Add test coverage for #10978 (#11029)

crates/uv/tests/it/pip_compile.rs

@@ -1344,6 +1344,90 @@ fn compile_python_312_annotation_line() -> Result<()> {
     Ok(())
 }
 
+/// Compile for 3.12 when only a different interpreter version is available.
+#[test]
+fn compile_fallback_interpreter() -> Result<()> {
+    let context = TestContext::new("3.10");
+    let requirements_in = context.temp_dir.child("requirements.in");
+    requirements_in.write_str("black==23.10.1")?;
+
+    uv_snapshot!(context.filters(), context.pip_compile()
+            .arg("requirements.in")
+            .arg("--python-version")
+            .arg("3.12"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    # This file was autogenerated by uv via the following command:
+    #    uv pip compile --cache-dir [CACHE_DIR] requirements.in --python-version 3.12
+    black==23.10.[X]
+        # via -r requirements.in
+    click==8.1.7
+        # via black
+    mypy-extensions==1.0.0
+        # via black
+    packaging==24.0
+        # via black
+    pathspec==0.12.1
+        # via black
+    platformdirs==4.2.0
+        # via black
+
+    ----- stderr -----
+    warning: The requested Python version 3.12 is not available; 3.10.[X] will be used to build dependencies instead.
+    Resolved 6 packages in [TIME]
+    "###
+    );
+
+    Ok(())
+}
+
+/// Compile for 3.12 when only a different interpreter version is available, there's also
+/// a broken interpreter in the PATH.
+#[test]
+#[cfg(unix)]
+fn compile_fallback_interpreter_broken_in_path() -> Result<()> {
+    use std::os::unix::fs::PermissionsExt;
+
+    let context = TestContext::new("3.10");
+    let requirements_in = context.temp_dir.child("requirements.in");
+    requirements_in.write_str("black==23.10.1")?;
+
+    // Create a "broken" Python executable in the test context `bin`
+    let contents = r"#!/bin/sh
+    echo 'error: intentionally broken python executable' >&2
+    exit 1";
+    let python = context
+        .bin_dir
+        .join(format!("python3{}", std::env::consts::EXE_SUFFIX));
+    fs_err::write(&python, contents).unwrap();
+
+    let mut perms = fs_err::metadata(&python).unwrap().permissions();
+    perms.set_mode(0o755);
+    fs_err::set_permissions(&python, perms).unwrap();
+
+    uv_snapshot!(context.filters(), context.pip_compile()
+            .arg("requirements.in")
+            .arg("--python-version")
+            .arg("3.12")
+            // In tests, we ignore `PATH` during Python discovery so we need to add the context `bin`
+            .env("UV_TEST_PYTHON_PATH", context.bin_dir.as_os_str()), @r###"
+    success: false
+    exit_code: 2
+    ----- stdout -----
+
+    ----- stderr -----
+    error: Failed to inspect Python interpreter from search path at `[BIN]/python3`
+      Caused by: Querying Python at `[BIN]/python3` failed with exit status exit status: 1
+
+    [stderr]
+    error: intentionally broken python executable
+    "###
+    );
+
+    Ok(())
+}
+
 /// Resolve a specific version of Black at Python 3.12 without deps.
 #[test]
 fn compile_python_312_no_deps() -> Result<()> {