feat: improve PackagePathNotExported error message with condition names

Improves the error message to match enhanced-resolve behavior from
commit f1bc1c2, providing better developer experience when debugging
export resolution failures.

Changes:
- Updated `ResolveError::PackagePathNotExported` to include condition names
- Added `ConditionNames` type with custom Display formatting
- Error message now shows which conditions were checked

Before:
  Package subpath './foo' is not defined by "exports" in /path/to/package.json

After:
  "./foo" is not exported under the conditions ["node", "import"] from package /path/to (see exports field in /path/to/package.json)

Fixes: Cross-reference with enhanced-resolve #460

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: Boshen

src/error.rs

@@ -88,8 +88,13 @@ pub enum ResolveError {
     #[error(r#"Invalid "exports" target "{0}" defined for '{1}' in the package config {2}"#)]
     InvalidPackageTarget(String, String, PathBuf),
 
-    #[error(r#"Package subpath '{0}' is not defined by "exports" in {1}"#)]
-    PackagePathNotExported(String, PathBuf),
+    #[error(r#""{subpath}" is not exported under {conditions} from package {package_path} (see exports field in {package_json_path})"#)]
+    PackagePathNotExported {
+        subpath: String,
+        package_path: PathBuf,
+        package_json_path: PathBuf,
+        conditions: ConditionNames,
+    },
 
     #[error(r#"Invalid package config "{0}", "exports" cannot contain some keys starting with '.' and some not. The exports object must either be an object of package subpath keys or an object of main entry condition name keys only."#)]
     InvalidPackageConfig(PathBuf),
@@ -201,6 +206,31 @@ impl From<Vec<PathBuf>> for CircularPathBufs {
     }
 }
 
+/// Helper type for formatting condition names in error messages
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct ConditionNames(Vec<String>);
+
+impl From<Vec<String>> for ConditionNames {
+    fn from(conditions: Vec<String>) -> Self {
+        Self(conditions)
+    }
+}
+
+impl Display for ConditionNames {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self.0.len() {
+            0 => write!(f, "no conditions"),
+            1 => write!(f, "the condition \"{}\"", self.0[0]),
+            _ => {
+                write!(f, "the conditions ")?;
+                let conditions_str =
+                    self.0.iter().map(|s| format!("\"{s}\"")).collect::<Vec<_>>().join(", ");
+                write!(f, "[{conditions_str}]")
+            }
+        }
+    }
+}
+
 #[test]
 fn test_into_io_error() {
     use std::io::{self, ErrorKind};

src/lib.rs

@@ -1718,10 +1718,12 @@ impl<Fs: FileSystem> ResolverGeneric<Fs> {
             }
         }
         // 4. Throw a Package Path Not Exported error.
-        Err(ResolveError::PackagePathNotExported(
-            subpath.to_string(),
-            package_url.path().join("package.json"),
-        ))
+        Err(ResolveError::PackagePathNotExported {
+            subpath: subpath.to_string(),
+            package_path: package_url.path().to_path_buf(),
+            package_json_path: package_url.path().join("package.json"),
+            conditions: self.options.condition_names.clone().into(),
+        })
     }
 
     /// PACKAGE_IMPORTS_RESOLVE(specifier, parentURL, conditions)
@@ -1977,10 +1979,12 @@ impl<Fs: FileSystem> ResolverGeneric<Fs> {
             // 1. If _target.length is zero, return null.
             if targets.is_empty() {
                 // Note: return PackagePathNotExported has the same effect as return because there are no matches.
-                return Err(ResolveError::PackagePathNotExported(
-                    pattern_match.unwrap_or(".").to_string(),
-                    package_url.path().join("package.json"),
-                ));
+                return Err(ResolveError::PackagePathNotExported {
+                    subpath: pattern_match.unwrap_or(".").to_string(),
+                    package_path: package_url.path().to_path_buf(),
+                    package_json_path: package_url.path().join("package.json"),
+                    conditions: self.options.condition_names.clone().into(),
+                });
             }
             // 2. For each item targetValue in target, do
             for (i, target_value) in targets.iter().enumerate() {

src/tests/exports_field.rs

@@ -71,13 +71,13 @@ fn test_simple() {
         ("relative path should not work with exports field", f.clone(), "./node_modules/exports-field/dist/main.js", ResolveError::NotFound("./node_modules/exports-field/dist/main.js".into())),
         ("backtracking should not work for request", f.clone(), "exports-field/dist/../../../a.js", ResolveError::InvalidPackageTarget("./lib/../../../a.js".to_string(), "./dist/".to_string(), p.clone())),
         ("backtracking should not work for exports field target", f.clone(), "exports-field/dist/a.js", ResolveError::InvalidPackageTarget("./../../a.js".to_string(), "./dist/a.js".to_string(), p.clone())),
-        ("not exported error", f.clone(), "exports-field/anything/else", ResolveError::PackagePathNotExported("./anything/else".to_string(), p.clone())),
-        ("request ending with slash #1", f.clone(), "exports-field/", ResolveError::PackagePathNotExported("./".to_string(), p.clone())),
-        ("request ending with slash #2", f.clone(), "exports-field/dist/", ResolveError::PackagePathNotExported("./dist/".to_string(), p.clone())),
-        ("request ending with slash #3", f.clone(), "exports-field/lib/", ResolveError::PackagePathNotExported("./lib/".to_string(), p)),
+        ("not exported error", f.clone(), "exports-field/anything/else", ResolveError::PackagePathNotExported { subpath: "./anything/else".to_string(), package_path: f.join("node_modules/exports-field"), package_json_path: p.clone(), conditions: vec!["webpack".into()].into() }),
+        ("request ending with slash #1", f.clone(), "exports-field/", ResolveError::PackagePathNotExported { subpath: "./".to_string(), package_path: f.join("node_modules/exports-field"), package_json_path: p.clone(), conditions: vec!["webpack".into()].into() }),
+        ("request ending with slash #2", f.clone(), "exports-field/dist/", ResolveError::PackagePathNotExported { subpath: "./dist/".to_string(), package_path: f.join("node_modules/exports-field"), package_json_path: p.clone(), conditions: vec!["webpack".into()].into() }),
+        ("request ending with slash #3", f.clone(), "exports-field/lib/", ResolveError::PackagePathNotExported { subpath: "./lib/".to_string(), package_path: f.join("node_modules/exports-field"), package_json_path: p, conditions: vec!["webpack".into()].into() }),
         ("should throw error if target is invalid", f4, "exports-field", ResolveError::InvalidPackageTarget("./a/../b/../../pack1/index.js".to_string(), ".".to_string(), p4)),
         ("throw error if exports field is invalid", f.clone(), "invalid-exports-field", ResolveError::InvalidPackageConfig(f.join("node_modules/invalid-exports-field/package.json"))),
-        ("should throw error if target is 'null'", f5, "m/features/internal/file.js", ResolveError::PackagePathNotExported("./features/internal/file.js".to_string(), p5)),
+        ("should throw error if target is 'null'", f5.clone(), "m/features/internal/file.js", ResolveError::PackagePathNotExported { subpath: "./features/internal/file.js".to_string(), package_path: f5.join("node_modules/m"), package_json_path: p5, conditions: vec!["webpack".into()].into() }),
     ];
 
     for (comment, path, request, error) in fail {
@@ -2560,7 +2560,7 @@ fn test_cases() {
         if let Some(expect) = case.expect {
             if expect.is_empty() {
                 assert!(
-                    matches!(resolved_path, Err(ResolveError::PackagePathNotExported(_, _))),
+                    matches!(resolved_path, Err(ResolveError::PackagePathNotExported { .. })),
                     "{} {:?}",
                     &case.name,
                     &resolved_path