workspace: introduce Workspace type

Having a concept of a "workspace" will be useful for adding support
for multiple workspaces (#13). You can think of the "workspace" as a
repo combined with a working copy. A workspace corresponds 1:1 with a
`.jj/` directory. It's pretty close to what other VCS simply call a
"repo", but I've ended up using the word "repo" for what Git calls a
"bare repo".

lib/src/lib.rs

@@ -59,3 +59,4 @@ pub mod tree;
 pub mod tree_builder;
 pub mod view;
 pub mod working_copy;
+pub mod workspace;

lib/src/repo.rs

@@ -419,6 +419,14 @@ impl RepoLoader {
         })
     }
 
+    pub fn repo_path(&self) -> &PathBuf {
+        &self.repo_path
+    }
+
+    pub fn working_copy_path(&self) -> &PathBuf {
+        &self.wc_path
+    }
+
     pub fn store(&self) -> &Arc<Store> {
         &self.store
     }

lib/src/workspace.rs

@@ -0,0 +1,54 @@
+// Copyright 2021 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+use std::path::PathBuf;
+
+use crate::repo::{RepoLoadError, RepoLoader};
+use crate::settings::UserSettings;
+
+/// Represents a workspace, i.e. what's typically the .jj/ directory and its
+/// parent.
+pub struct Workspace {
+    // Path to the workspace root (typically the parent of a .jj/ directory), which is where
+    // working copy files live.
+    workspace_root: PathBuf,
+    repo_loader: RepoLoader,
+}
+
+impl Workspace {
+    pub fn load(
+        user_settings: &UserSettings,
+        workspace_root: PathBuf,
+    ) -> Result<Self, RepoLoadError> {
+        // TODO: Move the find_repo_dir() call from RepoLoader::init() to here
+        let repo_loader = RepoLoader::init(user_settings, workspace_root)?;
+        let workspace_root = repo_loader.working_copy_path().clone();
+        Ok(Self {
+            workspace_root,
+            repo_loader,
+        })
+    }
+
+    pub fn workspace_root(&self) -> &PathBuf {
+        &self.workspace_root
+    }
+
+    pub fn repo_path(&self) -> &PathBuf {
+        self.repo_loader.repo_path()
+    }
+
+    pub fn repo_loader(&self) -> &RepoLoader {
+        &self.repo_loader
+    }
+}