Include newly-added optional dependencies in lockfile (#5686)

## Summary

When we add a new optional group in `uv add`, we never to update the
`pyproject.toml` before locking. Otherwise, we use the stale
`pyproject.toml` and omit the optional group.

Closes https://github.com/astral-sh/uv/issues/5687.

crates/uv-workspace/src/pyproject_mut.rs

@@ -24,6 +24,8 @@ pub enum Error {
     Parse(#[from] Box<TomlError>),
     #[error("Failed to serialize `pyproject.toml`")]
     Serialize(#[from] Box<toml::ser::Error>),
+    #[error("Failed to deserialize `pyproject.toml`")]
+    Deserialize(#[from] Box<toml::de::Error>),
     #[error("Dependencies in `pyproject.toml` are malformed")]
     MalformedDependencies,
     #[error("Sources in `pyproject.toml` are malformed")]
@@ -35,13 +37,18 @@ pub enum Error {
 }
 
 impl PyProjectTomlMut {
-    /// Initialize a `PyProjectTomlMut` from a `PyProjectToml`.
+    /// Initialize a [`PyProjectTomlMut`] from a [`PyProjectToml`].
     pub fn from_toml(pyproject: &PyProjectToml) -> Result<Self, Error> {
         Ok(Self {
             doc: pyproject.raw.parse().map_err(Box::new)?,
         })
     }
 
+    /// Initialize a [`PyProjectToml`] from a [`PyProjectTomlMut`].
+    pub fn to_toml(&self) -> Result<PyProjectToml, Error> {
+        Ok(toml::from_str(&self.doc.to_string()).map_err(Box::new)?)
+    }
+
     /// Adds a project to the workspace.
     pub fn add_workspace(&mut self, path: impl AsRef<Path>) -> Result<(), Error> {
         // Get or create `tool.uv.workspace.members`.

crates/uv-workspace/src/workspace.rs

@@ -197,6 +197,48 @@ impl Workspace {
         })
     }
 
+    /// Set the [`ProjectWorkspace`] for a given workspace member.
+    ///
+    /// Assumes that the project name is unchanged in the updated [`PyProjectToml`].
+    #[must_use]
+    pub fn with_pyproject_toml(
+        self,
+        package_name: &PackageName,
+        pyproject_toml: PyProjectToml,
+    ) -> Option<Self> {
+        let mut packages = self.packages;
+        let member = packages.get_mut(package_name)?;
+
+        if member.root == self.install_path {
+            // If the member is also the workspace root, update _both_ the member entry and the
+            // root `pyproject.toml`.
+            let workspace_pyproject_toml = pyproject_toml.clone();
+
+            // Refresh the workspace sources.
+            let workspace_sources = workspace_pyproject_toml
+                .tool
+                .clone()
+                .and_then(|tool| tool.uv)
+                .and_then(|uv| uv.sources)
+                .unwrap_or_default();
+
+            // Set the `pyproject.toml` for the member.
+            member.pyproject_toml = pyproject_toml;
+
+            Some(Self {
+                pyproject_toml: workspace_pyproject_toml,
+                sources: workspace_sources,
+                packages,
+                ..self
+            })
+        } else {
+            // Set the `pyproject.toml` for the member.
+            member.pyproject_toml = pyproject_toml;
+
+            Some(Self { packages, ..self })
+        }
+    }
+
     /// Returns the set of requirements that include all packages in the workspace.
     pub fn members_as_requirements(&self) -> Vec<Requirement> {
         self.packages
@@ -765,6 +807,19 @@ impl ProjectWorkspace {
         &self.workspace().packages[&self.project_name]
     }
 
+    /// Set the `pyproject.toml` for the current project.
+    ///
+    /// Assumes that the project name is unchanged in the updated [`PyProjectToml`].
+    #[must_use]
+    pub fn with_pyproject_toml(self, pyproject_toml: PyProjectToml) -> Option<Self> {
+        Some(Self {
+            workspace: self
+                .workspace
+                .with_pyproject_toml(&self.project_name, pyproject_toml)?,
+            ..self
+        })
+    }
+
     /// Find the workspace for a project.
     pub async fn from_project(
         install_path: &Path,