Allow [project] to be missing from a pyproject.toml (#14113)

Closes #8666 
Closes #6838

---------

Co-authored-by: Zanie Blue <[email protected]>

Author: Gankra

crates/uv-distribution/src/metadata/dependency_groups.rs

@@ -77,14 +77,15 @@ impl SourcedDependencyGroups {
                 SourceStrategy::Enabled => MemberDiscovery::default(),
                 SourceStrategy::Disabled => MemberDiscovery::None,
             },
+            ..DiscoveryOptions::default()
         };
 
         // The subsequent API takes an absolute path to the dir the pyproject is in
         let empty = PathBuf::new();
         let absolute_pyproject_path =
             std::path::absolute(pyproject_path).map_err(WorkspaceError::Normalize)?;
         let project_dir = absolute_pyproject_path.parent().unwrap_or(&empty);
-        let project = VirtualProject::discover_defaulted(project_dir, &discovery, cache).await?;
+        let project = VirtualProject::discover(project_dir, &discovery, cache).await?;
 
         // Collect the dependency groups.
         let dependency_groups =

crates/uv-distribution/src/metadata/requires_dist.rs

@@ -61,6 +61,7 @@ impl RequiresDist {
                 SourceStrategy::Enabled => MemberDiscovery::default(),
                 SourceStrategy::Disabled => MemberDiscovery::None,
             },
+            ..DiscoveryOptions::default()
         };
         let Some(project_workspace) =
             ProjectWorkspace::from_maybe_project_root(install_path, &discovery, cache).await?

crates/uv-workspace/src/lib.rs

@@ -1,6 +1,7 @@
 pub use workspace::{
-    DiscoveryOptions, Editability, MemberDiscovery, ProjectWorkspace, RequiresPythonSources,
-    VirtualProject, Workspace, WorkspaceCache, WorkspaceError, WorkspaceMember,
+    DiscoveryOptions, Editability, MemberDiscovery, ProjectDiscovery, ProjectWorkspace,
+    RequiresPythonSources, VirtualProject, Workspace, WorkspaceCache, WorkspaceError,
+    WorkspaceMember,
 };
 
 pub mod dependency_groups;

crates/uv-workspace/src/workspace.rs

@@ -94,12 +94,51 @@ pub enum MemberDiscovery {
     Ignore(BTreeSet<PathBuf>),
 }
 
+/// Whether a "project" must be defined via a `[project]` table.
+#[derive(Debug, Default, Clone, Hash, PartialEq, Eq)]
+pub enum ProjectDiscovery {
+    /// The `[project]` table is optional; when missing, the target is treated as virtual.
+    #[default]
+    Optional,
+    /// A `[project]` table must be defined, unless `[tool.uv.workspace]` is present indicating a
+    /// legacy non-project workspace root.
+    ///
+    /// If neither is defined, discovery will fail.
+    Legacy,
+    /// A `[project]` table must be defined.
+    ///
+    /// If not defined, discovery will fail.
+    Required,
+}
+
+impl ProjectDiscovery {
+    /// Whether a `[project]` table is required.
+    pub fn allows_implicit_workspace(&self) -> bool {
+        match self {
+            Self::Optional => true,
+            Self::Legacy => false,
+            Self::Required => false,
+        }
+    }
+
+    /// Whether a legacy workspace root is allowed.
+    pub fn allows_legacy_workspace(&self) -> bool {
+        match self {
+            Self::Optional => true,
+            Self::Legacy => true,
+            Self::Required => false,
+        }
+    }
+}
+
 #[derive(Debug, Default, Clone, Hash, PartialEq, Eq)]
 pub struct DiscoveryOptions {
     /// The path to stop discovery at.
     pub stop_discovery_at: Option<PathBuf>,
     /// The strategy to use when discovering workspace members.
     pub members: MemberDiscovery,
+    /// The strategy to use when discovering the project.
+    pub project: ProjectDiscovery,
 }
 
 pub type RequiresPythonSources = BTreeMap<(PackageName, Option<GroupName>), VersionSpecifiers>;
@@ -1561,13 +1600,13 @@ fn is_included_in_workspace(
 
 /// A project that can be discovered.
 ///
-/// The project could be a package within a workspace, a real workspace root, or a (legacy)
-/// non-project workspace root, which can define its own dev dependencies.
+/// The project could be a package within a workspace, a real workspace root, or a non-project
+/// workspace root, which can define its own dev dependencies.
 #[derive(Debug, Clone)]
 pub enum VirtualProject {
     /// A project (which could be a workspace root or member).
     Project(ProjectWorkspace),
-    /// A (legacy) non-project workspace root.
+    /// A non-project workspace root.
     NonProject(Workspace),
 }
 
@@ -1583,33 +1622,6 @@ impl VirtualProject {
         path: &Path,
         options: &DiscoveryOptions,
         cache: &WorkspaceCache,
-    ) -> Result<Self, WorkspaceError> {
-        Self::discover_impl(path, options, cache, false).await
-    }
-
-    /// Equivalent to [`VirtualProject::discover`] but consider it acceptable for
-    /// both `[project]` and `[tool.uv.workspace]` to be missing.
-    ///
-    /// If they are, we act as if an empty `[tool.uv.workspace]` was found.
-    pub async fn discover_defaulted(
-        path: &Path,
-        options: &DiscoveryOptions,
-        cache: &WorkspaceCache,
-    ) -> Result<Self, WorkspaceError> {
-        Self::discover_impl(path, options, cache, true).await
-    }
-
-    /// Find the current project or virtual workspace root, given the current directory.
-    ///
-    /// Similar to calling [`ProjectWorkspace::discover`] with a fallback to [`Workspace::discover`],
-    /// but avoids rereading the `pyproject.toml` (and relying on error-handling as control flow).
-    ///
-    /// This method requires an absolute path and panics otherwise.
-    async fn discover_impl(
-        path: &Path,
-        options: &DiscoveryOptions,
-        cache: &WorkspaceCache,
-        default_missing_workspace: bool,
     ) -> Result<Self, WorkspaceError> {
         assert!(
             path.is_absolute(),
@@ -1656,6 +1668,7 @@ impl VirtualProject {
             .as_ref()
             .and_then(|tool| tool.uv.as_ref())
             .and_then(|uv| uv.workspace.as_ref())
+            .filter(|_| options.project.allows_legacy_workspace())
         {
             // Otherwise, if it contains a `tool.uv.workspace` table, it's a non-project workspace
             // root.
@@ -1674,7 +1687,7 @@ impl VirtualProject {
             .await?;
 
             Ok(Self::NonProject(workspace))
-        } else if default_missing_workspace {
+        } else if options.project.allows_implicit_workspace() {
             // Otherwise it's a pyproject.toml that maybe contains dependency-groups
             // that we want to treat like a project/workspace to handle those uniformly
             let project_path = std::path::absolute(project_root)

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

@@ -358,7 +358,10 @@ async fn find_target(
         VirtualProject::Project(
             Workspace::discover(
                 project_dir,
-                &DiscoveryOptions::default(),
+                &DiscoveryOptions {
+                    project: uv_workspace::ProjectDiscovery::Required,
+                    ..DiscoveryOptions::default()
+                },
                 &WorkspaceCache::default(),
             )
             .await
@@ -369,7 +372,10 @@ async fn find_target(
     } else {
         VirtualProject::discover(
             project_dir,
-            &DiscoveryOptions::default(),
+            &DiscoveryOptions {
+                project: uv_workspace::ProjectDiscovery::Required,
+                ..DiscoveryOptions::default()
+            },
             &WorkspaceCache::default(),
         )
         .await