Improved diagnostics for misplaced AttributeList in variable declaration

[flashspys]

Previously the error messages for this code were quite bad. Now they are great!

struct A {
  var @State name: String
}

Part of this PR are some minor changes:

• The fixit message is shown in the diagnostics formatter
• The SyntaxRewriter now can use a given arena
• Do not print colors if run within Xcode (debatable, feel free to remove)

Best regards from Cocoaheads 😉

[ahoppen]

Thank you, Felix. Just a few minor comments but overall this looks very good to me.

Und viele Grüße aus Cupertino an CocoaHeads Aachen 🎉

[ahoppen]

Actually, I think it would be better if all nodes created by a SyntaxRewriter live in the same SyntaxArena. Creating a SyntaxArena is fairly costly because it creates a new bump pointer allocator and it’s a waste to create a new one for every node. So, what I’m saying is that arena can be non-optional and this could be

Suggested change

	public init(arena: SyntaxArena? = nil) {
	public init(arena: SyntaxArena = SyntaxArena()) {

[flashspys]

I tried that, but unfortunately the tests are failing. The MacroApplication SyntaxRewriter crashes internally with Precondition failed: an arena can't have a new child once it's owned by other arenas

[ahoppen]

@swift-ci Please test

[ahoppen]

@swift-ci Please test

[flashspys]

The SwiftSyntaxBuilder tests were failing, I fixed the part I could. They are still failing on my machine with some really weird error messages, but maybe this is already known?

[ahoppen]

@swift-ci Please test

[ahoppen]

Picking this up. Rebased on main and removed the changes to not print ASCII colors in Xcode terminal as well as displaying a hammer to indicate the presence of a Fix-It.

[ahoppen]

@swift-ci Please test

[ahoppen]

@swift-ci Please test

[ahoppen]

@swift-ci Please test Windows