fix(transport): libp2p compilation in Nim version-2-0

[tersec]

For example:

/Users/runner/work/nim-libp2p/nim-libp2p/nimbledeps/pkgs2/websock-0.1.0-94f836ae589056b2deb04bdfdcd614fff80adaf5/websock/http/client.nim(173, 5) template/generic instantiation of `async` from here
/Users/runner/work/nim-libp2p/nim-libp2p/nimbledeps/pkgs2/websock-0.1.0-94f836ae589056b2deb04bdfdcd614fff80adaf5/websock/http/client.nim(165, 1) Warning: The raises pragma doesn't work on async procedures - use `async: (raises: [...]) instead. [User]
/Users/runner/work/nim-libp2p/nim-libp2p/nimbledeps/pkgs2/websock-0.1.0-94f836ae589056b2deb04bdfdcd614fff80adaf5/websock/websock.nim(257, 5) template/generic instantiation of `async` from here
/Users/runner/work/nim-libp2p/nim-libp2p/nimbledeps/pkgs2/websock-0.1.0-94f836ae589056b2deb04bdfdcd614fff80adaf5/websock/websock.nim(251, 1) Warning: The raises pragma doesn't work on async procedures - use `async: (raises: [...]) instead. [User]
/Users/runner/work/nim-libp2p/nim-libp2p/libp2p/transports/wstransport.nim(77, 18) template/generic instantiation of `async` from here
/Users/runner/work/nim-libp2p/nim-libp2p/libp2p/transports/wstransport.nim(83, 10) template/generic instantiation of `setResult` from here
/Users/runner/work/nim-libp2p/nim-libp2p/libp2p/transports/wstransport.nim(78, 26) template/generic instantiation of `mapExceptions` from here
/Users/runner/work/nim-libp2p/nim-libp2p/nimbledeps/pkgs2/chronos-4.0.2-c5e9517b9189713210e2abab8b77a68da71ded12/chronos/internal/asyncmacro.nim(542, 60) Error: expression 'value(cast[type(recv(s.session, pbytes, nbytes))](chronosInternalRetFuture.internalChild))' is of type 'int' and has to be used (or discarded); start of expression here: /Users/runner/work/nim-libp2p/nim-libp2p/libp2p/transports/wstransport.nim(78, 26)
stack trace: (most recent call last)

from https://github.com/vacp2p/nim-libp2p/actions/runs/10655841970/job/29533846606?pr=1145

For minimal example of this:

template g(body: untyped) =
  try:
    body
  except CatchableError:
    raise newException(CatchableError, "")

discard g(0)

Also, even in 2.0.8, a variation doesn't work:

template g(body: untyped) = body
discard g(0)

[etan-status]

Thanks, that should fix the recent issue in #1145 and #1185 CI.

[diegomrsantos]

Thanks for the fix. What is the cause of the issue? Was it implemented in a wrong, but supported way? This code hasn't changed for more than 2 years.

[tersec]

Thanks for the fix. What is the cause of the issue? Was it implemented in a wrong, but supported way? This code hasn't changed for more than 2 years.

Some change from post-Nim 2.0.8 version-2-0 did trigger stricter checking in these instances of the return type of the template. The template, arguably, was always incorrect: it didn't indicate that it returned anything, and see examples, it only works because Nim got confused by the exception construction until recently.

For another, close example of Nim 2.0.8 already disallowing/being confused by this incorrect return type declaration:

template g(body: untyped) =
  try:
    body
  except CatchableError:
    raiseAssert ""

discard g(0)

yields

a.nim(7, 11) Error: expression '0' is of type 'int literal(0)' and has to be used (or discarded)

i.e. it was specifically that raise newException(...) bypassed this check somehow.

[diegomrsantos]

The error message seems a bit confusing. In the original error, it gets used, and your example is discarded.

[tersec]

The error message seems a bit confusing. In the original error, it gets used, and your example is discarded.

Not in a relevant way. Same error message:

template g(body: untyped) =
  try:
    body
  except CatchableError:
    raiseAssert ""

let t = g(0)

discard outside of the template is a red herring. The "discarding" it's complaining about it in the template g/mapExceptions itself, not in the code calling/invoking it.

[diegomrsantos]

The error message mentions the line where it is used.

[tersec]

The error message mentions the line where it is used.

Yes, it's a template. It doesn't really exist until it's instantiated.