Avoid un-strict syncing by-default for build isolation (#6606)

## Summary

Closes https://github.com/astral-sh/uv/issues/6580.

Closes https://github.com/astral-sh/uv/issues/6599.

crates/uv-cli/src/lib.rs

@@ -2261,10 +2261,7 @@ pub struct SyncArgs {
     /// Do not remove extraneous packages present in the environment.
     ///
     /// When enabled, uv will make the minimum necessary changes to satisfy the requirements.
-    ///
+    /// By default, syncing will remove any extraneous packages from the environment
-    /// By default, syncing will remove any extraneous packages from the environment, unless
-    /// `--no-build-isolation` is enabled, in which case extra packages are considered necessary for
-    /// builds.
     #[arg(long, overrides_with("exact"), alias = "no-exact")]
     pub inexact: bool,
 

crates/uv/src/settings.rs

@@ -621,6 +621,7 @@ impl PythonPinSettings {
         }
     }
 }
+
 /// The resolved settings to use for a `sync` invocation.
 #[allow(clippy::struct_excessive_bools, dead_code)]
 #[derive(Debug, Clone)]
@@ -668,16 +669,6 @@ impl SyncSettings {
             filesystem,
         );
 
-        let exact = flag(exact, inexact).unwrap_or(true);
-
-        // By default, sync with exact semantics, unless the user set `--no-build-isolation`;
-        // otherwise, we'll end up removing build dependencies.
-        let modifications = if !exact || settings.no_build_isolation {
-            Modifications::Sufficient
-        } else {
-            Modifications::Exact
-        };
-
         Self {
             locked,
             frozen,
@@ -689,7 +680,11 @@ impl SyncSettings {
             no_install_project,
             no_install_workspace,
             no_install_package,
-            modifications,
+            modifications: if flag(exact, inexact).unwrap_or(true) {
+                Modifications::Exact
+            } else {
+                Modifications::Sufficient
+            },
             package,
             python,
             refresh: Refresh::from(refresh),

crates/uv/tests/sync.rs

@@ -428,6 +428,7 @@ fn virtual_workspace_dev_dependencies() -> Result<()> {
     Ok(())
 }
 
+/// Use a `pip install` step to pre-install build dependencies for `--no-build-isolation`.
 #[test]
 fn sync_build_isolation() -> Result<()> {
     let context = TestContext::new("3.12");
@@ -479,9 +480,17 @@ fn sync_build_isolation() -> Result<()> {
     ----- stderr -----
     Resolved 2 packages in [TIME]
     Prepared 2 packages in [TIME]
+    Uninstalled 7 packages in [TIME]
     Installed 2 packages in [TIME]
+     - hatchling==1.22.4
+     - packaging==24.0
+     - pathspec==0.12.1
+     - pluggy==1.4.0
      + project==0.1.0 (from file://[TEMP_DIR]/)
+     - setuptools==69.2.0
      + source-distribution==0.0.1 (from https://files.pythonhosted.org/packages/10/1f/57aa4cce1b1abf6b433106676e15f9fa2c92ed2bd4cf77c3b50a9e9ac773/source_distribution-0.0.1.tar.gz)
+     - trove-classifiers==2024.3.3
+     - wheel==0.43.0
     "###);
 
     assert!(context.temp_dir.child("uv.lock").exists());
@@ -489,6 +498,7 @@ fn sync_build_isolation() -> Result<()> {
     Ok(())
 }
 
+/// Use a `pip install` step to pre-install build dependencies for `--no-build-isolation-package`.
 #[test]
 fn sync_build_isolation_package() -> Result<()> {
     let context = TestContext::new("3.12");
@@ -575,6 +585,118 @@ fn sync_build_isolation_package() -> Result<()> {
     Ok(())
 }
 
+/// Use dedicated extra groups to install dependencies for `--no-build-isolation-package`.
+#[test]
+fn sync_build_isolation_extra() -> 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 = []
+
+        [project.optional-dependencies]
+        build = ["hatchling"]
+        compile = ["source-distribution @ https://files.pythonhosted.org/packages/10/1f/57aa4cce1b1abf6b433106676e15f9fa2c92ed2bd4cf77c3b50a9e9ac773/source_distribution-0.0.1.tar.gz"]
+
+        [build-system]
+        requires = ["setuptools >= 40.9.0"]
+        build-backend = "setuptools.build_meta"
+
+        [tool.uv]
+        no-build-isolation-package = ["source-distribution"]
+        "#,
+    )?;
+
+    // Running `uv sync` should fail for the `compile` extra.
+    let filters = std::iter::once((r"exit code: 1", "exit status: 1"))
+        .chain(context.filters())
+        .collect::<Vec<_>>();
+    uv_snapshot!(&filters, context.sync().arg("--extra").arg("compile"), @r###"
+    success: false
+    exit_code: 2
+    ----- stdout -----
+
+    ----- stderr -----
+    Resolved 7 packages in [TIME]
+    error: Failed to prepare distributions
+      Caused by: Failed to fetch wheel: source-distribution @ https://files.pythonhosted.org/packages/10/1f/57aa4cce1b1abf6b433106676e15f9fa2c92ed2bd4cf77c3b50a9e9ac773/source_distribution-0.0.1.tar.gz
+      Caused by: Build backend failed to build wheel through `build_wheel()` with exit status: 1
+    --- stdout:
+
+    --- stderr:
+    Traceback (most recent call last):
+      File "<string>", line 8, in <module>
+    ModuleNotFoundError: No module named 'hatchling'
+    ---
+    "###);
+
+    // Running `uv sync` with `--all-extras` should also fail.
+    uv_snapshot!(&filters, context.sync().arg("--all-extras"), @r###"
+    success: false
+    exit_code: 2
+    ----- stdout -----
+
+    ----- stderr -----
+    Resolved 7 packages in [TIME]
+    error: Failed to prepare distributions
+      Caused by: Failed to fetch wheel: source-distribution @ https://files.pythonhosted.org/packages/10/1f/57aa4cce1b1abf6b433106676e15f9fa2c92ed2bd4cf77c3b50a9e9ac773/source_distribution-0.0.1.tar.gz
+      Caused by: Build backend failed to build wheel through `build_wheel()` with exit status: 1
+    --- stdout:
+
+    --- stderr:
+    Traceback (most recent call last):
+      File "<string>", line 8, in <module>
+    ModuleNotFoundError: No module named 'hatchling'
+    ---
+    "###);
+
+    // Install the build dependencies.
+    uv_snapshot!(context.filters(), context.sync().arg("--extra").arg("build"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Resolved 7 packages in [TIME]
+    Prepared 6 packages in [TIME]
+    Installed 6 packages in [TIME]
+     + hatchling==1.22.4
+     + packaging==24.0
+     + pathspec==0.12.1
+     + pluggy==1.4.0
+     + project==0.1.0 (from file://[TEMP_DIR]/)
+     + trove-classifiers==2024.3.3
+    "###);
+
+    // Running `uv sync` for the `compile` extra should succeed, and remove the build dependencies.
+    uv_snapshot!(context.filters(), context.sync().arg("--extra").arg("compile"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Resolved 7 packages in [TIME]
+    Prepared 1 package in [TIME]
+    Uninstalled 5 packages in [TIME]
+    Installed 1 package in [TIME]
+     - hatchling==1.22.4
+     - packaging==24.0
+     - pathspec==0.12.1
+     - pluggy==1.4.0
+     + source-distribution==0.0.1 (from https://files.pythonhosted.org/packages/10/1f/57aa4cce1b1abf6b433106676e15f9fa2c92ed2bd4cf77c3b50a9e9ac773/source_distribution-0.0.1.tar.gz)
+     - trove-classifiers==2024.3.3
+    "###);
+
+    assert!(context.temp_dir.child("uv.lock").exists());
+
+    Ok(())
+}
+
 /// Avoid using incompatible versions for build dependencies that are also part of the resolved
 /// environment. This is a very subtle issue, but: when locking, we don't enforce platform
 /// compatibility. So, if we reuse the resolver state to install, and the install itself has to

docs/reference/cli.md

@@ -1098,9 +1098,7 @@ uv sync [OPTIONS]
 
 </dd><dt><code>--inexact</code></dt><dd><p>Do not remove extraneous packages present in the environment.</p>
 
-<p>When enabled, uv will make the minimum necessary changes to satisfy the requirements.</p>
+<p>When enabled, uv will make the minimum necessary changes to satisfy the requirements. By default, syncing will remove any extraneous packages from the environment</p>
-
-<p>By default, syncing will remove any extraneous packages from the environment, unless <code>--no-build-isolation</code> is enabled, in which case extra packages are considered necessary for builds.</p>
 
 </dd><dt><code>--keyring-provider</code> <i>keyring-provider</i></dt><dd><p>Attempt to use <code>keyring</code> for authentication for index URLs.</p>