Linking empty rust file fails with "unable to find library -lc-debug"

[hoodmane]

The following:

touch file.rs
rustc file.rs --target wasm32-unknown-emscripten -C link-arg=-sSIDE_MODULE=1 --crate-type cdylib 

Results in
wasm-ld: error: unable to find library -lc-debug.
Rust always includes -sASSERTIONS=1 in the linker args, see rust-lang/rust#97928.

This seems to be a regression between emscripten version 2.0.27 and version 3.1.6. If I surgically set settings.ASSERTIONS=0, then it still fails with "unable to find library -lc". I don't think it should be trying to link -lc anyways because I am trying to make a side module. But I am not sure what the proper setting is to convince Rust or Emscripten to discard this.

[sbc100]

Can you show the failing wasm-ld comment?

I'm pretty sure emscripten will never add -lc or -lc-debug when linking a side module, so I can only imagine that the rust driver is (erroneously) adding it.

I think you best bet is to remove any/all explicit additions of -lc, since the compiler driver (emcc) will add it if/when its needed.

(I don't think this is actually and emscripten bug BTW, but a rustc driver bug)

[hoodmane]

Yes Rust is adding it. I agree it's probably not an emscripten bug. It's added by Rust in a way that makes it hard to remove, but if I patch emcc to delete -lc I am getting the following:

parse exception: attempted pop from empty stack / beyond block start boundary at 268272 (at 0:268272)]
          Fatal: error in parsing input
          emcc: error: emcc: error: '/home/hood/Documents/programming/emsdk/upstream/bin/wasm-emscripten-finalize --dyncalls-i64 --pass-arg=legalize-js-interface-export-originals --side-module main.wasm -o main.wasm --detect-features'

[sbc100]

That looks like an invalid wasm file... can you gram main.wasm.. does it pass wasm-validate ?

[hoodmane]

Nope.

[hoodmane]

Returns a lot of errors like:

main.wasm:00417f0: error: type mismatch in call, expected [i32, i32, i32, i32] but got [i32, i32, i32]
main.wasm:00417f1: error: type mismatch in drop, expected [any] but got []
main.wasm:0041b3f: error: type mismatch in function, expected [] but got [i32, i64]
main.wasm:004217b: error: type mismatch in function, expected [] but got [i32]
main.wasm:00422e4: error: type mismatch in function, expected [] but got [i32]
...

[hoodmane]

-C lto=thin seems to fix it.

[sbc100]

What is the default ? full LTO?

[sbc100]

Can you see which function is at those offsets? (wasm-objdump -D and then search for the hex offsets)

[hoodmane]

For the record, to get to this point I have applied the following patch:

diff --git a/emcc.py b/emcc.py
index b97321cde..66b13d744 100755
--- a/emcc.py
+++ b/emcc.py
@@ -3841,6 +3841,9 @@ def process_libraries(state, linker_inputs):
         state.forced_stdlibs.append(native_lib)
       continue
 
+    if lib == "-c":
+      continue
+
     # We don't need to resolve system libraries to absolute paths here, we can just
     # let wasm-ld handle that.  However, we do want to map to the correct variant.
     # For example we map `-lc` to `-lc-mt` if we are building with threading support.
diff --git a/tools/building.py b/tools/building.py
index cb015e18f..7879c868a 100644
--- a/tools/building.py
+++ b/tools/building.py
@@ -360,9 +360,9 @@ def link_lld(args, target, external_symbols=None):
 
   # Emscripten currently expects linkable output (SIDE_MODULE/MAIN_MODULE) to
   # include all archive contents.
-  if settings.LINKABLE and not any(arg.endswith(".rlib") for arg in args):
-    args.insert(0, '--whole-archive')
-    args.append('--no-whole-archive')
 
   if settings.STRICT:
     args.append('--fatal-warnings')

With that patch,

touch file.rs
rustc file.rs --target wasm32-unknown-emscripten -C link-arg=-sSIDE_MODULE=1 --crate-type cdylib -C lto=thin

works and produces a valid empty side module.