Drop prefer prefix from toolchain-preference values (#4602)

I think `--toolchain-preference system` is sufficiently clear and
`--toolchain-preference prefer-system` is excessively verbose. This was
discussed in the original pull request at
https://github.com/astral-sh/uv/pull/4424 but because we had a case for
preferring "installed managed" toolchains I was hesitant to change it.
Now that I've dropped that in #4601, I think we can drop the prefix.

crates/uv-toolchain/src/discovery.rs

@@ -57,16 +57,20 @@ pub enum ToolchainRequest {
 #[cfg_attr(feature = "clap", derive(clap::ValueEnum))]
 #[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
 pub enum ToolchainPreference {
-    /// Only use managed toolchains, never use system toolchains.
+    /// Only use managed toolchains; never use system toolchains.
     OnlyManaged,
     /// Prefer installed toolchains, only download managed toolchains if no system toolchain is found.
+    ///
+    /// Installed managed toolchains are still preferred over system toolchains.
     #[default]
     Installed,
-    /// Prefer managed toolchains over system toolchains, even if one needs to be downloaded.
+    /// Prefer managed toolchains over system toolchains, even if fetching is required.
-    PreferManaged,
+    Managed,
-    /// Prefer system toolchains, only use managed toolchains if no system toolchain is found.
+    /// Prefer system toolchains over managed toolchains.
-    PreferSystem,
+    ///
-    /// Only use system toolchains, never use managed toolchains.
+    /// If a system toolchain cannot be found, a managed toolchain can be used.
+    System,
+    /// Only use system toolchains; never use managed toolchains.
     OnlySystem,
 }
 
@@ -313,12 +317,12 @@ fn python_executables_from_installed<'a>(
 
     match preference {
         ToolchainPreference::OnlyManaged => Box::new(from_managed_toolchains),
-        ToolchainPreference::PreferManaged | ToolchainPreference::Installed => Box::new(
+        ToolchainPreference::Managed | ToolchainPreference::Installed => Box::new(
             from_managed_toolchains
                 .chain(from_search_path)
                 .chain(from_py_launcher),
         ),
-        ToolchainPreference::PreferSystem => Box::new(
+        ToolchainPreference::System => Box::new(
             from_search_path
                 .chain(from_py_launcher)
                 .chain(from_managed_toolchains),
@@ -1153,7 +1157,7 @@ impl ToolchainPreference {
 
         match self {
             ToolchainPreference::OnlyManaged => matches!(source, ToolchainSource::Managed),
-            Self::PreferManaged | Self::PreferSystem | Self::Installed => matches!(
+            Self::Managed | Self::System | Self::Installed => matches!(
                 source,
                 ToolchainSource::Managed
                     | ToolchainSource::SearchPath
@@ -1181,10 +1185,7 @@ impl ToolchainPreference {
     }
 
     pub(crate) fn allows_managed(self) -> bool {
-        matches!(
+        matches!(self, Self::Managed | Self::OnlyManaged | Self::Installed)
-            self,
-            Self::PreferManaged | Self::OnlyManaged | Self::Installed
-        )
     }
 }
 
@@ -1491,7 +1492,7 @@ impl fmt::Display for ToolchainPreference {
     fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
         let s = match self {
             Self::OnlyManaged => "managed toolchains",
-            Self::PreferManaged | Self::Installed | Self::PreferSystem => {
+            Self::Managed | Self::Installed | Self::System => {
                 if cfg!(windows) {
                     "managed toolchains, system path, or `py` launcher"
                 } else {

crates/uv-toolchain/src/toolchain.rs

@@ -88,9 +88,7 @@ impl Toolchain {
         let request = request.unwrap_or_default();
 
         // Perform a fetch aggressively if managed toolchains are preferred
-        if matches!(preference, ToolchainPreference::PreferManaged)
+        if matches!(preference, ToolchainPreference::Managed) && toolchain_fetch.is_automatic() {
-            && toolchain_fetch.is_automatic()
-        {
             if let Some(request) = PythonDownloadRequest::try_from_request(&request) {
                 return Self::fetch(request, client_builder, cache).await;
             }

crates/uv/tests/common/mod.rs

@@ -711,7 +711,7 @@ pub fn python_toolchains_for_versions(
             if let Ok(toolchain) = Toolchain::find(
                 &ToolchainRequest::parse(python_version),
                 EnvironmentPreference::OnlySystem,
-                ToolchainPreference::PreferManaged,
+                ToolchainPreference::Managed,
                 &cache,
             ) {
                 toolchain.into_interpreter().sys_executable().to_owned()

uv.schema.json

@@ -1218,35 +1218,35 @@
     "ToolchainPreference": {
       "oneOf": [
         {
-          "description": "Only use managed toolchains, never use system toolchains.",
+          "description": "Only use managed toolchains; never use system toolchains.",
           "type": "string",
           "enum": [
             "only-managed"
           ]
         },
         {
-          "description": "Prefer installed toolchains, only download managed toolchains if no system toolchain is found.",
+          "description": "Prefer installed toolchains, only download managed toolchains if no system toolchain is found.\n\nInstalled managed toolchains are still preferred over system toolchains.",
           "type": "string",
           "enum": [
             "installed"
           ]
         },
         {
-          "description": "Prefer managed toolchains over system toolchains, even if one needs to be downloaded.",
+          "description": "Prefer managed toolchains over system toolchains, even if fetching is required.",
           "type": "string",
           "enum": [
-            "prefer-managed"
+            "managed"
           ]
         },
         {
-          "description": "Prefer system toolchains, only use managed toolchains if no system toolchain is found.",
+          "description": "Prefer system toolchains over managed toolchains.\n\nIf a system toolchain cannot be found, a managed toolchain can be used.",
           "type": "string",
           "enum": [
-            "prefer-system"
+            "system"
           ]
         },
         {
-          "description": "Only use system toolchains, never use managed toolchains.",
+          "description": "Only use system toolchains; never use managed toolchains.",
           "type": "string",
           "enum": [
             "only-system"