`assertMacroExpansion()` gives different diagnostics than using a macro when attached to multiple variables

[fizker]

Description

This is reproduced with swift-syntax@603.0.0.

When testing macros attached to properties where there are multiple properties on the same var/let, assertMacroExpansion() gives an error before executing the macro. But when using the macro outside of assertMacroExpansion(), the macro is executed similarly to when there is only one property.

---

Testing macro

Using macro outside test

Steps to Reproduce

A minimal project showing the issue is here: https://github.com/fizker/swift-syntax-peer-macro-issue

To summarize:

We start with the following macro:

public struct MyMacro: PeerMacro {
	public static func expansion(
		of node: AttributeSyntax,
		providingPeersOf declaration: some DeclSyntaxProtocol,
		in context: some MacroExpansionContext,
	) throws -> [DeclSyntax] {
		return []
	}
}

We then use the macro

struct Foo {
  @MyMacro
  var a: Int, b: Int
}

If this code is placed in regularly executing Swift, it runs without issues and the return-statement is hit twice, for both a and b. But if it executed through assertMacroExpansion(), a diagnostic is raised stating that peer macro can only be applied to a single variable, and the macro code is never executed.