Using impl trait across crates results in linker error

[jplatte]

I have a small test case that doesn't work in its current state (split into two crates), producing a linker error; but works when everything is moved into one crate: https://github.com/jplatte/rust-impl-trait-across-crates

Error:

   Compiling bin v0.1.0 (file:///home/jplatte/Projekte/rust-impl-trait-across-crates/bin)
error: linking with `cc` failed: exit code: 1
  |
  = note: "cc" "-Wl,--as-needed" "-Wl,-z,noexecstack" "-m64" "-L" "/home/jplatte/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib" "/home/jplatte/Projekte/rust-impl-trait-across-crates/bin/target/debug/bin.0.o" "-o" "/home/jplatte/Projekte/rust-impl-trait-across-crates/bin/target/debug/bin" "-Wl,--gc-sections" "-pie" "-nodefaultlibs" "-L" "/home/jplatte/Projekte/rust-impl-trait-across-crates/bin/target/debug/deps" "-L" "/home/jplatte/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib" "-Wl,-Bstatic" "-Wl,-Bdynamic" "/home/jplatte/Projekte/rust-impl-trait-across-crates/bin/target/debug/deps/liblib.rlib" "/home/jplatte/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libstd-411f48d3.rlib" "/home/jplatte/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libpanic_unwind-411f48d3.rlib" "/home/jplatte/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libunwind-411f48d3.rlib" "/home/jplatte/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/librand-411f48d3.rlib" "/home/jplatte/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libcollections-411f48d3.rlib" "/home/jplatte/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/librustc_unicode-411f48d3.rlib" "/home/jplatte/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/liballoc-411f48d3.rlib" "/home/jplatte/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/liballoc_jemalloc-411f48d3.rlib" "/home/jplatte/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/liblibc-411f48d3.rlib" "/home/jplatte/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libcore-411f48d3.rlib" "-l" "dl" "-l" "pthread" "-l" "gcc_s" "-l" "pthread" "-l" "c" "-l" "m" "-l" "rt" "-l" "util" "-l" "compiler-rt"
  = note: /home/jplatte/Projekte/rust-impl-trait-across-crates/bin/target/debug/bin.0.o: In function `{{inlined-root}}::next<core::slice::Iter<u32>,closure>':
/buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/obj/../src/libcore/iter/mod.rs:953: undefined reference to `lib::Thing::even_elems::_$u7b$$u7b$closure$u7d$$u7d$::h337a400b104ae277'
collect2: error: ld returned 1 exit status


error: aborting due to previous error

error: Could not compile `bin`.

To learn more, run the command again with --verbose.

[eddyb]

cc @rust-lang/compiler I have seen some warnings along these lines, that we need to change what we internalize to not include what impl Trait "exports".

[jonas-schievink]

Due to this check, this can be worked around by marking even_elems as #[inline].

It looks like the actual issue is that privacy can't mark the returned type as reachable, since it's not yet known when privacy is checked. Normally, the privacy checker marks all types used in interfaces as reachable. This information is then used for computing the set of externally reachable symbols (which are then excluded from being internalized).

[eddyb]

@jonas-schievink That's syntactical, we should be able to make it simpler by running it after typeck.

[petrochenkov]

@eddyb

That's syntactical, we should be able to make it simpler by running it after typeck.

I don't think AccessLevel::Reachable^* is appropriate for private types leaked through impl Trait, it's more or less public, for example Reachable items require stability attributes + lints like missing copy/debug work on them.
You'd have to add one more AccessLevel for "public for linkage purpose only" items.

^*That FIXME is wrong.

[jplatte]

@jonas-schievink Thanks for providing a workaround! :)