Add testcase for custom build file warning

src/cargo/ops/cargo_package.rs

@@ -439,7 +439,7 @@ fn warn_custom_build_file_not_in_package(
         "the source file of {:?} target `{}` doesn't appear to be a path inside of the package.\n\
         It is at {}, whereas the root the package is {}.\n\
         This may cause issue during packaging, as modules resolution and resources included via macros are often relative to the path of source files.\n\
-        Please update the `build` setting in the manifest at `{}` and point to a path inside the root of the package.\n",
+        Please update the `build` setting in the manifest at `{}` and point to a path inside the root of the package.",
         target.kind(), target.name(), path.display(), pkg.root().display(),  pkg.manifest_path().display()
     );
 

tests/testsuite/package.rs

@@ -3304,3 +3304,57 @@ fn init_and_add_inner_target(p: ProjectBuilder) -> ProjectBuilder {
     .file("derp/target/foo.txt", "")
     .file("derp/not_target/foo.txt", "")
 }
+
+#[cargo_test]
+fn custom_build_warning() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+        [package]
+        name = "foo"
+        version = "0.0.1"
+        license = "MIT"
+        description = "foo"
+        authors = []
+        build = "../t_custom_build/custom_build.rs"
+        "#,
+        )
+        .file("src/main.rs", "fn main() {}")
+        .build();
+    p.cargo("package -l")
+        .with_stderr(format!(
+            "\
+warning: manifest has no documentation, homepage or repository.
+See https://doc.rust-lang.org/cargo/reference/manifest.html#package-metadata for more info.
+warning: build `{}/../t_custom_build/custom_build.rs` does not appear to exist.
+Please update the build setting in the manifest at `{}/Cargo.toml`
+This may become a hard error in the future.
+",
+            p.root().display(),
+            p.root().display()
+        ))
+        .run();
+
+    // crate custom_build.rs outside the package root
+    let custom_build_root = p.root().parent().unwrap().join("t_custom_build");
+    _ = fs::create_dir(&custom_build_root).unwrap();
+    _ = fs::write(&custom_build_root.join("custom_build.rs"), "fn main() {}");
+
+    p.cargo("package -l")
+        .with_stderr(format!(
+            "\
+warning: manifest has no documentation, homepage or repository.
+See https://doc.rust-lang.org/cargo/reference/manifest.html#package-metadata for more info.
+warning: the source file of \"custom-build\" target `build-script-custom_build` doesn't appear to be a path inside of the package.
+It is at {}/t_custom_build/custom_build.rs, whereas the root the package is {}.
+This may cause issue during packaging, as modules resolution and resources included via macros are often relative to the path of source files.
+Please update the `build` setting in the manifest at `{}/Cargo.toml` and point to a path inside the root of the package.
+",
+p.root().parent().unwrap().display(),
+            p.root().display(),
+            p.root().display()
+        ))
+        .run();
+    _ = fs::remove_dir_all(&custom_build_root).unwrap();
+}