Exited with hard error when custom build file no existence or not in package

src/cargo/ops/cargo_package.rs

@@ -7,6 +7,7 @@ use std::sync::Arc;
 use std::task::Poll;
 
 use crate::core::compiler::{BuildConfig, CompileMode, DefaultExecutor, Executor};
+use crate::core::manifest::Target;
 use crate::core::resolver::CliFeatures;
 use crate::core::{registry::PackageRegistry, resolver::HasDevUnits};
 use crate::core::{Feature, Shell, Verbosity, Workspace};
@@ -331,6 +332,29 @@ fn build_ar_list(
             warn_on_nonexistent_file(&pkg, &readme_path, "readme", &ws)?;
         }
     }
+
+    for t in pkg
+        .manifest()
+        .targets()
+        .iter()
+        .filter(|t| t.is_custom_build())
+    {
+        if let Some(custome_build_path) = t.src_path().path() {
+            let abs_custome_build_path =
+                paths::normalize_path(&pkg.root().join(custome_build_path));
+            if abs_custome_build_path.is_file() {
+                if !abs_custome_build_path
+                    .ancestors()
+                    .any(|ancestor| ancestor == pkg.root())
+                {
+                    warn_custom_build_file_not_in_package(pkg, &abs_custome_build_path, t, &ws)?
+                }
+            } else {
+                warn_on_nonexistent_file(&pkg, &custome_build_path, "build", &ws)?
+            }
+        }
+    }
+
     result.sort_unstable_by(|a, b| a.rel_path.cmp(&b.rel_path));
 
     Ok(result)
@@ -405,6 +429,23 @@ fn warn_on_nonexistent_file(
     ))
 }
 
+fn warn_custom_build_file_not_in_package(
+    pkg: &Package,
+    path: &Path,
+    target: &Target,
+    ws: &Workspace<'_>,
+) -> CargoResult<()> {
+    let msg = format!(
+        "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.",
+        target.kind(), target.name(), path.display(), pkg.root().display(),  pkg.manifest_path().display()
+    );
+
+    ws.config().shell().warn(&msg)
+}
+
 /// Construct `Cargo.lock` for the package to be published.
 fn build_lock(ws: &Workspace<'_>, orig_pkg: &Package) -> CargoResult<String> {
     let config = ws.config();

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();
+}