dotnet/roslyn

Collection expressions can perform covariant conversion of ReadOnlySpan in emit

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#71,106 opened on Dec 5, 2023

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 (6 comments) (1 reaction) (1 assignee)C# (4,257 forks)batch import
Area-CompilersCode Gen QualityFeature - Collection Expressionshelp wanted

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Description

The runtime doesn't provide a safe facility for covariant conversion of ReadOnlySpan, but it is supported to perform such conversions. If the compiler performed such conversions, it would enable more optimizations in scenarios like the following:

Span<Derived> span1 = ..., span2 = ...;
ReadOnlySpan<Base> spanBase1 = [..span1]; // emit as conversion to ReadOnlySpan<Base> then 'ToArray()'
ReadOnlySpan<Base> spanBase2 = [..span1, ..span2]; // emit as conversion then 'CopyTo()' the backing collection of 'spanBase2'

To start with, we are only going to perform ToArray and CopyTo optimizations in the case where the spread operator and the target collection have the same element type.

Contributor guide

Research direction
Investigate how collection expressions are emitted in the C# compiler, specifically the handling of ReadOnlySpan<T> conversions. Look at the existing optimization patterns for ToArray and CopyTo in spread expressions.
Tech stack
csharp
Domain
compilers
Issue type
Refactor
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-2 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Mostly clear
Prerequisites
C#
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
20