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ref fields spec updates #6539

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Oct 12, 2022
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ref fields spec updates #6539

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ref fields spec updates
jaredpar Oct 12, 2022
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10 changes: 6 additions & 4 deletions proposals/csharp-11.0/low-level-struct-improvements.md
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Expand Up @@ -156,9 +156,9 @@ Next the rules for ref reassignment need to be adjusted for the presence of `ref

The left operand of the `= ref` operator must be an expression that binds to a ref local variable, a ref parameter (other than `this`), an out parameter, **or a ref field**.

> For a ref reassignment in the form ...
> 1. `x.e1 = ref e2`: where `x` is *safe-to-escape* at least *return only* then `e2` must have *ref-safe-to-escape* at least as large as `x`
> 2. `e1 = ref e2`: where `e1` is a `ref` local or `ref` parameter then `e2` must have a *safe-to-escape* equal to *safe-to-escape* for `e1` and `e2` must have *ref-safe-to-escape* at least as large as *ref-safe-to-escape* of the *ref-safe-to-escape* of `e1`
> For a ref reassignment in the form `e1 = ref e2` both of the following must be true:
> 1. e2 must have ref-safe-to-escape at least as large as the ref-safe-to-escape of e1
> 2. e1 must have the same safe-to-escape as e2
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nit: backticks on e1 and e2
nit: I'm okay to keep a side-note on x.e1 case, since I agree it is a bit tricky how it falls out.

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Yeah formatting didn't copy correctly. Will add the sample.


That means the desired `Span<T>` constructor works without any extra annotation:

Expand Down Expand Up @@ -322,6 +322,8 @@ Any expression or statement which explicitly returns a value from a method or la

Likewise any assignment to an `out` must have a *safe-to-escape* of at least *return only*. This is not a special case though, this just follows from the existing assignment rules.

Note: An expression whose type is not a `ref struct` type always has a *safe-to-return* of *calling method*.

<a name="rules-method-invocation"></a>

The span safety rules for method invocation will be updated in several ways. The first is by recognizing the impact that `scoped` has on arguments. For a given argument `a` that is passed to parameter `p`:
Expand Down Expand Up @@ -1096,7 +1098,7 @@ The features outlined in this document don't need to be implemented in a single

What gets implemented in which release is merely a scoping exercise.

**Decision** Only (1) and (2) will make C# 11.0. The expectation is (3) and (4) are enabled very early in C# 12.0 to enable dogfooding by runtime throughout the .NET 8 cycle
**Decision** Only (1) and (2) made C# 11.0. The rest will be considered in future versions of C#.

## Future Considerations

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