from 'coc.nvim';
 import { randomBytes } from 'node:crypto';
-import { writeFileSync } from 'node:fs';
+import { existsSync, writeFileSync } from 'node:fs';
 import { tmpdir } from 'node:os';
 import { join } from 'node:path';
 import readline from 'node:readline';
@@ -695,9 +696,21 @@ export function openDocs(ctx: Ctx): Cmd {
       textDocument: { uri: document.uri },
       position,
     };
+    // TODO: Should 'cargo doc' run at this point? Or 'cargo doc --no-deps'?
     const doclink = await ctx.client.sendRequest(ra.openDocs, param);
-    if (doclink?.web) {
-      await commands.executeCommand('vscode.open', Uri.parse(doclink.web));
+    if (doclink) {
+      if (doclink.local) {
+        // remove leading 'file://'
+        const absolutePath = doclink.local.substring(7);
+        const isReadable = existsSync(absolutePath);
+        if (isReadable) {
+          await nvim.call('coc#ui#open_url', doclink.local);
+          return;
+        }
+      }
+      if (doclink.web) {
+        await commands.executeCommand('vscode.open', Uri.parse(doclink.web));
+      }
     }
   };
 }