Add free-threaded Python support

Author: messense

src/build_options.rs

@@ -1244,26 +1244,35 @@ fn find_interpreter_in_sysconfig(
     let mut interpreters = Vec::new();
     for interp in interpreter {
         let python = interp.display().to_string();
-        let (python_impl, python_ver) = if let Some(ver) = python.strip_prefix("pypy") {
-            (InterpreterKind::PyPy, ver.strip_prefix('-').unwrap_or(ver))
-        } else if let Some(ver) = python.strip_prefix("graalpy") {
+        let (python_impl, python_ver, abiflags) = if let Some(ver) = python.strip_prefix("pypy") {
             (
-                InterpreterKind::GraalPy,
+                InterpreterKind::PyPy,
                 ver.strip_prefix('-').unwrap_or(ver),
+                "",
             )
-        } else if let Some(ver) = python.strip_prefix("python") {
+        } else if let Some(ver) = python.strip_prefix("graalpy") {
             (
-                InterpreterKind::CPython,
+                InterpreterKind::GraalPy,
                 ver.strip_prefix('-').unwrap_or(ver),
+                "",
             )
+        } else if let Some(ver) = python.strip_prefix("python") {
+            // Also accept things like `python3.13t` for free-threaded python
+            let (ver, abiflags) =
+                if let Some(ver) = ver.strip_prefix('-').unwrap_or(ver).strip_suffix('t') {
+                    (ver, "t")
+                } else {
+                    (ver, "")
+                };
+            (InterpreterKind::CPython, ver, abiflags)
         } else if python
             .chars()
             .next()
             .map(|c| c.is_ascii_digit())
             .unwrap_or(false)
         {
             // Eg: -i 3.9 without interpreter kind, assume it's CPython
-            (InterpreterKind::CPython, &*python)
+            (InterpreterKind::CPython, &*python, "")
         } else {
             // if interpreter not known
             if std::path::Path::new(&python).is_file() {
@@ -1284,7 +1293,12 @@ fn find_interpreter_in_sysconfig(
         let ver_minor = ver_minor.parse::<usize>().with_context(|| {
             format!("Invalid python interpreter minor version '{ver_minor}', expect a digit")
         })?;
-        let sysconfig = InterpreterConfig::lookup_one(target, python_impl, (ver_major, ver_minor))
+
+        if (ver_major, ver_minor) < (3, 13) && abiflags == "t" {
+            bail!("Free-threaded Python interpreter is only supported on 3.13 and later.");
+        }
+
+        let sysconfig = InterpreterConfig::lookup_one(target, python_impl, (ver_major, ver_minor), abiflags)
             .with_context(|| {
                 format!("Failed to find a {python_impl} {ver_major}.{ver_minor} interpreter in known sysconfig")
             })?;