Remove special casing for base and default conda environment names (#17758)

Under the `special-conda-env-names` preview feature, instead, just using
our path-based logic.

---------

Co-authored-by: Claude <noreply@anthropic.com>

crates/uv-python/src/discovery.rs

@@ -285,8 +285,9 @@ pub enum Error {
 /// - Discovered virtual environment (e.g. `.venv` in a parent directory)
 ///
 /// Notably, "system" environments are excluded. See [`python_executables_from_installed`].
-fn python_executables_from_virtual_environments<'a>()
--> impl Iterator<Item = Result<(PythonSource, PathBuf), Error>> + 'a {
+fn python_executables_from_virtual_environments<'a>(
+    preview: Preview,
+) -> impl Iterator<Item = Result<(PythonSource, PathBuf), Error>> + 'a {
     let from_active_environment = iter::once_with(|| {
         virtualenv_from_env()
             .into_iter()
@@ -296,8 +297,8 @@ fn python_executables_from_virtual_environments<'a>()
     .flatten();
 
     // N.B. we prefer the conda environment over discovered virtual environments
-    let from_conda_environment = iter::once_with(|| {
-        conda_environment_from_env(CondaEnvironmentKind::Child)
+    let from_conda_environment = iter::once_with(move || {
+        conda_environment_from_env(CondaEnvironmentKind::Child, preview)
             .into_iter()
             .map(virtualenv_python_executable)
             .map(|path| Ok((PythonSource::CondaPrefix, path)))
@@ -524,15 +525,15 @@ fn python_executables<'a>(
     .flatten();
 
     // Check if the base conda environment is active
-    let from_base_conda_environment = iter::once_with(|| {
-        conda_environment_from_env(CondaEnvironmentKind::Base)
+    let from_base_conda_environment = iter::once_with(move || {
+        conda_environment_from_env(CondaEnvironmentKind::Base, preview)
             .into_iter()
             .map(virtualenv_python_executable)
             .map(|path| Ok((PythonSource::BaseCondaPrefix, path)))
     })
     .flatten();
 
-    let from_virtual_environments = python_executables_from_virtual_environments();
+    let from_virtual_environments = python_executables_from_virtual_environments(preview);
     let from_installed =
         python_executables_from_installed(version, implementation, platform, preference, preview);
 

crates/uv-python/src/lib.rs

@@ -136,7 +136,7 @@ mod tests {
     use temp_env::with_vars;
     use test_log::test;
     use uv_client::BaseClientBuilder;
-    use uv_preview::Preview;
+    use uv_preview::{Preview, PreviewFeature};
     use uv_static::EnvVars;
 
     use uv_cache::Cache;
@@ -1335,6 +1335,36 @@ mod tests {
             "We should not allow the base environment when looking for virtual environments"
         );
 
+        // With the `special-conda-env-names` preview feature, "base" is not special-cased
+        // and uses path-based heuristics instead. When the directory name matches the env name,
+        // it should be treated as a child environment.
+        let base_dir = context.tempdir.child("base");
+        TestContext::mock_conda_prefix(&base_dir, "3.12.6")?;
+        let python = context
+            .run_with_vars(
+                &[
+                    (EnvVars::CONDA_PREFIX, Some(base_dir.as_os_str())),
+                    (EnvVars::CONDA_DEFAULT_ENV, Some(&OsString::from("base"))),
+                    (EnvVars::CONDA_ROOT, None),
+                ],
+                || {
+                    find_python_installation(
+                        &PythonRequest::Default,
+                        EnvironmentPreference::OnlyVirtual,
+                        PythonPreference::OnlySystem,
+                        &context.cache,
+                        Preview::new(&[PreviewFeature::SpecialCondaEnvNames]),
+                    )
+                },
+            )?
+            .unwrap();
+
+        assert_eq!(
+            python.interpreter().python_full_version().to_string(),
+            "3.12.6",
+            "With special-conda-env-names preview, 'base' named env in matching dir should be treated as child"
+        );
+
         // When environment name matches directory name, it should be treated as a child environment
         let myenv_dir = context.tempdir.child("myenv");
         TestContext::mock_conda_prefix(&myenv_dir, "3.12.5")?;

crates/uv-python/src/virtualenv.rs

@@ -8,6 +8,7 @@ use std::{
 use fs_err as fs;
 use thiserror::Error;
 
+use uv_preview::{Preview, PreviewFeature};
 use uv_pypi_types::Scheme;
 use uv_static::EnvVars;
 
@@ -85,7 +86,7 @@ impl CondaEnvironmentKind {
     /// name, e.g., `base`, which does not match the `CONDA_PREFIX`, e.g., `/usr/local` instead of
     /// `/usr/local/conda/envs/<name>`. Note the name `CONDA_DEFAULT_ENV` is misleading, it's the
     /// active environment name, not a constant base environment name.
-    fn from_prefix_path(path: &Path) -> Self {
+    fn from_prefix_path(path: &Path, preview: Preview) -> Self {
         // Pixi never creates true "base" envs and names project envs "default", confusing our
         // heuristics, so treat Pixi prefixes as child envs outright.
         if is_pixi_environment(path) {
@@ -113,8 +114,10 @@ impl CondaEnvironmentKind {
         // If the environment name is "base" or "root", treat it as a base environment
         //
         // These are the expected names for the base environment; and is retained for backwards
-        // compatibility, but in a future breaking release we should remove this special-casing.
-        if current_env == "base" || current_env == "root" {
+        // compatibility, but can be removed with the `special-conda-env-names` preview feature.
+        if !preview.is_enabled(PreviewFeature::SpecialCondaEnvNames)
+            && (current_env == "base" || current_env == "root")
+        {
             return Self::Base;
         }
 
@@ -142,11 +145,14 @@ fn is_pixi_environment(path: &Path) -> bool {
 ///
 /// If `base` is true, the active environment must be the base environment or `None` is returned,
 /// and vice-versa.
-pub(crate) fn conda_environment_from_env(kind: CondaEnvironmentKind) -> Option<PathBuf> {
+pub(crate) fn conda_environment_from_env(
+    kind: CondaEnvironmentKind,
+    preview: Preview,
+) -> Option<PathBuf> {
     let dir = env::var_os(EnvVars::CONDA_PREFIX).filter(|value| !value.is_empty())?;
     let path = PathBuf::from(dir);
 
-    if kind != CondaEnvironmentKind::from_prefix_path(&path) {
+    if kind != CondaEnvironmentKind::from_prefix_path(&path, preview) {
         return None;
     }
 
@@ -353,7 +359,7 @@ mod tests {
 
         with_vars(vars, || {
             assert_eq!(
-                CondaEnvironmentKind::from_prefix_path(prefix),
+                CondaEnvironmentKind::from_prefix_path(prefix, Preview::default()),
                 CondaEnvironmentKind::Child
             );
         });