Avoid overwriting symlinks in pip compile output (#6487)

## Summary

Closes https://github.com/astral-sh/uv/issues/6485.

crates/uv/src/commands/pip/compile.rs

@@ -1,3 +1,4 @@
+use std::borrow::Cow;
 use std::env;
 use std::io::stdout;
 use std::path::Path;
@@ -643,6 +644,10 @@ impl<'a> OutputWriter<'a> {
     /// Commit the buffer to the output file.
     async fn commit(self) -> std::io::Result<()> {
         if let Some(output_file) = self.output_file {
+            // If the output file is an existing symlink, write to the destination instead.
+            let output_file = fs_err::read_link(output_file)
+                .map(Cow::Owned)
+                .unwrap_or(Cow::Borrowed(output_file));
             let stream = anstream::adapter::strip_bytes(&self.buffer).into_vec();
             uv_fs::write_atomic(output_file, &stream).await?;
         }

crates/uv/tests/pip_compile.rs

@@ -11907,3 +11907,54 @@ fn invalid_extra() -> Result<()> {
 
     Ok(())
 }
+
+/// Respect symlinks of output files.
+#[test]
+#[cfg(not(windows))]
+fn symlink() -> Result<()> {
+    let context = TestContext::new("3.8");
+    let requirements_in = context.temp_dir.child("requirements.in");
+    requirements_in.write_str("anyio")?;
+
+    // Create an output file.
+    let requirements_txt = context.temp_dir.child("requirements.txt");
+    requirements_txt.write_str("anyio")?;
+
+    // Create a symlink to the output file.
+    let symlink = context.temp_dir.child("requirements-symlink.txt");
+    symlink.symlink_to_file(requirements_txt.path())?;
+
+    // Write to the symlink.
+    uv_snapshot!(context.pip_compile()
+        .arg("requirements.in")
+        .arg("--output-file")
+        .arg("requirements-symlink.txt"), @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 --output-file requirements-symlink.txt
+    anyio==4.3.0
+        # via -r requirements.in
+    exceptiongroup==1.2.0
+        # via anyio
+    idna==3.6
+        # via anyio
+    sniffio==1.3.1
+        # via anyio
+    typing-extensions==4.10.0
+        # via anyio
+
+    ----- stderr -----
+    Resolved 5 packages in [TIME]
+    "###
+    );
+
+    // The symlink should still be a symlink.
+    assert!(symlink.path().symlink_metadata()?.file_type().is_symlink());
+
+    // The destination of the symlink should be the same as the output file.
+    assert_eq!(symlink.path().read_link()?, requirements_txt.path());
+
+    Ok(())
+}