Allow syncing to empty virtual environment directories (#9427)

As discussed in https://github.com/astral-sh/uv/issues/9423, it's
confusing that we do not allow `uv sync` just because the `.venv`
directory _exists_. This change matches `uv venv`.

crates/uv-python/src/environment.rs

@@ -43,6 +43,7 @@ pub struct InvalidEnvironment {
 #[derive(Debug, Clone)]
 pub enum InvalidEnvironmentKind {
     NotDirectory,
+    Empty,
     MissingExecutable(PathBuf),
 }
 
@@ -128,6 +129,7 @@ impl std::fmt::Display for InvalidEnvironmentKind {
             Self::MissingExecutable(path) => {
                 write!(f, "missing Python executable at `{}`", path.user_display())
             }
+            Self::Empty => write!(f, "directory is empty"),
         }
     }
 }
@@ -178,6 +180,14 @@ impl PythonEnvironment {
             .into());
         }
 
+        if venv.read_dir().is_ok_and(|mut dir| dir.next().is_none()) {
+            return Err(InvalidEnvironment {
+                path: venv,
+                kind: InvalidEnvironmentKind::Empty,
+            }
+            .into());
+        }
+
         let executable = virtualenv_python_executable(&venv);
 
         // Check if the executable exists before querying so we can provide a more specific error

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

@@ -689,6 +689,8 @@ impl ProjectInterpreter {
                             ));
                         }
                     }
+                    // If the environment is an empty directory, it's fine to use
+                    InvalidEnvironmentKind::Empty => {}
                 };
             }
             Err(uv_python::Error::Query(uv_python::InterpreterError::NotFound(path))) => {
@@ -807,10 +809,15 @@ pub(crate) async fn get_or_init_environment(
                 (Ok(false), Ok(false)) => false,
                 // If it's not a virtual environment, bail
                 (Ok(true), Ok(false)) => {
-                    return Err(ProjectError::InvalidProjectEnvironmentDir(
+                    // Unless it's empty, in which case we just ignore it
-                        venv,
+                    if venv.read_dir().is_ok_and(|mut dir| dir.next().is_none()) {
-                        "it is not a compatible environment but cannot be recreated because it is not a virtual environment".to_string(),
+                        false
-                    ));
+                    } else {
+                        return Err(ProjectError::InvalidProjectEnvironmentDir(
+                            venv,
+                            "it is not a compatible environment but cannot be recreated because it is not a virtual environment".to_string(),
+                        ));
+                    }
                 }
                 // Similarly, if we can't _tell_ if it exists we should bail
                 (_, Err(err)) | (Err(err), _) => {

crates/uv/tests/it/sync.rs

@@ -2795,13 +2795,17 @@ fn sync_empty_virtual_environment() -> Result<()> {
 
     // Running `uv sync` should work
     uv_snapshot!(context.filters(), context.sync(), @r###"
-    success: false
+    success: true
-    exit_code: 2
+    exit_code: 0
     ----- stdout -----
 
     ----- stderr -----
     Using CPython 3.12.[X] interpreter at: [PYTHON-3.12]
-    error: Project virtual environment directory `[VENV]/` cannot be used because it is not a compatible environment but cannot be recreated because it is not a virtual environment
+    Creating virtual environment at: .venv
+    Resolved 2 packages in [TIME]
+    Prepared 1 package in [TIME]
+    Installed 1 package in [TIME]
+     + iniconfig==2.0.0
     "###);
 
     Ok(())