Add error message when `uv build` is used with non-packaged projects (#8537)

## Summary

Closes https://github.com/astral-sh/uv/issues/8530.

crates/uv/src/commands/build_frontend.rs

@@ -187,14 +187,19 @@ async fn build_impl(
             }
         };
 
-        let project = workspace
+        let package = workspace
             .packages()
             .get(package)
-            .ok_or_else(|| anyhow::anyhow!("Package `{}` not found in workspace", package))?
-            .root();
+            .ok_or_else(|| anyhow::anyhow!("Package `{package}` not found in workspace"))?;
+
+        if !package.pyproject_toml().is_package() {
+            let name = &package.project().name;
+            let pyproject_toml = package.root().join("pyproject.toml");
+            return Err(anyhow::anyhow!("Package `{}` is missing a `{}`. For example, to build with `{}`, add the following to `{}`:\n```toml\n[build-system]\nrequires = [\"setuptools\"]\nbuild-backend = \"setuptools.build_meta\"\n```", name.cyan(), "build-system".green(), "setuptools".cyan(), pyproject_toml.user_display().cyan()));
+        }
 
         vec![AnnotatedSource::from(Source::Directory(Cow::Borrowed(
-            project,
+            package.root(),
         )))]
     } else if all {
         if matches!(src, Source::File(_)) {
@@ -211,6 +216,10 @@ async fn build_impl(
             }
         };
 
+        if workspace.packages().is_empty() {
+            return Err(anyhow::anyhow!("No packages found in workspace"));
+        }
+
         let packages: Vec<_> = workspace
             .packages()
             .values()
@@ -222,7 +231,10 @@ async fn build_impl(
             .collect();
 
         if packages.is_empty() {
-            return Err(anyhow::anyhow!("No packages found in workspace"));
+            let member = workspace.packages().values().next().unwrap();
+            let name = &member.project().name;
+            let pyproject_toml = member.root().join("pyproject.toml");
+            return Err(anyhow::anyhow!("Workspace does contain any buildable packages. For example, to build `{}` with `{}`, add a `{}` to `{}`:\n```toml\n[build-system]\nrequires = [\"setuptools\"]\nbuild-backend = \"setuptools.build_meta\"\n```", name.cyan(), "setuptools".cyan(), "build-system".green(), pyproject_toml.user_display().cyan()));
         }
 
         packages

crates/uv/tests/it/build.rs

@@ -1985,3 +1985,104 @@ fn git_boundary_in_dist_build() -> Result<()> {
 
     Ok(())
 }
+
+#[test]
+fn build_non_package() -> Result<()> {
+    let context = TestContext::new("3.12");
+    let filters = context
+        .filters()
+        .into_iter()
+        .chain([
+            (r"exit code: 1", "exit status: 1"),
+            (r"bdist\.[^/\\\s]+-[^/\\\s]+", "bdist.linux-x86_64"),
+            (r"\\\.", ""),
+            (r"\[project\]", "[PKG]"),
+            (r"\[member\]", "[PKG]"),
+        ])
+        .collect::<Vec<_>>();
+
+    let project = context.temp_dir.child("project");
+
+    let pyproject_toml = project.child("pyproject.toml");
+    pyproject_toml.write_str(
+        r#"
+        [project]
+        name = "project"
+        version = "0.1.0"
+        requires-python = ">=3.12"
+        dependencies = ["anyio==3.7.0"]
+
+        [tool.uv.workspace]
+        members = ["packages/*"]
+        "#,
+    )?;
+
+    project.child("src").child("__init__.py").touch()?;
+    project.child("README").touch()?;
+
+    let member = project.child("packages").child("member");
+    fs_err::create_dir_all(member.path())?;
+
+    member.child("pyproject.toml").write_str(
+        r#"
+        [project]
+        name = "member"
+        version = "0.1.0"
+        requires-python = ">=3.12"
+        dependencies = ["iniconfig"]
+        "#,
+    )?;
+
+    member.child("src").child("__init__.py").touch()?;
+    member.child("README").touch()?;
+
+    // Build the member.
+    uv_snapshot!(&filters, context.build().arg("--package").arg("member").current_dir(&project), @r###"
+    success: false
+    exit_code: 2
+    ----- stdout -----
+
+    ----- stderr -----
+    error: Package `member` is missing a `build-system`. For example, to build with `setuptools`, add the following to `packages/member/pyproject.toml`:
+    ```toml
+    [build-system]
+    requires = ["setuptools"]
+    build-backend = "setuptools.build_meta"
+    ```
+    "###);
+
+    project
+        .child("dist")
+        .child("member-0.1.0.tar.gz")
+        .assert(predicate::path::missing());
+    project
+        .child("dist")
+        .child("member-0.1.0-py3-none-any.whl")
+        .assert(predicate::path::missing());
+
+    // Build all packages.
+    uv_snapshot!(&filters, context.build().arg("--all").arg("--no-build-logs").current_dir(&project), @r###"
+    success: false
+    exit_code: 2
+    ----- stdout -----
+
+    ----- stderr -----
+    error: Workspace does contain any buildable packages. For example, to build `member` with `setuptools`, add a `build-system` to `packages/member/pyproject.toml`:
+    ```toml
+    [build-system]
+    requires = ["setuptools"]
+    build-backend = "setuptools.build_meta"
+    ```
+    "###);
+
+    project
+        .child("dist")
+        .child("member-0.1.0.tar.gz")
+        .assert(predicate::path::missing());
+    project
+        .child("dist")
+        .child("member-0.1.0-py3-none-any.whl")
+        .assert(predicate::path::missing());
+
+    Ok(())
+}