Champion: Unmanaged constructed types (16.3, Core 3)

[tannergooding]

Summary

Provide a way to take the address of a generic struct provided it does not contain any "managed" (GC-tracked) types.

For example, the following generic type is "unmanaged":

public struct MyStruct<T> where T : unmanaged
{
    public T field;
}

Motivation

Today, users have the ability to declare structs and take the address of it (in an unsafe context) provided that it is not considered a "managed" type.

• The compiler currently reports generic structs as "managed" types, even though they are not GC-tracked

However, there is nothing in the runtime preventing a user from taking the address of a generic struct and, in certain scenarios, it may be desirable to allow this.

One such example is the System.Runtime.Intrinsics.Vector128<T> type, which contains no GC tracked objects. The type is designed to be used in high-performance and generally unsafe scenarios, but there are certain operations (such as stackalloc, pinning, etc) which cannot be done in C# today.

Design

The C# 6 spec states that an unmanaged_type is:

isn't a reference_type or constructed type, and doesn't contain reference_type or constructed type fields at any level of nesting. In other words, an unmanaged_type is one of the following:

• sbyte, byte, short, ushort, int, uint, long, ulong, char, float, double, decimal, or bool.
• Any enum_type.
• Any pointer_type.
• Any user-defined struct_type that is not a constructed type and contains fields of unmanaged_types only.

This change would remove the restriction that unmanaged_type cannot be a constructed type. Instead, constructed types would be unmanaged if they meet the requirements of general user-defined struct types.

Drawbacks

The compiler may need to do additional validation in order to validate that a user-defined generic struct is "ok" to use.

Notes

The user will still not be able to take the address of any type, which is possible to do in IL code today.

[tannergooding]

CC. @jaredpar, @jkotas, @davidwrighton

[tannergooding]

Users can currently work around this using the System.Runtime.CompilerServices.Unsafe class, which allows converting a ref T to a void*, but this is less than ideal and still excludes some scenarios (such as stackalloc).

[jkotas]

The user will still not be able to take the address of any type, which is possible to do in IL code today.

This is less than ideal as well. Here is an example from our own CoreLib https://github.com/dotnet/coreclr/blob/master/src/System.Private.CoreLib/shared/System/Globalization/CultureData.Windows.cs#L286 :

Interop.Kernel32.EnumSystemLocalesEx(EnumAllSystemLocalesProc, flags, Unsafe.AsPointer(ref context), IntPtr.Zero)

My preference would be to allow taking address of any struct (generic and non-generic), and make taking address of managed structs produce on by default warning that one can suppress. It is fine for this to be a separate proposal if you would like.

[tannergooding]

@jkotas, thanks for the suggestion.

I do think this (allowing you to take the address of a "managed struct") would be better as a separate proposal.

In the example you linked, it looks like it is going a step beyond even "allow taking the address of a managed struct" (which would be a struct containing a GC-tracked type) and is turning it into, effectively, "take the address of anything" (in the example, EnumLocaleData is a class directly)

The current proposal mostly just requires relaxing an existing constraint in the compiler and some minor additional checking on T. While there may be some additional questions and design-work that would need to happen for both the "take the address of a managed struct" and the "take the address of anything" scenarios.

[jkotas]

"take the address of anything" (in the example, EnumLocaleData is a class directly)

That is left-over from times before we had ref locals. It should be a struct - other similar places in CoreLib are structs. I am fixing this in dotnet/coreclr#19193. I agree that "take the address of anything" would be hard to explain. "take the address of a managed struct" would be enough.

[alrz]

Isn't it useful to mark the type as unmanaged to ensure and intend that it's safe to take the address of it later on?

[tannergooding]

If you have a T that you want to take the address of sure. But, there a places (such as in the framework), where we cant use the unmanaged constraint (breaking change, etc) and where you still want to be able to take the address when it can statically be determined that a given struct is "unmanaged" and where we know it will always be unmanaged.

[alrz]

If you have a T that you want to take the address of sure.

I didn't mean the constraint. something like unmanaged struct S { ... }, and not as a requirement, just a piece of information and guarantee when you read the code.

[tannergooding]

I mean, as @jkotas pointed out, there isn't anything at the IL level preventing it and you can get a pointer to such a struct with workarounds today (such as S.R.CompilerServices.Unsafe, or having a ref assembly that doesn't include any fields that are reference types).

I think it might just be better to relax it entirely, but that probably requires more discussion and may be a slightly more controversial change...