vscode: Support opening local documentation if available

Displaying local instead of web docs can have many benefits:
- the web version may have different features enabled than locally selected
- the standard library may be a different version than is available online
- the user may not be online and therefore cannot access the web documentation
- the documentation may not be available online at all, for example because it
  is for a new feature in a library the user is currently developing

If the documentation is not available locally, the extension still falls back to
the web version.

editors/code/src/client.ts

@@ -389,6 +389,7 @@ class ExperimentalFeatures implements lc.StaticFeature {
             serverStatusNotification: true,
             colorDiagnosticOutput: true,
             openServerLogs: true,
+            localDocs: true,
             commands: {
                 commands: [
                     "rust-analyzer.runSingle",

editors/code/src/commands.ts

@@ -21,6 +21,7 @@ import type { LanguageClient } from "vscode-languageclient/node";
 import { LINKED_COMMANDS } from "./client";
 import type { DependencyId } from "./dependencies_provider";
 import { unwrapUndefinable } from "./undefinable";
+import { log } from "./util";
 
 export * from "./ast_inspector";
 export * from "./run";
@@ -947,7 +948,24 @@ export function openDocs(ctx: CtxInit): Cmd {
         const position = editor.selection.active;
         const textDocument = { uri: editor.document.uri.toString() };
 
-        const doclink = await client.sendRequest(ra.openDocs, { position, textDocument });
+        const doclinks = await client.sendRequest(ra.openDocs, { position, textDocument });
+
+        let doclink;
+        try {
+            if (
+                typeof doclinks.local === "string" &&
+                (await vscode.workspace.fs.stat(vscode.Uri.parse(doclinks.local)))
+            ) {
+                // file does exist locally
+                doclink = doclinks.local;
+            } else {
+                doclink = doclinks.web;
+            }
+        } catch (e) {
+            log.info("stat() threw error. Falling back to web version", e);
+
+            doclink = doclinks.web;
+        }
 
         if (doclink != null) {
             await vscode.env.openExternal(vscode.Uri.parse(doclink));

editors/code/src/lsp_ext.ts

@@ -135,7 +135,11 @@ export const onEnter = new lc.RequestType<lc.TextDocumentPositionParams, lc.Text
 export const openCargoToml = new lc.RequestType<OpenCargoTomlParams, lc.Location, void>(
     "experimental/openCargoToml",
 );
-export const openDocs = new lc.RequestType<lc.TextDocumentPositionParams, string | void, void>(
+export interface DocsUrls {
+    local: string | void;
+    web: string | void;
+}
+export const openDocs = new lc.RequestType<lc.TextDocumentPositionParams, DocsUrls, void>(
     "experimental/externalDocs",
 );
 export const parentModule = new lc.RequestType<