NIO just updated their package manifest to 5.6 and it started erroring when building the tests

[finagolfin]

Description

@tomerd made the change in apple/swift-nio#2235 and that now causes an error on linux and Android when compiling LinuxMain.swift. Workarounds are to revert that pull or add --enable-test-discovery, which avoids compiling LinuxMain.swift.

Expected behavior

The NIO tests to build and run.

Actual behavior

This compilation error:

/home/butta/swift-nio/Tests/LinuxMain.swift:146:1: error: expressions are not allowed at the top level
(LinuxMainRunnerImpl() as LinuxMainRunner).run()

Steps to reproduce

1. Check out swift-nio.
2. Build the tests with swift test.

Swift Package Manager version/commit hash

5.6.2 and latest 5.7 and trunk snapshots

Swift & OS version (output of swift --version && uname -a)

Swift 5.6.2/5.7/5.8 running on Ubuntu 20.04 x86_64 or Android 12 AArch64

[neonichu]

That's expected since we were assuming that any non-main.swift file is always using @main when using a 5.6+ tools-versions -- #5688 improves this behavior already, so I don't think there is more to do for SwiftPM here.

[tomerd]

#5687 should fix it on main
#5688 should fix it on 5.7
for 5.6 it will remain an issue

[finagolfin]

Alright, so people using 5.6 who set swift-tools-version to 5.6 are expected to patch that NIO manifest like this?

diff --git a/Tests/LinuxMain.swift b/Tests/LinuxMain.swift
index eacd4b02..a6a92e48 100644
--- a/Tests/LinuxMain.swift
+++ b/Tests/LinuxMain.swift
@@ -40,10 +40,9 @@ import XCTest
 // without the compiler noticing that we're calling a deprecated function.
 // This hack exists so we can deprecate individual tests which test deprecated functionality without
 // getting a compiler warning...
-protocol LinuxMainRunner { func run() }
-class LinuxMainRunnerImpl: LinuxMainRunner {
+@main class LinuxMainRunnerImpl {
    @available(*, deprecated, message: "not actually deprecated. Just deprecated to allow deprecated tests (which test deprecated functionality) without warnings")
-   func run() {
+   static func main() {
        XCTMain([
              testCase(AcceptBackoffHandlerTest.allTests),
              testCase(AdaptiveRecvByteBufferAllocatorTest.allTests),
@@ -144,5 +143,4 @@ class LinuxMainRunnerImpl: LinuxMainRunner {
         ])
     }
 }
-(LinuxMainRunnerImpl() as LinuxMainRunner).run()
 #endif

That patch works with both 5.6.2 and the latest Aug. 9 trunk snapshot build for linux, but I think you should probably get #5687 into a 5.6 patch release, or this @main requirement is going to break many CIs that test older Swift versions such as 5.6.

As I just noted on that NIO pull, #5687 does fix this issue, but 5.6 is currently not patched with it.

[tomerd]

this problem was introduced in 5.5 actually, and 5.6 also suffers from it. SwiftNIO adopting 5.6 brings to to bear because it using LinuxMain.swift, which is less common at this point. I think the workaround for folks that run swift-nio tests is to pass --enable-test-discovery not change LinuxMain.swift, as SwiftNIO still supports 5.4 and that does not have @main. @Lukasa thoughts?

[finagolfin]

this problem was introduced in 5.5 actually, and 5.6 also suffers from it.

You're right, I just tried it with Swift 5.5.3 and it errors the same.

SwiftNIO still supports 5.4 and that does not have @main

I think it does, but don't know if this patch woulddoesn't work with it, funnily giving a similar error:

swift-nio/Tests/LinuxMain.swift:43:1: error: 'main' attribute cannot be used in a module that contains top-level code

I guess checking for top-level code didn't work well with 5.4.3, as 5.5 and 5.6 accept the exact same source.

5.5 having this issue too definitely makes fixing 5.6 less important, but would be nice to get #5687 into a 5.6 patch release. If that's too late because of the impending 5.7, I understand.

[finagolfin]

Hmm, the fix was already reverted on 5.7.0, #5684, hope it makes it into 5.7.1.