Add withPosition instance method to allow reuse of resolved canonical file

See #4630

Author: lslonina

junit-platform-engine/src/main/java/org/junit/platform/engine/support/descriptor/FileSource.java

@@ -10,6 +10,7 @@
 
 package org.junit.platform.engine.support.descriptor;
 
+import static org.apiguardian.api.API.Status.EXPERIMENTAL;
 import static org.apiguardian.api.API.Status.STABLE;
 
 import java.io.File;
@@ -77,6 +78,11 @@ private FileSource(File file, @Nullable FilePosition filePosition) {
 		this.filePosition = filePosition;
 	}
 
+	private FileSource(FileSource fileSource, FilePosition filePosition) {
+		this.file = fileSource.file;
+		this.filePosition = filePosition;
+	}	
+
 	/**
 	 * Get the {@link URI} for the source {@linkplain #getFile file}.
 	 *
@@ -104,6 +110,20 @@ public Optional<FilePosition> getPosition() {
 		return Optional.ofNullable(this.filePosition);
 	}
 
+	/**
+	* Return a new {@code FileSource} based on this instance but with a different
+	* {@link FilePosition}. This avoids redundant canonical path resolution
+	* by reusing the already-canonical file.
+	*
+	* @param filePosition the new {@code FilePosition}; must not be {@code null}
+	* @return a new {@code FileSource} with the same file and updated position
+	*/
+	@API(status = EXPERIMENTAL, since = "6.0")
+	public FileSource withPosition(FilePosition filePosition) {
+		Preconditions.notNull(filePosition, "filePosition must not be null");
+		return new FileSource(this, filePosition);
+	}
+
 	@Override
 	public boolean equals(Object o) {
 		if (this == o) {

platform-tests/src/test/java/org/junit/platform/engine/support/descriptor/FileSystemSourceTests.java

@@ -76,6 +76,22 @@ void fileWithPosition() {
 		assertThat(source.getPosition()).hasValue(position);
 	}
 
+	@Test
+	void fileReuseWithPosition() {
+		var file = new File("test.txt");
+		var position = FilePosition.from(42, 23);
+		var source = FileSource.from(file);
+		var sourceWithPosition = source.withPosition(position);
+
+		assertThat(source.getUri()).isEqualTo(file.getAbsoluteFile().toURI());
+		assertThat(source.getFile()).isEqualTo(file.getAbsoluteFile());
+		assertThat(source.getPosition()).isEmpty();
+
+		assertThat(source).isNotSameAs(sourceWithPosition);
+		assertThat(source.getFile()).isSameAs(sourceWithPosition.getFile());
+		assertThat(sourceWithPosition.getPosition()).hasValue(position);
+	}
+
 	@Test
 	void equalsAndHashCodeForFileSource() {
 		var file1 = new File("foo.txt");