Split preview mode into separate feature flags (#14823)

I think this would give us better hygiene than a global flag. It makes
it easier for users to opt-in to overlapping features, such as Python
upgrades and Python bin installations and to disable warnings for
preview mode without opting in to a bunch of other features. In general,
I want to reduce the burden for putting something under preview.

The `--preview` and `--no-preview` flags are retained as global
overrides. A new `--preview-features` option is added which accepts
comma separated features or can be passed multiple times, e.g.,
`--preview-features add-bounds,pylock`. There's a `UV_PREVIEW_FEATURES`
environment variable for that option (I'm not sure if we should overload
`UV_PREVIEW`, but could be convinced).

crates/uv-python/src/discovery.rs

@@ -8,7 +8,7 @@ use std::{env, io, iter};
 use std::{path::Path, path::PathBuf, str::FromStr};
 use thiserror::Error;
 use tracing::{debug, instrument, trace};
-use uv_configuration::PreviewMode;
+use uv_configuration::Preview;
 use which::{which, which_all};
 
 use uv_cache::Cache;
@@ -335,7 +335,7 @@ fn python_executables_from_installed<'a>(
     implementation: Option<&'a ImplementationName>,
     platform: PlatformRequest,
     preference: PythonPreference,
-    preview: PreviewMode,
+    preview: Preview,
 ) -> Box<dyn Iterator<Item = Result<(PythonSource, PathBuf), Error>> + 'a> {
     let from_managed_installations = iter::once_with(move || {
         ManagedPythonInstallations::from_settings(None)
@@ -485,7 +485,7 @@ fn python_executables<'a>(
     platform: PlatformRequest,
     environments: EnvironmentPreference,
     preference: PythonPreference,
-    preview: PreviewMode,
+    preview: Preview,
 ) -> Box<dyn Iterator<Item = Result<(PythonSource, PathBuf), Error>> + 'a> {
     // Always read from `UV_INTERNAL__PARENT_INTERPRETER` — it could be a system interpreter
     let from_parent_interpreter = iter::once_with(|| {
@@ -705,7 +705,7 @@ fn python_interpreters<'a>(
     environments: EnvironmentPreference,
     preference: PythonPreference,
     cache: &'a Cache,
-    preview: PreviewMode,
+    preview: Preview,
 ) -> impl Iterator<Item = Result<(PythonSource, Interpreter), Error>> + 'a {
     python_interpreters_from_executables(
         // Perform filtering on the discovered executables based on their source. This avoids
@@ -1053,7 +1053,7 @@ pub fn find_python_installations<'a>(
     environments: EnvironmentPreference,
     preference: PythonPreference,
     cache: &'a Cache,
-    preview: PreviewMode,
+    preview: Preview,
 ) -> Box<dyn Iterator<Item = Result<FindPythonResult, Error>> + 'a> {
     let sources = DiscoveryPreferences {
         python_preference: preference,
@@ -1254,7 +1254,7 @@ pub(crate) fn find_python_installation(
     environments: EnvironmentPreference,
     preference: PythonPreference,
     cache: &Cache,
-    preview: PreviewMode,
+    preview: Preview,
 ) -> Result<FindPythonResult, Error> {
     let installations =
         find_python_installations(request, environments, preference, cache, preview);
@@ -1353,7 +1353,7 @@ pub(crate) fn find_best_python_installation(
     environments: EnvironmentPreference,
     preference: PythonPreference,
     cache: &Cache,
-    preview: PreviewMode,
+    preview: Preview,
 ) -> Result<FindPythonResult, Error> {
     debug!("Starting Python discovery for {}", request);