Allow uv in PEP 517 build hooks

crates/uv-build-frontend/src/lib.rs

@@ -1229,6 +1229,7 @@ impl PythonRunner {
             .args(["-c", script])
             .current_dir(source_tree.simplified())
             .envs(environment_variables)
+            .env(EnvVars::UV_INTERNAL__BUILD_DIR, source_tree)
             .env(EnvVars::PATH, modified_path)
             .env(EnvVars::VIRTUAL_ENV, venv.root())
             // NOTE: it would be nice to get colored output from build backends,

crates/uv-static/src/env_vars.rs

@@ -658,6 +658,11 @@ impl EnvVars {
     #[attr_added_in("0.2.0")]
     pub const UV_INTERNAL__PARENT_INTERPRETER: &'static str = "UV_INTERNAL__PARENT_INTERPRETER";
 
+    /// Used to identify the source tree when invoking PEP 517 build hooks.
+    #[attr_hidden]
+    #[attr_added_in("0.11.22")]
+    pub const UV_INTERNAL__BUILD_DIR: &'static str = "UV_INTERNAL__BUILD_DIR";
+
     /// Used to force showing the derivation tree during resolver error reporting.
     #[attr_hidden]
     #[attr_added_in("0.3.0")]

crates/uv/src/lib.rs

@@ -538,21 +538,36 @@ async fn run(cli: Cli) -> Result<ExitStatus> {
     // the settings that go into the cache constructor, but the check happens before the first
     // workspace discovery that's used beyond settings discovery.
     let cache_dir = std::path::absolute(cache.root())?;
-    if project_dir.starts_with(&cache_dir) {
-        bail!(
-            "The project directory `{}` is inside the cache directory `{}`",
-            project_dir.user_display(),
-            cache_dir.user_display()
-        );
-    } else if let Ok(cache_dir) = fs_err::canonicalize(&cache_dir)
-        && let Ok(project_dir) = fs_err::canonicalize(&*project_dir)
-        && project_dir.starts_with(&cache_dir)
-    {
-        bail!(
-            "The project directory `{}` is inside the cache directory `{}`",
-            project_dir.user_display(),
-            cache_dir.user_display()
-        );
+    // PEP 517 hooks run from uv-managed source trees, including source distributions extracted
+    // into the cache, and can invoke uv recursively.
+    let project_is_in_build_dir =
+        std::env::var_os(EnvVars::UV_INTERNAL__BUILD_DIR).is_some_and(|build_dir| {
+            std::path::absolute(build_dir).is_ok_and(|build_dir| {
+                project_dir.starts_with(&build_dir)
+                    || fs_err::canonicalize(&*project_dir).is_ok_and(|project_dir| {
+                        fs_err::canonicalize(build_dir)
+                            .is_ok_and(|build_dir| project_dir.starts_with(build_dir))
+                    })
+            })
+        });
+    if !project_is_in_build_dir {
+        if project_dir.starts_with(&cache_dir) {
+            bail!(
+                "The project directory `{}` is inside the cache directory `{}`",
+                project_dir.user_display(),
+                cache_dir.user_display()
+            );
+        }
+        if let Ok(cache_dir) = fs_err::canonicalize(&cache_dir)
+            && let Ok(project_dir) = fs_err::canonicalize(&*project_dir)
+            && project_dir.starts_with(&cache_dir)
+        {
+            bail!(
+                "The project directory `{}` is inside the cache directory `{}`",
+                project_dir.user_display(),
+                cache_dir.user_display()
+            );
+        }
     }
 
     let workspace_cache = WorkspaceCache::default();

crates/uv/tests/build/build.rs

@@ -10,7 +10,7 @@ use std::env::current_dir;
 use std::path::Path;
 use url::Url;
 use uv_static::EnvVars;
-use uv_test::{DEFAULT_PYTHON_VERSION, apply_filters, uv_snapshot};
+use uv_test::{DEFAULT_PYTHON_VERSION, apply_filters, get_bin, uv_snapshot};
 use wiremock::{
     Mock, MockServer, ResponseTemplate,
     matchers::{method, path as url_path},
@@ -148,6 +148,66 @@ fn build_basic() -> Result<()> {
     Ok(())
 }
 
+/// Build hooks can invoke uv while building a wheel from an extracted source distribution.
+/// Regression test for <https://github.com/astral-sh/uv/issues/19878>.
+#[test]
+fn build_hook_invokes_uv() -> Result<()> {
+    let context = uv_test::test_context!("3.12");
+    let project = context.temp_dir.child("project");
+
+    project.child("pyproject.toml").write_str(indoc! {r#"
+        [project]
+        name = "project"
+        version = "0.1.0"
+        requires-python = ">=3.12"
+
+        [build-system]
+        requires = ["hatchling"]
+        build-backend = "hatchling.build"
+
+        [tool.hatch.build.hooks.custom]
+        path = "hatch_build.py"
+    "#})?;
+    project.child("src/project/__init__.py").touch()?;
+    project.child("hatch_build.py").write_str(&formatdoc! {r#"
+        import subprocess
+
+        from hatchling.builders.hooks.plugin.interface import BuildHookInterface
+
+
+        class CustomBuildHook(BuildHookInterface):
+            def initialize(self, version, build_data):
+                subprocess.run(
+                    [
+                        {uv:?},
+                        "export",
+                        "--quiet",
+                        "--no-dev",
+                        "--no-editable",
+                        "--no-emit-project",
+                        "--output-file",
+                        "requirements.txt",
+                    ],
+                    check=True,
+                    cwd=self.root,
+                )
+    "#, uv = get_bin!().display() })?;
+
+    uv_snapshot!(context.filters(), context.build().current_dir(&project), @"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Building source distribution...
+    Building wheel from source distribution...
+    Successfully built dist/project-0.1.0.tar.gz
+    Successfully built dist/project-0.1.0-py3-none-any.whl
+    ");
+
+    Ok(())
+}
+
 /// A source distribution must include an in-tree build backend referenced by `backend-path`.
 /// Regression test for <https://github.com/astral-sh/uv/issues/19771>.
 #[test]