dotnet/roslyn
Unnecessary defensive copies in `string` + implicit `object` operator
Aberta
#72.044 aberto em 10 de fev. de 2024
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() => "";
}
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.