`rustc --target wasm32-unknown-unknown -Zinstrument-coverage` fails with "can't find crate for `profiler_builtins`"

[anp]

I'm trying to add support for code coverage to wasm-bindgen tests with the new -Zinstrument-coverage flag, and hitting an error that I only partly understand.

I'm building this with anp/moxie@20920ca, if it would be helpful I can make a smaller repro. Here's the command I'm running and the output I'm seeing (repeated error messages snipped):

❯ env RUSTFLAGS=-Zinstrument-coverage cargo +nightly build --target wasm32-unknown-unknown --package moxie-dom
   Compiling cfg-if v1.0.0
   Compiling once_cell v1.5.2
   Compiling futures-sink v0.3.8
   Compiling futures-core v0.3.8
   Compiling scopeguard v1.1.0
   Compiling pin-utils v0.1.0
   Compiling slab v0.4.2
   Compiling futures-io v0.3.8
error[E0463]: can't find crate for `profiler_builtins`
  |
  = note: the compiler may have been built without the profiler runtime

error: aborting due to previous error

For more information about this error, try `rustc --explain E0463`.
...
error: could not compile `futures-sink`

To learn more, run the command again with --verbose.
warning: build failed, waiting for other jobs to finish...
...

I've also tried using the wasm32-wasi target, but that also appears to be missing profiler_builtins. Does anyone have guidance on what code needs to be extended to support the instrument-coverage flag for wasm targets?

Meta

rustc --version --verbose:

❯ rustc +nightly --version --verbose
rustc 1.51.0-nightly (04caa632d 2021-01-30)
binary: rustc
commit-hash: 04caa632dd10c2bf64b69524c7f9c4c30a436877
commit-date: 2021-01-30
host: aarch64-apple-darwin
release: 1.51.0-nightly
LLVM version: 11.0.1

[anp]

From the dev guide it seems that the config.toml for the rustup toolchains may need to be updated.

[anp]

If I set profiler = true in my config.toml and add wasm32-unknown-unknown to the targets, I get a failure in building compiler_rt for wasm:

error: failed to run custom build command for `profiler_builtins v0.0.0 (/Users/adam/code/rust/library/profiler_builtins)`

Caused by:
  process didn't exit successfully: `/Users/adam/code/rust/build/aarch64-apple-darwin/stage1-std/release/build/profiler_builtins-91d370bd947d85af/build-script-build` (exit code: 1)
  --- stdout
  TARGET = Some("wasm32-unknown-unknown")
  OPT_LEVEL = Some("3")
  HOST = Some("aarch64-apple-darwin")
  CC_wasm32-unknown-unknown = Some("clang")
  CFLAGS_wasm32-unknown-unknown = Some("-ffunction-sections -fdata-sections -fPIC --target=wasm32-unknown-unknown")
  CRATE_CC_NO_DEFAULTS = None
  DEBUG = Some("false")
  running: "clang" "-O3" "-ffunction-sections" "-fdata-sections" "-fPIC" "--target=wasm32-unknown-unknown" "-ffunction-sections" "-fdata-sections" "-fPIC" "--target=wasm32-unknown-unknown" "-I" "../../src/llvm-project/compiler-rt/include" "-fno-builtin" "-fomit-frame-pointer" "-fvisibility=hidden" "-DVISIBILITY_HIDDEN" "-DCOMPILER_RT_HAS_UNAME=1" "-DCOMPILER_RT_HAS_ATOMICS=1" "-o" "/Users/adam/code/rust/build/aarch64-apple-darwin/stage1-std/wasm32-unknown-unknown/release/build/profiler_builtins-614c077fb1787a99/out/../../src/llvm-project/compiler-rt/lib/profile/GCDAProfiling.o" "-c" "../../src/llvm-project/compiler-rt/lib/profile/GCDAProfiling.c"
  cargo:warning=../../src/llvm-project/compiler-rt/lib/profile/GCDAProfiling.c:24:10: fatal error: 'errno.h' file not found
  cargo:warning=#include <errno.h>
  cargo:warning=         ^~~~~~~~~
  cargo:warning=1 error generated.
  exit code: 1

  --- stderr


  error occurred: Command "clang" "-O3" "-ffunction-sections" "-fdata-sections" "-fPIC" "--target=wasm32-unknown-unknown" "-ffunction-sections" "-fdata-sections" "-fPIC" "--target=wasm32-unknown-unknown" "-I" "../../src/llvm-project/compiler-rt/include" "-fno-builtin" "-fomit-frame-pointer" "-fvisibility=hidden" "-DVISIBILITY_HIDDEN" "-DCOMPILER_RT_HAS_UNAME=1" "-DCOMPILER_RT_HAS_ATOMICS=1" "-o" "/Users/adam/code/rust/build/aarch64-apple-darwin/stage1-std/wasm32-unknown-unknown/release/build/profiler_builtins-614c077fb1787a99/out/../../src/llvm-project/compiler-rt/lib/profile/GCDAProfiling.o" "-c" "../../src/llvm-project/compiler-rt/lib/profile/GCDAProfiling.c" with args "clang" did not execute successfully (status code exit code: 1).

Perhaps LLVM doesn't yet support the profiler runtime in webassembly.

[anp]

Perhaps LLVM doesn't yet support the profiler runtime in webassembly.

This seems to be the case. I have a couple of patches which seem pretty close to getting things compiling for wasm32-wasi, not sure it will work yet though.

[gilescope]

Did it work in the end? ( @richkadel you might be interested in this? )
Would be super cool to be able to do coverage on wasm code.

[richkadel]

Yes I'm interested. I didn't expecting coverage to work for WASM, but very excited if Adam has a workable path forward!

[anp]

I think the WASI path could work but I'm a bit skeptical -- the coverage runtime relies on mmap and some other functions which didn't have full implementations in WASI when I last checked. It might end up blocked on some gnarly POSIX compatibility issue.

I think the best path forward for wasm-bindgen is probably be to find a way to use minicov as suggested by @Amanieu. I think this would require either

1. allowing binaries to bring their own profiler_builtins crate from crates.io -- then wasm-bindgen-test could depend on minicov when built for coverage
2. building profiler_builtins with minicov on wasm32-unknown-unknown and exposing some JS-accessible hooks for wasm-bindgen-test to use to grab the traces to ship out of the browser process

I'm not sure that both of these are actually viable, and if they are, not sure which would be preferable.

[gilescope]

I love the idea of being able to plug in builtins (ignoring the oxymoron). Feels very rusty - prior art: we can plug in global allocators.