add more info to exports errors in package.json

internal/bundler_tests/bundler_packagejson_test.go

@@ -1362,7 +1362,7 @@ func TestPackageJsonExportsErrorInvalidPackageTarget(t *testing.T) {
 				{ "exports": { ".": "invalid" } }
 			`,
 			"/Users/user/project/node_modules/pkg2/package.json": `
-				{ "exports": { ".": "../pkg3" } }
+				{ "exports": { ".": "./../pkg3" } }
 			`,
 			"/Users/user/project/node_modules/pkg3/package.json": `
 				{ "exports": { ".": "./node_modules/pkg" } }
@@ -1374,13 +1374,13 @@ func TestPackageJsonExportsErrorInvalidPackageTarget(t *testing.T) {
 			AbsOutputFile: "/Users/user/project/out.js",
 		},
 		expectedScanLog: `Users/user/project/src/entry.js: ERROR: Could not resolve "pkg1"
-Users/user/project/node_modules/pkg1/package.json: NOTE: The package target "invalid" is invalid:
+Users/user/project/node_modules/pkg1/package.json: NOTE: The package target "invalid" is invalid because it doesn't start with "./":
 NOTE: You can mark the path "pkg1" as external to exclude it from the bundle, which will remove this error.
 Users/user/project/src/entry.js: ERROR: Could not resolve "pkg2"
-Users/user/project/node_modules/pkg2/package.json: NOTE: The package target "../pkg3" is invalid:
+Users/user/project/node_modules/pkg2/package.json: NOTE: The package target "./../pkg3" is invalid because it contains invalid segment "..":
 NOTE: You can mark the path "pkg2" as external to exclude it from the bundle, which will remove this error.
 Users/user/project/src/entry.js: ERROR: Could not resolve "pkg3"
-Users/user/project/node_modules/pkg3/package.json: NOTE: The package target "./node_modules/pkg" is invalid:
+Users/user/project/node_modules/pkg3/package.json: NOTE: The package target "./node_modules/pkg" is invalid because it contains invalid segment "node_modules":
 NOTE: You can mark the path "pkg3" as external to exclude it from the bundle, which will remove this error.
 `,
 	})
@@ -1853,7 +1853,7 @@ func TestPackageJsonExportsErrorMissingTrailingSlash(t *testing.T) {
 			AbsOutputFile: "/Users/user/project/out.js",
 		},
 		expectedScanLog: `Users/user/project/src/entry.js: ERROR: Could not resolve "pkg1/foo/bar"
-Users/user/project/node_modules/pkg1/package.json: NOTE: The module specifier "./test" is invalid:
+Users/user/project/node_modules/pkg1/package.json: NOTE: The module specifier "./test" is invalid because it doesn't end in "/":
 NOTE: You can mark the path "pkg1/foo/bar" as external to exclude it from the bundle, which will remove this error.
 `,
 	})

internal/resolver/package_json.go

@@ -777,6 +777,10 @@ func (status pjStatus) isUndefined() bool {
 }
 
 type pjDebug struct {
+	// If the status is "pjStatusInvalidPackageTarget" or "pjStatusInvalidModuleSpecifier",
+	// then this is the reason. It always starts with " because".
+	invalidBecause string
+
 	// If the status is "pjStatusUndefinedNoConditionsMatch", this is the set of
 	// conditions that didn't match, in the order that they were found in the file.
 	// This information is used for error messages.
@@ -1018,9 +1022,12 @@ func (r resolverQuery) esmPackageTargetResolve(
 		// does not end with "/", throw an Invalid Module Specifier error.
 		if !pattern && subpath != "" && !strings.HasSuffix(target.strData, "/") {
 			if r.debugLogs != nil {
-				r.debugLogs.addNote(fmt.Sprintf("The target %q is invalid because it doesn't end \"/\"", target.strData))
+				r.debugLogs.addNote(fmt.Sprintf("The target %q is invalid because it doesn't end in \"/\"", target.strData))
+			}
+			return target.strData, pjStatusInvalidModuleSpecifier, pjDebug{
+				token:          target.firstToken,
+				invalidBecause: " because it doesn't end in \"/\"",
 			}
-			return target.strData, pjStatusInvalidModuleSpecifier, pjDebug{token: target.firstToken}
 		}
 
 		// If target does not start with "./", then...
@@ -1042,7 +1049,10 @@ func (r resolverQuery) esmPackageTargetResolve(
 			if r.debugLogs != nil {
 				r.debugLogs.addNote(fmt.Sprintf("The target %q is invalid because it doesn't start with \"./\"", target.strData))
 			}
-			return target.strData, pjStatusInvalidPackageTarget, pjDebug{token: target.firstToken}
+			return target.strData, pjStatusInvalidPackageTarget, pjDebug{
+				token:          target.firstToken,
+				invalidBecause: " because it doesn't start with \"./\"",
+			}
 		}
 
 		// If target split on "/" or "\" contains any ".", ".." or "node_modules"
@@ -1051,7 +1061,10 @@ func (r resolverQuery) esmPackageTargetResolve(
 			if r.debugLogs != nil {
 				r.debugLogs.addNote(fmt.Sprintf("The target %q is invalid because it contains invalid segment %q", target.strData, invalidSegment))
 			}
-			return target.strData, pjStatusInvalidPackageTarget, pjDebug{token: target.firstToken}
+			return target.strData, pjStatusInvalidPackageTarget, pjDebug{
+				token:          target.firstToken,
+				invalidBecause: fmt.Sprintf(" because it contains invalid segment %q", invalidSegment),
+			}
 		}
 
 		// Let resolvedTarget be the URL resolution of the concatenation of packageURL and target.
@@ -1063,7 +1076,10 @@ func (r resolverQuery) esmPackageTargetResolve(
 			if r.debugLogs != nil {
 				r.debugLogs.addNote(fmt.Sprintf("The path %q is invalid because it contains invalid segment %q", subpath, invalidSegment))
 			}
-			return subpath, pjStatusInvalidModuleSpecifier, pjDebug{token: target.firstToken}
+			return subpath, pjStatusInvalidModuleSpecifier, pjDebug{
+				token:          target.firstToken,
+				invalidBecause: fmt.Sprintf(" because it contains invalid segment %q", invalidSegment),
+			}
 		}
 
 		if pattern {

internal/resolver/resolver.go

@@ -2326,14 +2326,14 @@ func (r resolverQuery) finalizeImportsExportsResult(
 	switch status {
 	case pjStatusInvalidModuleSpecifier:
 		r.debugMeta.notes = []logger.MsgData{tracker.MsgData(debug.token,
-			fmt.Sprintf("The module specifier %q is invalid:", resolvedPath))}
+			fmt.Sprintf("The module specifier %q is invalid%s:", resolvedPath, debug.invalidBecause))}
 
 	case pjStatusInvalidPackageConfiguration:
 		r.debugMeta.notes = []logger.MsgData{tracker.MsgData(debug.token,
 			"The package configuration has an invalid value here:")}
 
 	case pjStatusInvalidPackageTarget:
-		why := fmt.Sprintf("The package target %q is invalid:", resolvedPath)
+		why := fmt.Sprintf("The package target %q is invalid%s:", resolvedPath, debug.invalidBecause)
 		if resolvedPath == "" {
 			// "PACKAGE_TARGET_RESOLVE" is specified to throw an "Invalid
 			// Package Target" error for what is actually an invalid package