This document lists known breaking changes in Roslyn after .NET 8 all the way to .NET 9.

InlineArray attribute on a record struct type is no longer allowed.

Introduced in Visual Studio 2022 version 17.11

[System.Runtime.CompilerServices.InlineArray(10)] // error CS9259: Attribute 'System.Runtime.CompilerServices.InlineArray' cannot be applied to a record struct.
record struct Buffer1()
{
    private int _element0;
}

[System.Runtime.CompilerServices.InlineArray(10)] // error CS9259: Attribute 'System.Runtime.CompilerServices.InlineArray' cannot be applied to a record struct.
record struct Buffer2(int p1)
{
}

Iterators introduce safe context in C# 13 and newer

Introduced in Visual Studio 2022 version 17.11

Although the language spec states that iterators introduce a safe context, Roslyn does not implement that in C# 12 and lower. This will change in C# 13 as part of a feature which allows unsafe code in iterators. The change does not break normal scenarios as it was disallowed to use unsafe constructs directly in iterators anyway. However, it can break scenarios where an unsafe context was previously inherited into nested local functions, for example:

unsafe class C // unsafe context
{
    System.Collections.Generic.IEnumerable<int> M() // an iterator
    {
        yield return 1;
        local();
        void local()
        {
            int* p = null; // allowed in C# 12; error in C# 13
        }
    }
}

You can work around the break simply by adding the unsafe modifier to the local function.