less.js/lib/less/tree/debug-info.js

var debugInfo = function(context, ctx, lineSeparator) {
    var result = "";
    if (context.dumpLineNumbers && !context.compress) {
        switch(context.dumpLineNumbers) {
            case 'comments':
                result = debugInfo.asComment(ctx);
                break;
            case 'mediaquery':
                result = debugInfo.asMediaQuery(ctx);
                break;
            case 'all':
                result = debugInfo.asComment(ctx) + (lineSeparator || "") + debugInfo.asMediaQuery(ctx);
                break;
        }
    }
    return result;
};

debugInfo.asComment = function(ctx) {
    return '/* line ' + ctx.debugInfo.lineNumber + ', ' + ctx.debugInfo.fileName + ' */\n';
};

debugInfo.asMediaQuery = function(ctx) {
    var filenameWithProtocol = ctx.debugInfo.fileName;
    if (!/^[a-z]+:\/\//i.test(filenameWithProtocol)) {
        filenameWithProtocol = 'file://' + filenameWithProtocol;
    }
    return '@media -sass-debug-info{filename{font-family:' +
        filenameWithProtocol.replace(/([.:\/\\])/g, function (a) {
            if (a == '\\') {
                a = '\/';
            }
            return '\\' + a;
        }) +
        '}line{font-family:\\00003' + ctx.debugInfo.lineNumber + '}}\n';
};

module.exports = debugInfo;