First-Class Span Types

• Proposed
• Prototype: Not Started
• Implementation: test plan
• Specification: link

Summary

We introduce first-class support for Span<T> and ReadOnlySpan<T> in the language, including new implicit conversion types and consider them in more places,
allowing more natural programming with these integral types.

Motivation

Since their introduction in C# 7.2, Span<T> and ReadOnlySpan<T> have worked their way into the language and base class library (BCL) in many key ways. This is great for
developers, as their introduction improves performance without costing developer safety. However, the language has held these types at arm's length in a few key ways,
which makes it hard to express the intent of APIs and leads to a significant amount of surface area duplication for new APIs. For example, the BCL has added a number of new
tensor primitive APIs in .NET 9, but these APIs are all offered on ReadOnlySpan<T>. Because C# doesn't recognize the
relationship between ReadOnlySpan<T>, Span<T>, and T[], it means that any developers looking to use those APIs with anything other than a ReadOnlySpan<T> have to explicitly
convert to a ReadOnlySpan<T>. Further, it also means that they don't have IDE tooling guiding them to use these APIs, since nothing will indicate to the IDE that it is valid
to pass them after conversion. There are also issues with generic inference in these scenarios. In order to provide maximum usability for this style of API, the BCL will have to
define an entire set of Span<T> and T[] overloads, which is a lot of duplicate surface area to maintain for no real gain. This proposal seeks to address the problem by
having the language more directly recognize these types and conversions.

Detailed Design

https://github.com/dotnet/csharplang/blob/main/proposals/first-class-span-types.md

Design meetings

• https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-02-05.md#first-class-span-types
• https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-05-13.md#first-class-span-types-questions
• https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-06-24.md#first-class-spans
• https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-07-15.md#field-keyword
• https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-07-15.md#first-class-spans-open-question
• https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-09-11.md#first-class-span-types