librustc_span has a concurrency bug

[kdy1]

librustc_span contains a concurrency bug. Even though, rustc works without any problem.
(I guess rustc process does not use multiples threads to load file to sourcemap.)

rust/src/librustc_span/source_map.rs

Lines 216 to 261 in 0176a9e

	fn try_new_source_file(
	&self,
	filename: FileName,
	src: String,
	) -> Result<Lrc<SourceFile>, OffsetOverflowError> {
	let start_pos = self.next_start_pos();
	// The path is used to determine the directory for loading submodules and
	// include files, so it must be before remapping.
	// Note that filename may not be a valid path, eg it may be `<anon>` etc,
	// but this is okay because the directory determined by `path.pop()` will
	// be empty, so the working directory will be used.
	let unmapped_path = filename.clone();
	let (filename, was_remapped) = match filename {
	FileName::Real(filename) => {
	let (filename, was_remapped) = self.path_mapping.map_prefix(filename);
	(FileName::Real(filename), was_remapped)
	}
	other => (other, false),
	};
	let file_id =
	StableSourceFileId::new_from_pieces(&filename, was_remapped, Some(&unmapped_path));
	let lrc_sf = match self.source_file_by_stable_id(file_id) {
	Some(lrc_sf) => lrc_sf,
	None => {
	let source_file = Lrc::new(SourceFile::new(
	filename,
	was_remapped,
	unmapped_path,
	src,
	Pos::from_usize(start_pos),
	)?);
	let mut files = self.files.borrow_mut();
	files.source_files.push(source_file.clone());
	files.stable_id_to_source_file.insert(file_id, source_file.clone());
	source_file
	}
	};
	Ok(lrc_sf)
	}

When multiple threads invoked this method at a time, it results in overlapping spans.
(Because the lock is released while analyzing the source file)

Meta

Simple fix: https://github.com/swc-project/swc/pull/672/files#diff-8d69cef1163ff9667c130830b15da62bL181-L188

[Mark-Simulacrum]

cc @Zoxc