fix: improve workspace detection for workspace members

- Enhanced CargoMetadata::new() to preserve manifest path context
- Added comprehensive find_target_package() method that handles both standalone packages and workspace members
- Improved error messages with detailed workspace member information
- Maintains full backward compatibility for standalone packages

Fixes #909

Assisted-by: Claude 4

Author: naoNao89

src/config/manifest.rs

@@ -18,21 +18,23 @@ pub struct CargoMetadata {
 impl CargoMetadata {
     // Create a new instance from the Cargo.toml at the given path.
     pub async fn new(manifest: &Path) -> Result<Self> {
+        let manifest_path = dunce::simplified(manifest).to_path_buf();
         let mut cmd = MetadataCommand::new();
-        cmd.manifest_path(dunce::simplified(manifest));
+        cmd.manifest_path(&manifest_path);
         let metadata = spawn_blocking(move || cmd.exec())
             .await
             .context("error awaiting spawned cargo metadata task")?
             .context("error getting cargo metadata")?;
 
-        Self::from_metadata(metadata)
+        Self::from_metadata_with_manifest_path(metadata, manifest_path)
     }
 
-    pub(crate) fn from_metadata(metadata: Metadata) -> Result<Self> {
-        let package = metadata
-            .root_package()
-            .cloned()
-            .context("could not find the root package of the target crate")?;
+
+
+    /// Create a new instance from metadata with a known manifest path.
+    /// This is the preferred method as it can better handle workspace scenarios.
+    pub(crate) fn from_metadata_with_manifest_path(metadata: Metadata, original_manifest_path: std::path::PathBuf) -> Result<Self> {
+        let package = Self::find_target_package(&metadata, Some(&original_manifest_path))?;
 
         // Get the path to the Cargo.toml manifest.
         let manifest_path = package.manifest_path.to_string();
@@ -43,4 +45,65 @@ impl CargoMetadata {
             manifest_path,
         })
     }
+
+    /// Find the target package from metadata, handling both standalone packages and workspace members.
+    fn find_target_package(metadata: &Metadata, original_manifest_path: Option<&std::path::Path>) -> Result<Package> {
+        // First, try the traditional approach for standalone packages
+        if let Some(package) = metadata.root_package() {
+            return Ok(package.clone());
+        }
+
+        // If no root package exists, we're likely in a workspace.
+        // In this case, we need to find the package that corresponds to the manifest path
+        // that was used to generate this metadata.
+
+        let workspace_packages = metadata.workspace_packages();
+
+        if workspace_packages.is_empty() {
+            anyhow::bail!(
+                "could not find the root package of the target crate: no root package found and no workspace members available"
+            );
+        }
+
+        // If we have the original manifest path, try to find the exact matching package
+        if let Some(original_path) = original_manifest_path {
+            // Canonicalize the original path for comparison
+            let canonical_original = dunce::canonicalize(original_path)
+                .with_context(|| format!("failed to canonicalize manifest path: {}", original_path.display()))?;
+
+            for package in &workspace_packages {
+                // Canonicalize the package's manifest path for comparison
+                if let Ok(canonical_package) = dunce::canonicalize(&package.manifest_path) {
+                    if canonical_original == canonical_package {
+                        return Ok((*package).clone());
+                    }
+                }
+            }
+
+            // If we couldn't find an exact match, provide helpful error information
+            let package_names: Vec<String> = workspace_packages.iter()
+                .map(|p| format!("{} ({})", p.name, p.manifest_path))
+                .collect();
+            anyhow::bail!(
+                "could not find the root package of the target crate: manifest path '{}' does not match any workspace member. \
+                Available workspace members: [{}]",
+                original_path.display(),
+                package_names.join(", ")
+            );
+        }
+
+        // If there's only one workspace package, use it
+        if workspace_packages.len() == 1 {
+            return Ok(workspace_packages[0].clone());
+        }
+
+        // If there are multiple workspace packages and we don't have a specific manifest path,
+        // we can't determine which one to use.
+        let package_names: Vec<&str> = workspace_packages.iter().map(|p| p.name.as_str()).collect();
+        anyhow::bail!(
+            "could not find the root package of the target crate: multiple workspace members found: [{}]. \
+            Consider running trunk from within a specific package directory or using a more specific manifest path.",
+            package_names.join(", ")
+        );
+    }
 }