Add preview warning

crates/uv/src/commands/project/lock.rs

@@ -24,7 +24,7 @@ use uv_distribution_types::{
 use uv_git::ResolvedRepositoryReference;
 use uv_normalize::{GroupName, PackageName};
 use uv_pep440::Version;
-use uv_pypi_types::{Conflicts, SupportedEnvironments};
+use uv_pypi_types::{ConflictKind, Conflicts, SupportedEnvironments};
 use uv_python::{Interpreter, PythonDownloads, PythonEnvironment, PythonPreference, PythonRequest};
 use uv_requirements::ExtrasResolver;
 use uv_requirements::upgrade::{LockedRequirements, read_lock_requirements};
@@ -476,6 +476,21 @@ async fn do_lock(
         }
     }
 
+    // Check if any conflicts contain project-level conflicts
+    if preview.is_disabled() {
+        for set in conflicts.iter() {
+            if set
+                .iter()
+                .any(|item| matches!(item.kind(), ConflictKind::Project))
+            {
+                warn_user_once!(
+                    "Declaring conflicts for packages (`package = ...`) is experimental and may change without warning. Pass `--preview` to disable this warning."
+                );
+                break;
+            }
+        }
+    }
+
     // Collect the list of supported environments.
     let environments = {
         let environments = target.environments();

crates/uv/tests/it/lock.rs

@@ -2947,6 +2947,7 @@ fn lock_conflicting_workspace_members() -> Result<()> {
     ----- stdout -----
 
     ----- stderr -----
+    warning: Declaring conflicts for packages (`package = ...`) is experimental and may change without warning. Pass `--preview` to disable this warning.
     Resolved 4 packages in [TIME]
     ");
 
@@ -3123,6 +3124,7 @@ fn lock_conflicting_workspace_members_depends_direct() -> Result<()> {
     ----- stdout -----
 
     ----- stderr -----
+    warning: Declaring conflicts for packages (`package = ...`) is experimental and may change without warning. Pass `--preview` to disable this warning.
       × No solution found when resolving dependencies:
       ╰─▶ Because subexample depends on sortedcontainers==2.4.0 and example depends on sortedcontainers==2.3.0, we can conclude that example and subexample are incompatible.
           And because example depends on subexample and your workspace requires example, we can conclude that your workspace's requirements are unsatisfiable.
@@ -3202,6 +3204,7 @@ fn lock_conflicting_workspace_members_depends_direct_extra() -> Result<()> {
     ----- stdout -----
 
     ----- stderr -----
+    warning: Declaring conflicts for packages (`package = ...`) is experimental and may change without warning. Pass `--preview` to disable this warning.
     Resolved 4 packages in [TIME]
     ");
 
@@ -3418,6 +3421,7 @@ fn lock_conflicting_workspace_members_depends_transitive() -> Result<()> {
     ----- stdout -----
 
     ----- stderr -----
+    warning: Declaring conflicts for packages (`package = ...`) is experimental and may change without warning. Pass `--preview` to disable this warning.
       × No solution found when resolving dependencies:
       ╰─▶ Because subexample depends on sortedcontainers==2.4.0 and indirection depends on subexample, we can conclude that indirection depends on sortedcontainers==2.4.0.
           And because example depends on sortedcontainers==2.3.0, we can conclude that example and indirection are incompatible.
@@ -3518,6 +3522,7 @@ fn lock_conflicting_workspace_members_depends_transitive_extra() -> Result<()> {
     ----- stdout -----
 
     ----- stderr -----
+    warning: Declaring conflicts for packages (`package = ...`) is experimental and may change without warning. Pass `--preview` to disable this warning.
     Resolved 5 packages in [TIME]
     ");