Show user-facing warning when falling back to copy installs (#4880)

## Summary

This has come up a few times including in a recent email to me.

Cargo.lock

@@ -1854,6 +1854,7 @@ dependencies = [
  "tracing",
  "uv-fs",
  "uv-normalize",
+ "uv-warnings",
  "walkdir",
  "zip",
 ]

crates/install-wheel-rs/Cargo.toml

@@ -23,9 +23,10 @@ name = "install_wheel_rs"
 distribution-filename = { workspace = true }
 pep440_rs = { workspace = true }
 platform-tags = { workspace = true }
-uv-normalize = { workspace = true }
-uv-fs = { workspace = true }
 pypi-types = { workspace = true }
+uv-fs = { workspace = true }
+uv-normalize = { workspace = true }
+uv-warnings = { workspace = true }
 
 clap = { workspace = true, optional = true, features = ["derive"] }
 configparser = { workspace = true }

crates/install-wheel-rs/src/linker.rs

@@ -16,6 +16,7 @@ use distribution_filename::WheelFilename;
 use pep440_rs::Version;
 use pypi_types::DirectUrl;
 use uv_normalize::PackageName;
+use uv_warnings::warn_user_once;
 
 use crate::script::{scripts_from_ini, Script};
 use crate::wheel::{
@@ -355,7 +356,8 @@ fn clone_recursive(
                             debug!(
                                 "Failed to clone `{}` to temporary location `{}`, attempting to copy files as a fallback",
                                 from.display(),
-                                tempfile.display());
+                                tempfile.display(),
+                            );
                             *attempt = Attempt::UseCopyFallback;
                             fs::copy(&from, &to)?;
                         }
@@ -401,6 +403,7 @@ fn clone_recursive(
             } else {
                 fs::copy(&from, &to)?;
             }
+            warn_user_once!("Failed to clone files; falling back to full copy. This may lead to degraded performance. If this is intentional, use `--link-mode=copy` to suppress this warning.\n\nhint: If the cache and target directories are on different filesystems, reflinking may not be supported.");
         }
     }
 
@@ -524,6 +527,7 @@ fn hardlink_wheel_files(
             }
             Attempt::UseCopyFallback => {
                 fs::copy(path, &out_path)?;
+                warn_user_once!("Failed to hardlink files; falling back to full copy. This may lead to degraded performance. If this is intentional, use `--link-mode=copy` to suppress this warning.\n\nhint: If the cache and target directories are on different filesystems, hardlinking may not be supported.");
             }
         }
 

crates/uv/tests/pip_sync.rs

@@ -5104,6 +5104,10 @@ fn target_no_build_isolation() -> Result<()> {
 
 /// Sync to a `--prefix` directory.
 #[test]
+#[cfg_attr(
+    target_os = "macos",
+    ignore = "On macOS, we fail to reflink due to a non-existent site-packages directory"
+)]
 fn prefix() -> Result<()> {
     let context = TestContext::new("3.12");
 
@@ -5111,10 +5115,12 @@ fn prefix() -> Result<()> {
     let requirements_in = context.temp_dir.child("requirements.in");
     requirements_in.write_str("iniconfig==2.0.0")?;
 
+    let prefix = context.temp_dir.child("prefix");
+
     uv_snapshot!(context.pip_sync()
         .arg("requirements.in")
         .arg("--prefix")
-        .arg("prefix"), @r###"
+        .arg(prefix.path()), @r###"
     success: true
     exit_code: 0
     ----- stdout -----
@@ -5149,7 +5155,7 @@ fn prefix() -> Result<()> {
     uv_snapshot!(context.pip_sync()
         .arg("requirements.in")
         .arg("--prefix")
-        .arg("prefix"), @r###"
+        .arg(prefix.path()), @r###"
     success: true
     exit_code: 0
     ----- stdout -----