Analyzer/codefix to transform older GroupJoin/SelectMany pattern to the new .NET 10 LeftJoin

[roji]

.NET 10 is introducing LINQ LeftJoin and RightJoin operators, similar to the Join operator (see dotnet/runtime#110292). The new LeftJoin operator is intended to replace the traditional way of expressing a left join operation, as shown in the LINQ docs:

var query = students
    .GroupJoin(
        departments,
        student => student.DepartmentID,
        department => department.ID,
        (student, departmentList) => new { student, subgroup = departmentList })
    .SelectMany(
        joinedSet => joinedSet.subgroup.DefaultIfEmpty(),
        (student, department) => new
        {
            student.student.FirstName,
            student.student.LastName,
            Department = department.Name
        });

This combination of GroupJoin/SelectMany/DefaultIfEmpty() is both comlex/verbose, and inefficient (see #110292 for some benchmark figures). In .NET 10, the above can be replaced with the following simpler (and more efficient) code:

var query = students.LeftJoin(
    departments,
    s => s.DepartmentID,
    d => d.Id,
    (s, d) => new
    {
        s.FirstName,
        s.LastName,
        DepartmentName = d.Name
    });

We can introduce an analyzer to pattern-match the GroupJoin/SelectMany/DefaultIfEmpty construct, and a codefix to transform it to the below LeftJoin construct.

In addition, dotnet/csharplang#8947 proposes a corresponding C# language change, adding left join (and right join) within LINQ query (comprehension) syntax:

from student in students
left join department in departments on student.DepartmentID equals department.ID
select new { student.Name, Department = department?.Name }

Again, this can replace the older pattern:

var query =
    from student in students
    join department in departments on student.DepartmentID equals department.ID into gj
    from subgroup in gj.DefaultIfEmpty()
    select new
    {
        student.Name,
        Department = subgroup?.Name
    };

Assuming this makes sense, it's something I'd be happy to work on.