Add warning for invalid custom build file

Author: linyihai

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.\n",
+        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();