byref and inref for loops

[Happypig375]

I propose we allow this:

let zero (span : System.Span<int>) =
    for item : int byref in span do item <- 0
let useInref (span : System.ReadOnlySpan<int>) =
    for item : int inref in span do System.ReadOnlySpan (&item)

This is the equivalent of C#'s

If the enumerator's Current property returns a reference return value (ref T where T is the type of a collection element), you can declare an iteration variable with the ref or ref readonly modifier

Since byref/inref/outref cannot be an inferred type from type inference, this should not be a breaking change to type inference.

The existing way of approaching this problem in F# is to work around the absence of this feature entirely, for example using integer iterators, which is verbose and not applicable to collections that are not randomly indexable. Alternatively, writing manual while loops with direct invocation of Current and MoveNext which is more error-prone than for loops.

Pros and Cons

The advantages of making this adjustment to F# are

1. C# parity
2. Conciseness
3. Encouraging writing performant code that is safe

The disadvantage of making this adjustment to F# is implementation complexity.

Extra information

Estimated cost (XS, S, M, L, XL, XXL): S

Related suggestions: (put links to related suggestions here)

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• This is not a question (e.g. like one you might ask on StackOverflow) and I have searched StackOverflow for discussions of this issue
• This is a language change and not purely a tooling change (e.g. compiler bug, editor support, warning/error messages, new warning, non-breaking optimisation) belonging to the compiler and tooling repository
• This is not something which has obviously "already been decided" in previous versions of F#. If you're questioning a fundamental design decision that has obviously already been taken (e.g. "Make F# untyped") then please don't submit it
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Please tick all that apply:

• This is not a breaking change to the F# language design
• I or my company would be willing to help implement and/or test this

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[jl0pd]

It would be nice for this feature to enabled for arrays as well. C# for some reason doesn't allow this:

void Foo(int[] ar)
{
    foreach (ref int x in ar) {}
}

But allows this

void Foo(int[] ar)
{
    foreach (ref int x in ar.AsSpan()) {}
}