This commit add Cargo-style project discovery for Buck and Bazel users.

This feature requires the user to add a command that generates a
`rust-project.json` from a set of files. Project discovery can be invoked
in two ways:

1. At extension activation time, which includes the generated
   `rust-project.json` as part of the linkedProjects argument in
    InitializeParams
2. Through a new command titled "Add current file to workspace", which
   makes use of a new, rust-analyzer specific LSP request that adds
   the workspace without erasing any existing workspaces.

I think that the command-running functionality _could_ merit being
placed into its own extension (and expose it via extension contribution
points), if only provide build-system idiomatic progress reporting and
status handling, but I haven't (yet) made an extension that does this.

editors/code/src/util.ts

@@ -150,9 +150,11 @@ export function memoizeAsync<Ret, TThis, Param extends string>(
 
 /** Awaitable wrapper around `child_process.exec` */
 export function execute(command: string, options: ExecOptions): Promise<string> {
+    log.info(`running command: ${command}`)
     return new Promise((resolve, reject) => {
         exec(command, options, (err, stdout, stderr) => {
             if (err) {
+                log.error(err);
                 reject(err);
                 return;
             }
@@ -167,6 +169,21 @@ export function execute(command: string, options: ExecOptions): Promise<string>
     });
 }
 
+export function executeDiscoverProject(command: string, options: ExecOptions): Promise<string> {
+    log.info(`running command: ${command}`)
+    return new Promise((resolve, reject) => {
+        exec(command, options, (err, stdout, _) => {
+            if (err) {
+                log.error(err);
+                reject(err);
+                return;
+            }
+
+            resolve(stdout.trimEnd());
+        });
+    });
+}
+
 export class LazyOutputChannel implements vscode.OutputChannel {
     constructor(name: string) {
         this.name = name;