Fails to compile if proc-macro2 is a dependency

[lukaslueg]

I'm unsure which tree to bark up against, so I'm reporting this here:

If a crate has a dependency on proc-macro2 (any version, currently resolved to 0.4.9), compilation via cargo web build (on 0.6.4) fails with can't find crate for 'proc_macro' while compiling proc-macro2. The offending line in proc-macro2 is extern crate proc_macro;.

It fails for all targets, except native targets, on stable and nightly-2018-07-23.

[koute]

This happens because the proc_macro crate is not distributed for the asmjs/wasm targets, i.e. here we have it:

$ ls ~/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib | grep proc_macro
libproc_macro-1ea4ba9e4f6cdbc1.so

and here we don't:

$ ls ~/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/wasm32-unknown-unknown/lib | grep proc_macro

You can easily reproduce this issue using only cargo:

$ cargo new foobar
$ cd foobar
$ echo 'proc-macro2 = "*"' >> Cargo.toml
$ cargo build --target=wasm32-unknown-unknown
   Compiling unicode-xid v0.1.0                                                                                                                                                                                                              
   Compiling proc-macro2 v0.4.9
error[E0463]: can't find crate for `proc_macro`
  --> /home/kou/.cargo/registry/src/github.com-1ecc6299db9ec823/proc-macro2-0.4.9/src/lib.rs:50:1
   |
50 | extern crate proc_macro;
   | ^^^^^^^^^^^^^^^^^^^^^^^^ can't find crate

So if this is a problem for you then you should report it as an Rust issue.

However, why would you actually want to use the proc_macro2 crate with the web targets? AFAIK that crate is only useful when writing procedural plugins which use your native target instead of the one for which you're compiling for.

[lukaslueg]

proc_macro was being pulled in due to a dependency on syn, which pulled proc_macro2, which pulled in proc_macro. There is a knob in syn to turn this off, so problem solved for now.

[lukaslueg]

Reopening because I still think it's a bug with cargo-web:

This builds using cargo web build

[dependencies]
proc-macro2 = { version = "0.4", default_features = false }

This also builds using cargo web build

[dependencies]
stdweb = { version = "0.4" }

But with both dependencies

[dependencies]
proc-macro2 = { version = "0.4", default_features = false }
stdweb = { version = "0.4" }

... cargo web build fails due to missing proc_macro-crate. It does, however, build using cargo build.

[lukaslueg]

Re-closing because after some tinkering I now think it's actually cargo's fault :-/