Use of `static var` constants instead of `static let`

[Måns Bernhardt]

When building code using -warn-concurrency, the compiler will return warnings when using "constants" from Euclid, as they are declared static var instead of static let such as e.g.:

public extension Color {
    static var clear = Color(0, 0)
    static var black = Color(0)
    static var white = Color(1)
    static let gray = Color(0.5)

returns this warning when accessing white:

Reference to static property 'white' is not concurrency-safe because it involves shared mutable state

I suggest updating all constants to use let instead of var.

Related to this, are there any plans to upgrade Euclid for concurrency such as adding Sendable conformances?

[Nick Lockwood]

@mansbernhardt thanks for reporting this. I've fixed that and also added Sendable conformances on the develop branch. I had to add unchecked conformance for the Material handling (which isn't truly thread safe), but I'll consider how that might be improved in future.

[Måns Bernhardt]

Thanks @nicklockwood!