Enable `--dry-run` with `--locked` / `--frozen` for `uv sync` (#12778)

## Summary
Closes #12687. 

<!-- What's the purpose of the change? What does it do, and why? -->

## Test Plan

<!-- How was it tested? -->
Added the corresponding integration tests for:
- `uv sync --dry-run --locked`
  - [x] Preview lock changes
  - [x] Errors if lockfile is out-of-date
- `uv sync --dry-run --frozen`
  - [x] Preview lock changes

---------

Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>

crates/uv-cli/src/lib.rs

@@ -3229,7 +3229,7 @@ pub struct SyncArgs {
     ///
     /// In dry-run mode, uv will resolve the project's dependencies and report on the resulting
     /// changes to both the lockfile and the project environment, but will not modify either.
-    #[arg(long, conflicts_with = "locked", conflicts_with = "frozen")]
+    #[arg(long)]
     pub dry_run: bool,
 
     #[command(flatten)]

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

@@ -333,7 +333,7 @@ impl<'env> LockOperation<'env> {
 
                 // If the lockfile changed, return an error.
                 if matches!(result, LockResult::Changed(_, _)) {
-                    return Err(ProjectError::LockMismatch);
+                    return Err(ProjectError::LockMismatch(Box::new(result.into_lock())));
                 }
 
                 Ok(result)

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

@@ -72,7 +72,7 @@ pub(crate) mod tree;
 #[derive(thiserror::Error, Debug)]
 pub(crate) enum ProjectError {
     #[error("The lockfile at `uv.lock` needs to be updated, but `--locked` was provided. To update the lockfile, run `uv lock`.")]
-    LockMismatch,
+    LockMismatch(Box<Lock>),
 
     #[error(
         "Unable to find lockfile at `uv.lock`. To create a lockfile, run `uv lock` or `uv sync`."

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

@@ -24,7 +24,7 @@ use uv_normalize::{DefaultGroups, PackageName};
 use uv_pep508::{MarkerTree, VersionOrUrl};
 use uv_pypi_types::{ParsedArchiveUrl, ParsedGitUrl, ParsedUrl};
 use uv_python::{PythonDownloads, PythonEnvironment, PythonPreference, PythonRequest};
-use uv_resolver::{FlatIndex, Installable};
+use uv_resolver::{FlatIndex, Installable, Lock};
 use uv_scripts::{Pep723ItemRef, Pep723Script};
 use uv_settings::PythonInstallMirrors;
 use uv_types::{BuildIsolation, HashStrategy};
@@ -327,7 +327,7 @@ pub(crate) async fn sync(
         SyncTarget::Script(script) => LockTarget::from(script),
     };
 
-    let lock = match LockOperation::new(
+    let outcome = match LockOperation::new(
         mode,
         &settings.resolver,
         &network_settings,
@@ -379,68 +379,24 @@ pub(crate) async fn sync(
                     }
                 }
             }
-            result.into_lock()
+            Outcome::Success(result.into_lock())
         }
         Err(ProjectError::Operation(err)) => {
             return diagnostics::OperationDiagnostic::native_tls(network_settings.native_tls)
                 .report(err)
                 .map_or(Ok(ExitStatus::Failure), |err| Err(err.into()))
         }
+        Err(ProjectError::LockMismatch(lock)) if dry_run.enabled() => {
+            // The lockfile is mismatched, but we're in dry-run mode. We should proceed with the
+            // sync operation, but exit with a non-zero status.
+            Outcome::LockMismatch(lock)
+        }
         Err(err) => return Err(err.into()),
     };
 
     // Identify the installation target.
-    let sync_target = match &target {
+    let sync_target =
-        SyncTarget::Project(project) => {
+        identify_installation_target(&target, outcome.lock(), all_packages, package.as_ref());
-            match &project {
-                VirtualProject::Project(project) => {
-                    if all_packages {
-                        InstallTarget::Workspace {
-                            workspace: project.workspace(),
-                            lock: &lock,
-                        }
-                    } else if let Some(package) = package.as_ref() {
-                        InstallTarget::Project {
-                            workspace: project.workspace(),
-                            name: package,
-                            lock: &lock,
-                        }
-                    } else {
-                        // By default, install the root package.
-                        InstallTarget::Project {
-                            workspace: project.workspace(),
-                            name: project.project_name(),
-                            lock: &lock,
-                        }
-                    }
-                }
-                VirtualProject::NonProject(workspace) => {
-                    if all_packages {
-                        InstallTarget::NonProjectWorkspace {
-                            workspace,
-                            lock: &lock,
-                        }
-                    } else if let Some(package) = package.as_ref() {
-                        InstallTarget::Project {
-                            workspace,
-                            name: package,
-                            lock: &lock,
-                        }
-                    } else {
-                        // By default, install the entire workspace.
-                        InstallTarget::NonProjectWorkspace {
-                            workspace,
-                            lock: &lock,
-                        }
-                    }
-                }
-            }
-        }
-        SyncTarget::Script(script) => InstallTarget::Script {
-            script,
-            lock: &lock,
-        },
-    };
 
     let state = state.fork();
 
@@ -475,7 +431,79 @@ pub(crate) async fn sync(
         Err(err) => return Err(err.into()),
     }
 
-    Ok(ExitStatus::Success)
+    match outcome {
+        Outcome::Success(..) => Ok(ExitStatus::Success),
+        Outcome::LockMismatch(lock) => Err(ProjectError::LockMismatch(lock).into()),
+    }
+}
+
+/// The outcome of a `lock` operation within a `sync` operation.
+#[derive(Debug)]
+enum Outcome {
+    /// The `lock` operation was successful.
+    Success(Lock),
+    /// The `lock` operation successfully resolved, but failed due to a mismatch (e.g., with `--locked`).
+    LockMismatch(Box<Lock>),
+}
+
+impl Outcome {
+    /// Return the [`Lock`] associated with this outcome.
+    fn lock(&self) -> &Lock {
+        match self {
+            Self::Success(lock) => lock,
+            Self::LockMismatch(lock) => lock,
+        }
+    }
+}
+
+fn identify_installation_target<'a>(
+    target: &'a SyncTarget,
+    lock: &'a Lock,
+    all_packages: bool,
+    package: Option<&'a PackageName>,
+) -> InstallTarget<'a> {
+    match &target {
+        SyncTarget::Project(project) => {
+            match &project {
+                VirtualProject::Project(project) => {
+                    if all_packages {
+                        InstallTarget::Workspace {
+                            workspace: project.workspace(),
+                            lock,
+                        }
+                    } else if let Some(package) = package {
+                        InstallTarget::Project {
+                            workspace: project.workspace(),
+                            name: package,
+                            lock,
+                        }
+                    } else {
+                        // By default, install the root package.
+                        InstallTarget::Project {
+                            workspace: project.workspace(),
+                            name: project.project_name(),
+                            lock,
+                        }
+                    }
+                }
+                VirtualProject::NonProject(workspace) => {
+                    if all_packages {
+                        InstallTarget::NonProjectWorkspace { workspace, lock }
+                    } else if let Some(package) = package {
+                        InstallTarget::Project {
+                            workspace,
+                            name: package,
+                            lock,
+                        }
+                    } else {
+                        // By default, install the entire workspace.
+                        InstallTarget::NonProjectWorkspace { workspace, lock }
+                    }
+                }
+            }
+        }
+        SyncTarget::Script(script) => InstallTarget::Script { script, lock },
+    }
 }
 
 #[derive(Debug, Clone)]

crates/uv/tests/it/sync.rs

@@ -7896,6 +7896,108 @@ fn sync_dry_run() -> Result<()> {
     Ok(())
 }
 
+#[test]
+fn sync_dry_run_and_locked() -> Result<()> {
+    let context = TestContext::new("3.12");
+
+    let pyproject_toml = context.temp_dir.child("pyproject.toml");
+    pyproject_toml.write_str(
+        r#"
+        [project]
+        name = "project"
+        version = "0.1.0"
+        requires-python = ">=3.12"
+        dependencies = ["anyio==3.7.0"]
+        "#,
+    )?;
+
+    // Lock the initial requirements.
+    context.lock().assert().success();
+
+    let existing = context.read("uv.lock");
+
+    // Update the requirements.
+    pyproject_toml.write_str(
+        r#"
+        [project]
+        name = "project"
+        version = "0.1.0"
+        requires-python = ">=3.12"
+        dependencies = ["iniconfig"]
+        "#,
+    )?;
+
+    // Running with `--locked` and `--dry-run` should error.
+    uv_snapshot!(context.filters(), context.sync().arg("--locked").arg("--dry-run"), @r"
+    success: false
+    exit_code: 2
+    ----- stdout -----
+
+    ----- stderr -----
+    Discovered existing environment at: .venv
+    Resolved 2 packages in [TIME]
+    Would download 1 package
+    Would install 1 package
+     + iniconfig==2.0.0
+    error: The lockfile at `uv.lock` needs to be updated, but `--locked` was provided. To update the lockfile, run `uv lock`.
+    ");
+
+    let updated = context.read("uv.lock");
+
+    // And the lockfile should be unchanged.
+    assert_eq!(existing, updated);
+
+    Ok(())
+}
+
+#[test]
+fn sync_dry_run_and_frozen() -> Result<()> {
+    let context = TestContext::new("3.12");
+
+    let pyproject_toml = context.temp_dir.child("pyproject.toml");
+    pyproject_toml.write_str(
+        r#"
+        [project]
+        name = "project"
+        version = "0.1.0"
+        requires-python = ">=3.12"
+        dependencies = ["anyio==3.7.0"]
+        "#,
+    )?;
+
+    // Lock the initial requirements.
+    context.lock().assert().success();
+
+    // Update the requirements.
+    pyproject_toml.write_str(
+        r#"
+        [project]
+        name = "project"
+        version = "0.1.0"
+        requires-python = ">=3.12"
+        dependencies = ["iniconfig"]
+        "#,
+    )?;
+
+    // Running with `--frozen` with `--dry-run` should preview dependencies to be installed.
+    uv_snapshot!(context.filters(), context.sync().arg("--frozen").arg("--dry-run"), @r"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Discovered existing environment at: .venv
+    Found up-to-date lockfile at: uv.lock
+    Would download 3 packages
+    Would install 3 packages
+     + anyio==3.7.0
+     + idna==3.6
+     + sniffio==1.3.1
+    ");
+
+    Ok(())
+}
+
 #[test]
 fn sync_script() -> Result<()> {
     let context = TestContext::new_with_versions(&["3.8", "3.12"]);