[Embedded] Impossible to link `@_expose` symbols from swift dependencies without explicitly referencing them

[sliemeobn]

Description

Example use case:
JavaScriptKit provides interop via @_expose(wasm) declarations.

However, when an embedded swift app is depending on it, these symbols are missing from the final binary. After a brief discussion with @rauhul it is pretty clear why:

• all swift module dependencies are actually only "compiled" into the final module via swiftmodules
• the object files of the swift dependencies cannot be used for linking (otherwise we'd get duplicate symbols)
• since the app module does not actually reference the swift declaration, it will not end up in an object file used for linking

I suspect this is true for any "unused" top-level swift declaration that should end up in the binary, and I guess this is also the reason for @_cdecls not working (see #74328)

Reproduction

see a package with this issue here (with the work-around of referencing the expose as an underscored public swift symbol)
https://github.com/swiftwasm/JavaScriptKit/blob/dd0c977dde0ad056b5b0665bc66335b8c77ce59e/Examples/Embedded/Sources/EmbeddedApp/_thingsThatShouldNotBeNeeded.swift#L3-L8

Expected behavior

ideally all @_expose and @_cdecl symbols end up "somewhere" so they are available for linking.
maybe it is possible to "just" add them to an object file in the entry module?

we also talked about the @_used attribute, but I am not sure it should be needed. (eg: a cdecl can always NOT be linked, that is fine, but it needs to be available at link-time)

Environment

Apple Swift version 6.1-dev (LLVM 5c0d283c37132bf, Swift b5af518)
Target: arm64-apple-macosx15.0

Additional information

forum post with all the context
https://forums.swift.org/t/web-app-with-embedded-swift-poc-demo/75486

[rauhul]

cc @kubamracek @eeckstein

[MaxDesiatov]

@sliemeobn would you be able to confirm that the issues is fixed in DEVELOPMENT-SNAPSHOT-2025-03-28-a or a later tag for you?

[sliemeobn]

@MaxDesiatov
since I need to roll my own linker invocation for now (SwiftPM would cause duplicate symbol errors with wasm-ld otherwise), I am not entire sure how?

in which object file are these symbols supposed to end up in?

edit:
Just to be sure I tried adding @_used to the [JavaScriptKit exposed funcs] (swiftwasm/JavaScriptKit@main...sliemeobn:JavaScriptKit:embedded/used-attribute) and commenting out the manual references in the app module - does not work (exports do not show up in the final wasm)

[kubamracek]

@sliemeobn we have recently changed the behavior of @_used, and declarations annotated with @_used should now be getting linked even from other modules (see #80185). Would you be able to try the @_used attribute once again, using a fresh nightly "main" toolchain?