Fix uv python pin 3.13t failure when parsing version for project re…

…quires check (#8056)

Closes #7964

We can probably do some restructuring to avoid unwrapping here in the
future, but this just fixes the bug quick.

crates/uv-python/src/discovery.rs

@@ -1952,6 +1952,29 @@ impl VersionRequest {
             | Self::Range(_, variant) => variant == &PythonVariant::Freethreaded,
         }
     }
+
+    /// Return a new [`VersionRequest`] with the [`PythonVariant`] if it has one.
+    ///
+    /// This is useful for converting the string representation to pep440.
+    #[must_use]
+    pub fn without_python_variant(self) -> Self {
+        // TODO(zanieb): Replace this entire function with a utility that casts this to a version
+        // without using `VersionRequest::to_string`.
+        match self {
+            Self::Any | Self::Default => self,
+            Self::Major(major, _) => Self::Major(major, PythonVariant::Default),
+            Self::MajorMinor(major, minor, _) => {
+                Self::MajorMinor(major, minor, PythonVariant::Default)
+            }
+            Self::MajorMinorPatch(major, minor, patch, _) => {
+                Self::MajorMinorPatch(major, minor, patch, PythonVariant::Default)
+            }
+            Self::MajorMinorPrerelease(major, minor, prerelease, _) => {
+                Self::MajorMinorPrerelease(major, minor, prerelease, PythonVariant::Default)
+            }
+            Self::Range(specifiers, _) => Self::Range(specifiers, PythonVariant::Default),
+        }
+    }
 }
 
 impl FromStr for VersionRequest {

crates/uv/src/commands/python/pin.rs

@@ -169,8 +169,12 @@ fn pep440_version_from_request(request: &PythonRequest) -> Option<uv_pep440::Ver
         return None;
     }
 
-    // SAFETY: converting `VersionRequest` to `Version` is guaranteed to succeed if not a `Range`.
-    Some(uv_pep440::Version::from_str(&version_request.to_string()).unwrap())
+    // SAFETY: converting `VersionRequest` to `Version` is guaranteed to succeed if not a `Range`
+    // and does not have a Python variant (e.g., freethreaded) attached.
+    Some(
+        uv_pep440::Version::from_str(&version_request.clone().without_python_variant().to_string())
+            .unwrap(),
+    )
 }
 
 /// Check if pinned request is compatible with the workspace/project's `Requires-Python`.

crates/uv/tests/python_pin.rs

@@ -222,7 +222,8 @@ fn python_pin_no_python() {
 
 #[test]
 fn python_pin_compatible_with_requires_python() -> anyhow::Result<()> {
-    let context: TestContext = TestContext::new_with_versions(&["3.10", "3.11"]);
+    let context: TestContext =
+        TestContext::new_with_versions(&["3.10", "3.11"]).with_filtered_python_sources();
     let pyproject_toml = context.temp_dir.child("pyproject.toml");
     pyproject_toml.write_str(
         r#"
@@ -294,12 +295,23 @@ fn python_pin_compatible_with_requires_python() -> anyhow::Result<()> {
     ----- stderr -----
     "###);
 
+    // Request a version that is compatible and uses a Python variant
+    uv_snapshot!(context.filters(), context.python_pin().arg("3.13t"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    Updated `.python-version` from `3.11` -> `3.13t`
+
+    ----- stderr -----
+    warning: No interpreter found for Python 3.13t in [PYTHON SOURCES]
+    "###);
+
     // Request a implementation version that is compatible
     uv_snapshot!(context.filters(), context.python_pin().arg("[email protected]"), @r###"
     success: true
     exit_code: 0
     ----- stdout -----
-    Updated `.python-version` from `3.11` -> `[email protected]`
+    Updated `.python-version` from `3.13t` -> `[email protected]`
 
     ----- stderr -----
     "###);