Enable projects to opt-out of workspace management (#4565)

## Summary

You can now add `managed = false` under `[tool.uv]` in a
`pyproject.toml` to explicitly opt out of the project and workspace
APIs.

If a project sets `managed = false`, we will (1) _not_ discover it as a
workspace root, and (2) _not_ discover it as a workspace member (similar
to using `exclude` in the workspace parent).

Closes https://github.com/astral-sh/uv/issues/4551.

crates/uv-distribution/src/pyproject.rs

@@ -74,7 +74,11 @@ pub struct Tool {
 #[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
 pub struct ToolUv {
     pub sources: Option<BTreeMap<PackageName, Source>>,
+    /// The workspace definition for the project, if any.
     pub workspace: Option<ToolUvWorkspace>,
+    /// Whether the project is managed by `uv`. If `false`, `uv` will ignore the project when
+    /// `uv run` is invoked.
+    pub managed: Option<bool>,
     #[cfg_attr(
         feature = "schemars",
         schemars(

crates/uv-distribution/src/workspace.rs

@@ -25,6 +25,8 @@ pub enum WorkspaceError {
     MissingProject(PathBuf),
     #[error("No workspace found for: `{}`", _0.simplified_display())]
     MissingWorkspace(PathBuf),
+    #[error("The project is marked as unmanaged: `{}`", _0.simplified_display())]
+    NonWorkspace(PathBuf),
     #[error("pyproject.toml section is declared as dynamic, but must be static: `{0}`")]
     DynamicNotAllowed(&'static str),
     #[error("Failed to find directories for glob: `{0}`")]
@@ -83,6 +85,21 @@ impl Workspace {
             .map_err(WorkspaceError::Normalize)?
             .to_path_buf();
 
+        // Check if the project is explicitly marked as unmanaged.
+        if pyproject_toml
+            .tool
+            .as_ref()
+            .and_then(|tool| tool.uv.as_ref())
+            .and_then(|uv| uv.managed)
+            == Some(false)
+        {
+            debug!(
+                "Project `{}` is marked as unmanaged",
+                project_path.simplified_display()
+            );
+            return Err(WorkspaceError::NonWorkspace(project_path));
+        }
+
         // Check if the current project is also an explicit workspace root.
         let explicit_root = pyproject_toml
             .tool
@@ -326,6 +343,21 @@ impl Workspace {
                 let pyproject_toml = PyProjectToml::from_string(contents)
                     .map_err(|err| WorkspaceError::Toml(pyproject_path, Box::new(err)))?;
 
+                // Check if the current project is explicitly marked as unmanaged.
+                if pyproject_toml
+                    .tool
+                    .as_ref()
+                    .and_then(|tool| tool.uv.as_ref())
+                    .and_then(|uv| uv.managed)
+                    == Some(false)
+                {
+                    debug!(
+                        "Project `{}` is marked as unmanaged; omitting from workspace members",
+                        pyproject_toml.project.as_ref().unwrap().name
+                    );
+                    continue;
+                }
+
                 // Extract the package name.
                 let Some(project) = pyproject_toml.project.clone() else {
                     return Err(WorkspaceError::MissingProject(member_root));
@@ -586,6 +618,18 @@ impl ProjectWorkspace {
             .map_err(WorkspaceError::Normalize)?
             .to_path_buf();
 
+        // Check if workspaces are explicitly disabled for the project.
+        if project_pyproject_toml
+            .tool
+            .as_ref()
+            .and_then(|tool| tool.uv.as_ref())
+            .and_then(|uv| uv.managed)
+            == Some(false)
+        {
+            debug!("Project `{}` is marked as unmanaged", project.name);
+            return Err(WorkspaceError::NonWorkspace(project_path));
+        }
+
         // Check if the current project is also an explicit workspace root.
         let mut workspace = project_pyproject_toml
             .tool

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

@@ -143,6 +143,7 @@ pub(crate) async fn run(
             match VirtualProject::discover(&std::env::current_dir()?, None).await {
                 Ok(project) => Some(project),
                 Err(WorkspaceError::MissingPyprojectToml) => None,
+                Err(WorkspaceError::NonWorkspace(_)) => None,
                 Err(err) => return Err(err.into()),
             }
         };

crates/uv/tests/run.rs

@@ -284,3 +284,34 @@ fn run_script() -> Result<()> {
 
     Ok(())
 }
+
+/// With `managed = false`, we should avoid installing the project itself.
+#[test]
+fn run_managed_false() -> Result<()> {
+    let context = TestContext::new("3.12");
+
+    let pyproject_toml = context.temp_dir.child("pyproject.toml");
+    pyproject_toml.write_str(indoc! { r#"
+        [project]
+        name = "foo"
+        version = "1.0.0"
+        requires-python = ">=3.8"
+        dependencies = ["anyio"]
+
+        [tool.uv]
+        managed = false
+        "#
+    })?;
+
+    uv_snapshot!(context.filters(), context.run().arg("python").arg("--version"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    Python 3.12.[X]
+
+    ----- stderr -----
+    warning: `uv run` is experimental and may change without warning.
+    "###);
+
+    Ok(())
+}

crates/uv/tests/workspace.rs

@@ -167,6 +167,21 @@ fn test_albatross_project_in_excluded() {
     );
 
     context.assert_file(current_dir.join("check_installed_bird_feeder.py"));
+
+    let current_dir = workspaces_dir()
+        .join("albatross-project-in-excluded")
+        .join("packages")
+        .join("seeds");
+    uv_snapshot!(context.filters(), install_workspace(&context, &current_dir), @r###"
+    success: false
+    exit_code: 2
+    ----- stdout -----
+
+    ----- stderr -----
+    error: Failed to download and build: `seeds @ file://[WORKSPACE]/scripts/workspaces/albatross-project-in-excluded/packages/seeds`
+      Caused by: The project is marked as unmanaged: `[WORKSPACE]/scripts/workspaces/albatross-project-in-excluded/packages/seeds`
+    "###
+    );
 }
 
 #[test]