ref-safe-to-escape rule for in parameter

[gafter]

Looking at

https://github.com/dotnet/csharplang/blob/master/proposals/csharp-7.2/span-safety.md#method-invocation

• For each in parameter of the method, if there is a corresponding expression that is an lvalue, its ref-safe-to-escape, otherwise the nearest enclosing scope

This is only correct if the argument has an identity conversion to the type of the parameter. For any other type of conversion, I think it should be "nearest enclosing scope" because a copy will be made.

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Associated WorkItem - 52211

[gafter]

/cc @OmarTawfik

[BillWagner]

Moving to dotnet/csharpstandard

[BillWagner]

tagging @RexJaeschke as this is in an area you're working on now for C# 7.x

[jskeet]

Whoops - the link from PR 381 should have been to issue 269, not this one...