@main shouldn't allow @available

[KeithBauerANZ]

Description

Like the soundness hole with stored properties and @available fixed in Swift 5.9, there's a similar hole with @main:

@main
@available(macOS 99, *)
struct Main {
    static func main() {
        print("oops")
    }
}

(on macOS:)

swiftc -parse-as-library available-main.swift && ./available-main
oops

This can allow code explicitly requiring a newer OS to be run on an older OS.

Either mixing @main with @available should be disallowed, or the availability information should be forwarded to the linker.

Environment

swift-driver version: 1.75.2 Apple Swift version 5.8 (swiftlang-5.8.0.124.2 clang-1403.0.22.11.100)
Target: arm64-apple-macosx13.0

and

swift-driver version: 1.85 Apple Swift version 5.9 (swiftlang-5.9.0.120.7 clang-1500.0.34.3)
Target: arm64-apple-macosx13.0

show the same behavior.