Allow use of free-threaded variants in Python 3.14+ without explicit opt-in (#16142)

Closes #15739
Closes #12445

Author: zanieb

crates/uv-python/src/discovery.rs

@@ -1668,9 +1668,19 @@ fn is_windows_store_shim(_path: &Path) -> bool {
 impl PythonVariant {
     fn matches_interpreter(self, interpreter: &Interpreter) -> bool {
         match self {
-            // TODO(zanieb): Right now, we allow debug interpreters to be selected by default for
-            // backwards compatibility, but we may want to change this in the future.
-            Self::Default => !interpreter.gil_disabled(),
+            Self::Default => {
+                // TODO(zanieb): Right now, we allow debug interpreters to be selected by default for
+                // backwards compatibility, but we may want to change this in the future.
+                if (interpreter.python_major(), interpreter.python_minor()) >= (3, 14) {
+                    // For Python 3.14+, the free-threaded build is not considered experimental
+                    // and can satisfy the default variant without opt-in
+                    true
+                } else {
+                    // In Python 3.13 and earlier, the free-threaded build is considered
+                    // experimental and requires explicit opt-in
+                    !interpreter.gil_disabled()
+                }
+            }
             Self::Debug => interpreter.debug_enabled(),
             Self::Freethreaded => interpreter.gil_disabled(),
             Self::FreethreadedDebug => interpreter.gil_disabled() && interpreter.debug_enabled(),

crates/uv/tests/it/python_find.rs

@@ -1,3 +1,4 @@
+use assert_cmd::assert::OutputAssertExt;
 use assert_fs::prelude::{FileTouch, PathChild};
 use assert_fs::{fixture::FileWriteStr, prelude::PathCreateDir};
 use indoc::indoc;
@@ -1256,3 +1257,81 @@ fn python_find_path() {
     error: No interpreter found at path `foobar`
     ");
 }
+
+#[test]
+fn python_find_freethreaded_313() {
+    let context: TestContext = TestContext::new_with_versions(&[])
+        .with_filtered_python_keys()
+        .with_filtered_python_sources()
+        .with_managed_python_dirs()
+        .with_python_download_cache()
+        .with_filtered_python_install_bin()
+        .with_filtered_python_names()
+        .with_filtered_exe_suffix();
+
+    context
+        .python_install()
+        .arg("--preview")
+        .arg("3.13t")
+        .assert()
+        .success();
+
+    // Request Python 3.13 (without opt-in)
+    uv_snapshot!(context.filters(), context.python_find().arg("3.13"), @r"
+    success: false
+    exit_code: 2
+    ----- stdout -----
+
+    ----- stderr -----
+    error: No interpreter found for Python 3.13 in [PYTHON SOURCES]
+    ");
+
+    // Request Python 3.13t (with explicit opt-in)
+    uv_snapshot!(context.filters(), context.python_find().arg("3.13t"), @r"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    [TEMP_DIR]/managed/cpython-3.13+freethreaded-[PLATFORM]/[INSTALL-BIN]/[PYTHON]
+
+    ----- stderr -----
+    ");
+}
+
+#[test]
+fn python_find_freethreaded_314() {
+    let context: TestContext = TestContext::new_with_versions(&[])
+        .with_filtered_python_keys()
+        .with_filtered_python_sources()
+        .with_managed_python_dirs()
+        .with_python_download_cache()
+        .with_filtered_python_install_bin()
+        .with_filtered_python_names()
+        .with_filtered_exe_suffix();
+
+    context
+        .python_install()
+        .arg("--preview")
+        .arg("3.14t")
+        .assert()
+        .success();
+
+    // Request Python 3.14 (without opt-in)
+    uv_snapshot!(context.filters(), context.python_find().arg("3.14"), @r"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    [TEMP_DIR]/managed/cpython-3.14+freethreaded-[PLATFORM]/[INSTALL-BIN]/[PYTHON]
+
+    ----- stderr -----
+    ");
+
+    // Request Python 3.14t (with explicit opt-in)
+    uv_snapshot!(context.filters(), context.python_find().arg("3.14t"), @r"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    [TEMP_DIR]/managed/cpython-3.14+freethreaded-[PLATFORM]/[INSTALL-BIN]/[PYTHON]
+
+    ----- stderr -----
+    ");
+}