Add basic task support

This adds basic support for running `cargo build`, `cargo run`, etc.

editors/code/src/tasks.ts

@@ -0,0 +1,60 @@
+import {
+    Disposable,
+    ShellExecution,
+    Task,
+    TaskGroup,
+    TaskProvider,
+    tasks,
+    WorkspaceFolder,
+} from 'vscode';
+
+// This ends up as the `type` key in tasks.json. RLS also uses `cargo` and
+// our configuration should be compatible with it so use the same key.
+const TASK_TYPE = 'cargo';
+
+export function activateTaskProvider(target: WorkspaceFolder): Disposable {
+    const provider: TaskProvider = {
+        // Detect Rust tasks. Currently we do not do any actual detection
+        // of tasks (e.g. aliases in .cargo/config) and just return a fixed
+        // set of tasks that always exist. These tasks cannot be removed in
+        // tasks.json - only tweaked.
+        provideTasks: () => getStandardCargoTasks(target),
+
+        // We don't need to implement this.
+        resolveTask: () => undefined,
+    };
+
+    return tasks.registerTaskProvider(TASK_TYPE, provider);
+}
+
+function getStandardCargoTasks(target: WorkspaceFolder): Task[] {
+    return [
+        { command: 'build', group: TaskGroup.Build },
+        { command: 'check', group: TaskGroup.Build },
+        { command: 'test', group: TaskGroup.Test },
+        { command: 'clean', group: TaskGroup.Clean },
+        { command: 'run', group: undefined },
+    ]
+        .map(({ command, group }) => {
+            const vscodeTask = new Task(
+                // The contents of this object end up in the tasks.json entries.
+                {
+                    type: TASK_TYPE,
+                    command,
+                },
+                // The scope of the task - workspace or specific folder (global
+                // is not supported).
+                target,
+                // The task name, and task source. These are shown in the UI as
+                // `${source}: ${name}`, e.g. `rust: cargo build`.
+                `cargo ${command}`,
+                'rust',
+                // What to do when this command is executed.
+                new ShellExecution('cargo', [command]),
+                // Problem matchers.
+                ['$rustc'],
+            );
+            vscodeTask.group = group;
+            return vscodeTask;
+        });
+}