dotnet/roslyn

Unnecessary defensive copies in `string` + implicit `object` operator

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#72,044 opened on Feb 10, 2024

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Area-CompilersBughelp wanted

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Description

When string + something (not a string) is encountered, the compiler converts the something to a string using the ToString method, instead of boxing it. However, it does not currently recognise that defensive copies are unnecessary if the ToString method is marked readonly.

Version Used:

Sharplab.io Also reproduces with the compiler included in .NET SDK 8.0.100

Steps to Reproduce:

using System;
public class C
{
    public static string M1(in S1 s)
    {
        return "" + s; //defensive copy (unnecessary)
    }

    public static string M2(ref S1 s)
    {
        return "" + s; //no defensive copy
    }
}
public struct S1
{
    public readonly override string ToString() => "";
}

Link

Expected Behavior:

s.ToString() is emitted without a defensive copy of s in both cases, since the method is marked as readonly.

Actual Behavior:

s.ToString() is emitted with a defensive copy of s in M1.

Contributor guide

Research direction
Investigate how the compiler lowers string concatenation involving structs with a readonly ToString method and locate where defensive copies are generated. Determine why the readonly modifier is not being checked to suppress the copy.
Tech stack
csharp
Domain
compilers
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
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.
Clear
Prerequisites
C#Git
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
35