working copy: add reset() function to the backend trait

This includes documenting the new function and the other types moved
to the `working_copy` module.

cli/src/cli_util.rs

@@ -41,9 +41,7 @@ use jj_lib::git_backend::GitBackend;
 use jj_lib::gitignore::GitIgnoreFile;
 use jj_lib::hex_util::to_reverse_hex;
 use jj_lib::id_prefix::IdPrefixContext;
-use jj_lib::local_working_copy::{
-    LocalWorkingCopy, LockedLocalWorkingCopy, ResetError, WorkingCopyStateError,
-};
+use jj_lib::local_working_copy::{LocalWorkingCopy, LockedLocalWorkingCopy, WorkingCopyStateError};
 use jj_lib::matchers::{EverythingMatcher, Matcher, PrefixMatcher, Visit};
 use jj_lib::merged_tree::{MergedTree, MergedTreeBuilder};
 use jj_lib::op_heads_store::{self, OpHeadResolutionError, OpHeadsStore};
@@ -63,7 +61,7 @@ use jj_lib::settings::{ConfigResultExt as _, UserSettings};
 use jj_lib::transaction::Transaction;
 use jj_lib::tree::TreeMergeError;
 use jj_lib::working_copy::{
-    CheckoutStats, LockedWorkingCopy, SnapshotError, SnapshotOptions, WorkingCopy,
+    CheckoutStats, LockedWorkingCopy, ResetError, SnapshotError, SnapshotOptions, WorkingCopy,
 };
 use jj_lib::workspace::{
     LockedWorkspace, Workspace, WorkspaceInitError, WorkspaceLoadError, WorkspaceLoader,

lib/src/local_working_copy.rs

@@ -59,7 +59,7 @@ use crate::settings::HumanByteSize;
 use crate::store::Store;
 use crate::tree::Tree;
 use crate::working_copy::{
-    CheckoutError, CheckoutStats, LockedWorkingCopy, SnapshotError, SnapshotOptions,
+    CheckoutError, CheckoutStats, LockedWorkingCopy, ResetError, SnapshotError, SnapshotOptions,
     SnapshotProgress, WorkingCopy,
 };
 
@@ -308,24 +308,6 @@ struct FsmonitorMatcher {
     watchman_clock: Option<crate::protos::working_copy::WatchmanClock>,
 }
 
-#[derive(Debug, Error)]
-pub enum ResetError {
-    // The current working-copy commit was deleted, maybe by an overly aggressive GC that happened
-    // while the current process was running.
-    #[error("Current working-copy commit not found: {source}")]
-    SourceNotFound {
-        source: Box<dyn std::error::Error + Send + Sync>,
-    },
-    #[error("Internal error: {0}")]
-    InternalBackendError(#[from] BackendError),
-    #[error("{message}: {err:?}")]
-    Other {
-        message: String,
-        #[source]
-        err: Box<dyn std::error::Error + Send + Sync>,
-    },
-}
-
 struct DirectoryToVisit {
     dir: RepoPath,
     disk_dir: PathBuf,
@@ -1549,6 +1531,18 @@ impl LockedWorkingCopy for LockedLocalWorkingCopy {
         self.tree_state_dirty = true;
         Ok(stats)
     }
+
+    fn reset(&mut self, new_tree: &MergedTree) -> Result<(), ResetError> {
+        self.wc
+            .tree_state_mut()
+            .map_err(|err| ResetError::Other {
+                message: "Failed to read the working copy state".to_string(),
+                err: err.into(),
+            })?
+            .reset(new_tree)?;
+        self.tree_state_dirty = true;
+        Ok(())
+    }
 }
 
 impl LockedLocalWorkingCopy {
@@ -1564,18 +1558,6 @@ impl LockedLocalWorkingCopy {
         Ok(())
     }
 
-    pub fn reset(&mut self, new_tree: &MergedTree) -> Result<(), ResetError> {
-        self.wc
-            .tree_state_mut()
-            .map_err(|err| ResetError::Other {
-                message: "Failed to read the working copy state".to_string(),
-                err: err.into(),
-            })?
-            .reset(new_tree)?;
-        self.tree_state_dirty = true;
-        Ok(())
-    }
-
     pub fn sparse_patterns(&self) -> Result<&[RepoPath], WorkingCopyStateError> {
         self.wc.sparse_patterns()
     }

lib/src/working_copy.rs

@@ -66,6 +66,9 @@ pub trait LockedWorkingCopy {
     /// Check out the specified tree in the working copy.
     // TODO: Pass a Commit object here because some implementations need that.
     fn check_out(&mut self, new_tree: &MergedTree) -> Result<CheckoutStats, CheckoutError>;
+
+    /// Update to another tree without touching the files in the working copy.
+    fn reset(&mut self, new_tree: &MergedTree) -> Result<(), ResetError>;
 }
 
 /// An error while snapshotting the working copy.
@@ -193,3 +196,27 @@ pub enum CheckoutError {
         err: Box<dyn std::error::Error + Send + Sync>,
     },
 }
+
+/// An error while resetting the working copy.
+#[derive(Debug, Error)]
+pub enum ResetError {
+    /// The current working-copy commit was deleted, maybe by an overly
+    /// aggressive GC that happened while the current process was running.
+    #[error("Current working-copy commit not found: {source}")]
+    SourceNotFound {
+        /// The underlying error.
+        source: Box<dyn std::error::Error + Send + Sync>,
+    },
+    /// Reading or writing from the commit backend failed.
+    #[error("Internal error: {0}")]
+    InternalBackendError(#[from] BackendError),
+    /// Some other error happened while checking out the working copy.
+    #[error("{message}: {err:?}")]
+    Other {
+        /// Error message.
+        message: String,
+        #[source]
+        /// The underlying error.
+        err: Box<dyn std::error::Error + Send + Sync>,
+    },
+}