Attempt to use reflink on Windows

.github/workflows/test.yml

@@ -245,6 +245,13 @@ jobs:
           # Write pending snapshots to a separate directory for artifact upload
           INSTA_UPDATE: new
           INSTA_PENDING_DIR: ${{ github.workspace }}/pending-snapshots
+          # Dev Drive is ReFS which supports copy-on-write
+          # Note: trailing backslash is required — `D:` alone is a relative
+          # path on Windows (current directory on that drive), not the root.
+          UV_INTERNAL__TEST_COW_FS: "${{ env.DEV_DRIVE }}\\"
+          # C: is NTFS which does not support copy-on-write
+          UV_INTERNAL__TEST_NOCOW_FS: "C:\\"
+          UV_INTERNAL__TEST_ALT_FS: "C:\\"
         shell: bash
         run: |
           cargo nextest run \

crates/uv-fs/src/link.rs

@@ -14,7 +14,7 @@ use walkdir::WalkDir;
 ///
 /// Defaults to [`Clone`](LinkMode::Clone) on macOS and Linux (which support copy-on-write on
 /// APFS and btrfs/xfs/bcachefs respectively), and [`Hardlink`](LinkMode::Hardlink) on other
-/// platforms.
+/// platforms. On Windows, clone can be enabled via the `reflink-windows` preview flag.
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 #[cfg_attr(

crates/uv-preview/src/lib.rs

@@ -194,6 +194,7 @@ pub enum PreviewFeature {
     PublishRequireNormalized = 1 << 25,
     Audit = 1 << 26,
     ProjectDirectoryMustExist = 1 << 27,
+    ReflinkWindows = 1 << 28,
 }
 
 impl PreviewFeature {
@@ -228,6 +229,7 @@ impl PreviewFeature {
             Self::PublishRequireNormalized => "publish-require-normalized",
             Self::Audit => "audit",
             Self::ProjectDirectoryMustExist => "project-directory-must-exist",
+            Self::ReflinkWindows => "reflink-windows",
         }
     }
 }
@@ -275,6 +277,7 @@ impl FromStr for PreviewFeature {
             "publish-require-normalized" => Self::PublishRequireNormalized,
             "audit" => Self::Audit,
             "project-directory-must-exist" => Self::ProjectDirectoryMustExist,
+            "reflink-windows" => Self::ReflinkWindows,
             _ => return Err(PreviewFeatureParseError),
         })
     }
@@ -524,6 +527,7 @@ mod tests {
             PreviewFeature::ProjectDirectoryMustExist.as_str(),
             "project-directory-must-exist"
         );
+        assert_eq!(PreviewFeature::ReflinkWindows.as_str(), "reflink-windows");
     }
 
     #[test]

crates/uv/src/commands/project/add.rs

@@ -449,7 +449,7 @@ pub(crate) async fn add(
                 build_isolation,
                 &extra_build_requires,
                 &extra_build_variables,
-                settings.resolver.link_mode,
+                crate::effective_link_mode(settings.resolver.link_mode, preview),
                 &settings.resolver.build_options,
                 &build_hasher,
                 settings.resolver.exclude_newer.clone(),

crates/uv/src/commands/project/mod.rs

@@ -2237,6 +2237,7 @@ pub(crate) async fn sync_environment(
         build_options,
         sources,
     } = settings;
+    let link_mode = crate::effective_link_mode(link_mode, preview);
 
     let client_builder = client_builder.clone().keyring(keyring_provider);
 

crates/uv/src/commands/project/sync.rs

@@ -647,6 +647,7 @@ pub(super) async fn do_sync(
         build_options,
         sources,
     } = settings;
+    let link_mode = crate::effective_link_mode(link_mode, preview);
 
     // Lower the extra build dependencies with source resolution.
     let extra_build_requires = match &target {

crates/uv/src/lib.rs

@@ -37,6 +37,7 @@ use uv_client::BaseClientBuilder;
 use uv_configuration::min_stack_size;
 use uv_flags::EnvironmentFlags;
 use uv_fs::{CWD, Simplified};
+use uv_install_wheel::LinkMode;
 #[cfg(feature = "self-update")]
 use uv_pep440::release_specifiers_to_ranges;
 use uv_pep508::VersionOrUrl;
@@ -69,6 +70,22 @@ pub(crate) mod settings;
 #[cfg(windows)]
 mod windows_exception;
 
+/// Returns the effective [`LinkMode`], applying the `reflink-windows` preview override.
+///
+/// On Windows, the default link mode is [`LinkMode::Hardlink`]. When the `reflink-windows`
+/// preview feature is enabled, this overrides the default to [`LinkMode::Clone`] so that
+/// copy-on-write is attempted on ReFS/Dev Drive volumes.
+pub(crate) fn effective_link_mode(link_mode: LinkMode, preview: uv_preview::Preview) -> LinkMode {
+    if cfg!(target_os = "windows")
+        && link_mode == LinkMode::Hardlink
+        && preview.is_enabled(PreviewFeature::ReflinkWindows)
+    {
+        LinkMode::Clone
+    } else {
+        link_mode
+    }
+}
+
 #[instrument(skip_all)]
 async fn run(mut cli: Cli) -> Result<ExitStatus> {
     // Enable flag to pick up warnings generated by workspace loading.
@@ -737,7 +754,7 @@ async fn run(mut cli: Cli) -> Result<ExitStatus> {
                 args.settings.exclude_newer,
                 args.settings.sources,
                 args.settings.annotation_style,
-                args.settings.link_mode,
+                effective_link_mode(args.settings.link_mode, globals.preview),
                 args.settings.python,
                 args.settings.system,
                 globals.python_preference,
@@ -798,7 +815,7 @@ async fn run(mut cli: Cli) -> Result<ExitStatus> {
                 &args.settings.extras,
                 &groups,
                 args.settings.reinstall,
-                args.settings.link_mode,
+                effective_link_mode(args.settings.link_mode, globals.preview),
                 args.settings.compile_bytecode,
                 args.settings.hash_checking,
                 args.settings.index_locations,
@@ -963,7 +980,7 @@ async fn run(mut cli: Cli) -> Result<ExitStatus> {
                 args.settings.keyring_provider,
                 &client_builder.subcommand(vec!["pip".to_owned(), "install".to_owned()]),
                 args.settings.reinstall,
-                args.settings.link_mode,
+                effective_link_mode(args.settings.link_mode, globals.preview),
                 args.settings.compile_bytecode,
                 args.settings.hash_checking,
                 globals.installer_metadata,
@@ -1301,7 +1318,7 @@ async fn run(mut cli: Cli) -> Result<ExitStatus> {
                 args.settings.install_mirrors,
                 globals.python_preference,
                 globals.python_downloads,
-                args.settings.link_mode,
+                effective_link_mode(args.settings.link_mode, globals.preview),
                 &args.settings.index_locations,
                 args.settings.index_strategy,
                 args.settings.dependency_metadata,