Avoid creating .venv in uv add --frozen and uv add --no-sync` (#8980)

## Summary

Closes https://github.com/astral-sh/uv/issues/8977.

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

@@ -44,7 +44,8 @@ use crate::commands::pip::loggers::{
 use crate::commands::pip::operations::Modifications;
 use crate::commands::project::lock::LockMode;
 use crate::commands::project::{
-    init_script_python_requirement, validate_script_requires_python, ProjectError, ScriptPython,
+    init_script_python_requirement, validate_script_requires_python, ProjectError,
+    ProjectInterpreter, ScriptPython,
 };
 use crate::commands::reporters::{PythonDownloadReporter, ResolverReporter};
 use crate::commands::{diagnostics, pip, project, ExitStatus, SharedState};
@@ -214,22 +215,43 @@ pub(crate) async fn add(
             }
         }
 
-        // Discover or create the virtual environment.
-        let venv = project::get_or_init_environment(
-            project.workspace(),
-            python.as_deref().map(PythonRequest::parse),
-            python_preference,
-            python_downloads,
-            connectivity,
-            native_tls,
-            allow_insecure_host,
-            no_config,
-            cache,
-            printer,
-        )
-        .await?;
+        if frozen || no_sync {
+            // Discover the interpreter.
+            let interpreter = ProjectInterpreter::discover(
+                project.workspace(),
+                project_dir,
+                python.as_deref().map(PythonRequest::parse),
+                python_preference,
+                python_downloads,
+                connectivity,
+                native_tls,
+                allow_insecure_host,
+                no_config,
+                cache,
+                printer,
+            )
+            .await?
+            .into_interpreter();
 
-        Target::Project(project, venv)
+            Target::Project(project, Box::new(PythonTarget::Interpreter(interpreter)))
+        } else {
+            // Discover or create the virtual environment.
+            let venv = project::get_or_init_environment(
+                project.workspace(),
+                python.as_deref().map(PythonRequest::parse),
+                python_preference,
+                python_downloads,
+                connectivity,
+                native_tls,
+                allow_insecure_host,
+                no_config,
+                cache,
+                printer,
+            )
+            .await?;
+
+            Target::Project(project, Box::new(PythonTarget::Environment(venv)))
+        }
     };
 
     let client_builder = BaseClientBuilder::new()
@@ -576,8 +598,8 @@ pub(crate) async fn add(
         }
     };
 
-    let (project, venv) = match target {
-        Target::Project(project, venv) => (project, venv),
+    let (project, environment) = match target {
+        Target::Project(project, environment) => (project, environment),
         // If `--script`, exit early. There's no reason to lock and sync.
         Target::Script(script, _) => {
             writeln!(
@@ -627,10 +649,9 @@ pub(crate) async fn add(
         project,
         &mut toml,
         &edits,
-        &venv,
+        &environment,
         state,
         locked,
-        no_sync,
         &dependency_type,
         raw_sources,
         settings.as_ref(),
@@ -688,10 +709,9 @@ async fn lock_and_sync(
     mut project: VirtualProject,
     toml: &mut PyProjectTomlMut,
     edits: &[DependencyEdit],
-    venv: &PythonEnvironment,
+    environment: &PythonTarget,
     state: SharedState,
     locked: bool,
-    no_sync: bool,
     dependency_type: &DependencyType,
     raw_sources: bool,
     settings: ResolverInstallerSettingsRef<'_>,
@@ -704,9 +724,9 @@ async fn lock_and_sync(
     printer: Printer,
 ) -> Result<(), ProjectError> {
     let mode = if locked {
-        LockMode::Locked(venv.interpreter())
+        LockMode::Locked(environment.interpreter())
     } else {
-        LockMode::Write(venv.interpreter())
+        LockMode::Write(environment.interpreter())
     };
 
     let mut lock = project::lock::do_safe_lock(
@@ -846,9 +866,10 @@ async fn lock_and_sync(
         }
     }
 
-    if no_sync {
+    let PythonTarget::Environment(venv) = environment else {
+        // If we're not syncing, exit early.
         return Ok(());
-    }
+    };
 
     // Sync the environment.
     let (extras, dev) = match dependency_type {
@@ -1024,8 +1045,9 @@ fn resolve_requirement(
 enum Target {
     /// A PEP 723 script, with inline metadata.
     Script(Pep723Script, Box<Interpreter>),
+
     /// A project with a `pyproject.toml`.
-    Project(VirtualProject, PythonEnvironment),
+    Project(VirtualProject, Box<PythonTarget>),
 }
 
 impl Target {
@@ -1038,6 +1060,24 @@ impl Target {
     }
 }
 
+/// A Python [`Interpreter`] or [`PythonEnvironment`] for a project.
+#[derive(Debug)]
+#[allow(clippy::large_enum_variant)]
+enum PythonTarget {
+    Interpreter(Interpreter),
+    Environment(PythonEnvironment),
+}
+
+impl PythonTarget {
+    /// Return the [`Interpreter`] for the project.
+    fn interpreter(&self) -> &Interpreter {
+        match self {
+            Self::Interpreter(interpreter) => interpreter,
+            Self::Environment(venv) => venv.interpreter(),
+        }
+    }
+}
+
 #[derive(Debug, Clone)]
 struct DependencyEdit {
     dependency_type: DependencyType,

crates/uv/tests/it/edit.rs

@@ -3675,6 +3675,9 @@ fn add_puts_default_indentation_in_pyproject_toml_if_not_observed() -> Result<()
 fn add_frozen() -> Result<()> {
     let context = TestContext::new("3.12");
 
+    // Remove the virtual environment.
+    fs_err::remove_dir_all(&context.venv)?;
+
     let pyproject_toml = context.temp_dir.child("pyproject.toml");
     pyproject_toml.write_str(indoc! {r#"
         [project]
@@ -3694,6 +3697,7 @@ fn add_frozen() -> Result<()> {
     ----- stdout -----
 
     ----- stderr -----
+    Using CPython 3.12.[X] interpreter at: [PYTHON-3.12]
     "###);
 
     let pyproject_toml = context.read("pyproject.toml");
@@ -3719,6 +3723,7 @@ fn add_frozen() -> Result<()> {
     });
 
     assert!(!context.temp_dir.join("uv.lock").exists());
+    assert!(!context.venv.exists());
 
     Ok(())
 }
@@ -3728,6 +3733,9 @@ fn add_frozen() -> Result<()> {
 fn add_no_sync() -> Result<()> {
     let context = TestContext::new("3.12");
 
+    // Remove the virtual environment.
+    fs_err::remove_dir_all(&context.venv)?;
+
     let pyproject_toml = context.temp_dir.child("pyproject.toml");
     pyproject_toml.write_str(indoc! {r#"
         [project]
@@ -3747,6 +3755,7 @@ fn add_no_sync() -> Result<()> {
     ----- stdout -----
 
     ----- stderr -----
+    Using CPython 3.12.[X] interpreter at: [PYTHON-3.12]
     Resolved 4 packages in [TIME]
     "###);
 
@@ -3773,6 +3782,7 @@ fn add_no_sync() -> Result<()> {
     });
 
     assert!(context.temp_dir.join("uv.lock").exists());
+    assert!(!context.venv.exists());
 
     Ok(())
 }