multiple matching crates when deps are duplicated

[colin-kiegel]

I get a weird error when I use the combination of:

• rust-skeptic
• cargo check
• cargo test
• on a proc_macro crate

This looks like a skeptic (or cargo check) bug and it is dependent on the order of cargo check and cargo test.

error[E0464]: multiple matching crates for `cargo_check_bug`
 --> /tmp/rust-skeptic.U89a9gBDNE36/test.rs:1:1
  |
1 | extern crate cargo_check_bug;
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: candidates:
  = note: path: /home/colin/projects/rust/cargo_check_bug/target/debug/deps/libcargo_check_bug-0b76af5b42a4f158.so
  = note: crate name: cargo_check_bug
  = note: crate name: cargo_check_bug

Steps to reprodroduce

1. git clone https://github.com/colin-kiegel/cargo_check_bug.git
2. cd cargo_check_bug
3. rustup override set nightly-2017-03-03
4. cargo clean && cargo test && cargo check
5. cargo clean && cargo check && cargo test

Result of Step 4

Everything OK!

   Compiling getopts v0.2.14
   Compiling libc v0.2.21
   Compiling bitflags v0.5.0
   Compiling rand v0.3.15
   Compiling pulldown-cmark v0.0.8
   Compiling tempdir v0.3.5
   Compiling skeptic v0.7.1
   Compiling cargo_check_bug v0.1.0 (file:///home/colin/projects/rust/cargo_check_bug)
    Finished dev [unoptimized + debuginfo] target(s) in 5.40 secs
     Running target/debug/deps/cargo_check_bug-bc124838e21e68e1
running 0 tests
test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
 Running target/debug/deps/skeptic-c3b458a20ebee641

running 1 test

warning: proc macro crates and #[no_link] crates have no effect without #[macro_use]

--> /tmp/rust-skeptic.9IqFqNCvwCPZ/test.rs:1:1

|

1 | extern crate cargo_check_bug;

| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
test readme_0 ... ok
test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
Doc-tests cargo_check_bug
running 0 tests
test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
Compiling cargo_check_bug v0.1.0 (file:///home/colin/projects/rust/cargo_check_bug)

Finished dev [unoptimized + debuginfo] target(s) in 0.12 secs

Result of Step 5

error[E0464]: multiple matching crates for `cargo_check_bug`

   Compiling bitflags v0.5.0
   Compiling getopts v0.2.14
   Compiling libc v0.2.21
   Compiling rand v0.3.15
   Compiling pulldown-cmark v0.0.8
   Compiling tempdir v0.3.5
   Compiling skeptic v0.7.1
   Compiling cargo_check_bug v0.1.0 (file:///home/colin/projects/rust/cargo_check_bug)
    Finished dev [unoptimized + debuginfo] target(s) in 4.54 secs
   Compiling cargo_check_bug v0.1.0 (file:///home/colin/projects/rust/cargo_check_bug)
    Finished dev [unoptimized + debuginfo] target(s) in 0.97 secs
     Running target/debug/deps/cargo_check_bug-bc124838e21e68e1
running 0 tests
test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
 Running target/debug/deps/skeptic-c3b458a20ebee641

running 1 test

error[E0464]: multiple matching crates for cargo_check_bug

--> /tmp/rust-skeptic.U89a9gBDNE36/test.rs:1:1

|

1 | extern crate cargo_check_bug;

| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

|

= note: candidates:

= note: path: /home/colin/projects/rust/cargo_check_bug/target/debug/deps/libcargo_check_bug-0b76af5b42a4f158.so

= note: crate name: cargo_check_bug

= note: crate name: cargo_check_bug
error[E0463]: can't find crate for cargo_check_bug

--> /tmp/rust-skeptic.U89a9gBDNE36/test.rs:1:1

|

1 | extern crate cargo_check_bug;

| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ can't find crate
error: aborting due to 2 previous errors
test readme_0 ... FAILED
failures:
---- readme_0 stdout ----

thread 'readme_0' panicked at 'Command failed:

"rustc" "/tmp/rust-skeptic.U89a9gBDNE36/test.rs" "--verbose" "-o" "/tmp/rust-skeptic.U89a9gBDNE36/out.exe" "--crate-type=bin" "-L" "/home/colin/projects/rust/cargo_check_bug/target/debug" "-L" "/home/colin/projects/rust/cargo_check_bug/target/debug/deps" "--extern" "pulldown_cmark=/home/colin/projects/rust/cargo_check_bug/target/debug/deps/libpulldown_cmark-ecfb67e72dcb6fb2.rlib" "--extern" "skeptic=/home/colin/projects/rust/cargo_check_bug/target/debug/deps/libskeptic-74b6fcba1ef2d3dc.rlib" "--extern" "getopts=/home/colin/projects/rust/cargo_check_bug/target/debug/deps/libgetopts-3facdbd0235704b0.rlib" "--extern" "tempdir=/home/colin/projects/rust/cargo_check_bug/target/debug/deps/libtempdir-0ee757a585f719dd.rlib" "--extern" "libc=/home/colin/projects/rust/cargo_check_bug/target/debug/deps/liblibc-5dc7b85e748840b4.rlib" "--extern" "bitflags=/home/colin/projects/rust/cargo_check_bug/target/debug/deps/libbitflags-8510a47beebe00a6.rlib" "--extern" "rand=/home/colin/projects/rust/cargo_check_bug/target/debug/deps/librand-c9d9fbdab2355ee4.rlib"', /home/colin/.cargo/registry/src/github.com-1ecc6299db9ec823/skeptic-0.7.1/lib.rs:391

note: Run with RUST_BACKTRACE=1 for a backtrace.
failures:

readme_0
test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured
error: test failed

Cargo.toml

[package]
name = "cargo_check_bug"
version = "0.1.0"
build = "build.rs"

[lib]
proc-macro = true

[dependencies]

[build-dependencies]
skeptic = "0.7"

[dev-dependencies]
skeptic = "0.7"

build.rs

extern crate skeptic;

fn main() {
    skeptic::generate_doc_tests(&["README.md"]);
}

README.md

```rust
extern crate cargo_check_bug;

fn main() {
}
```

tests/skeptic.rs

include!(concat!(env!("OUT_DIR"), "/skeptic-tests.rs"));

src/lib.rs

#![crate_type = "proc-macro"]
extern crate proc_macro;
use proc_macro::TokenStream;

#[proc_macro_derive(Foo)]
pub fn derive(_input: TokenStream) -> TokenStream {
    unimplemented!()
}

[colin-kiegel]

Do you think this might be a bug in cargo check or in rust-skeptic?

PS: I can't locate the file /tmp/rust-skeptic.ocUVTbTQ1DCk/test.rs:1:1 from the error message above. All I can find is this

./target/debug/build/cargo_check_bug-899f203356c46842/out/skeptic-tests.rs

extern crate skeptic;
#[test] fn readme_0() {
    let ref s = format!("{}", r####"extern crate cargo_check_bug;

fn main() {
}
"####);
    skeptic::rt::run_test(r#"/home/colin/projects/rust/cargo_check_bug/target/debug/build/cargo_check_bug-899f203356c46842/out"#, s);
}

[colin-kiegel]

ok, looks like something has changed between rust-skeptic v0.6.x and v0.7.1.

https://github.com/brson/rust-skeptic/compare/master@%7B2016-08-12%7D...master@%7B2017-02-26%7D

It looks like this commit brson@205f426 from @oli-obk fixed it for ordinary libs. But I can still reproduce this error if the library is a proc-macro!

Since an update of skeptic changed the error, I believe the remaining error is also related to skeptic (and not cargo check).

PS: I can confirm, that removing test/skeptic.rs eliminates the error - both in this toy crate and in derive_builder.

[colin-kiegel]

Any news on this?

It's a bit problematic that I get nonsense failing tests, if I activate skeptic tests (+cargo cache) on travis
https://travis-ci.org/colin-kiegel/rust-derive-builder/jobs/219789090#L644

I'm already using skeptic 0.8.1.

The only workaround I know is to always run cargo clean before cargo test, but I would prefer to benefit from faster travis builds using its cargo cache feature.

[brson]

@colin-kiegel Thanks for the great report and investigation, and again sorry I haven't responded earlier.

This kind of sporadic crate resolution failure is because skeptic's tests aren't passing the --extern flags to rustc needed to disambiguate between multiple crate candidates, like

--extern skeptic_readme=/mnt2/dev/rust-skeptic/target/debug/deps/libskeptic_readme-77286c6085960cab.rlib 

This happens when versions change and cargo leaves old deps around. Normally this doesn't matter because cargo properly passes --extern flags to everything it builds. Skeptic though runs rustc itself as part of cargo test, and doesn't actually know the correct values (cargo does not communicate these values to build scripts).

This is a pretty big limitation of skeptic presently, and the workaround is to cargo clean beforehand.

I'm surprised this looks like a regression to you, since I consider it a long-standing bug, put probably something about skeptic and/or cargo changed to exacerbate the problem, and there may be especially weird stuff going on with proc macros.

The best solution would be for cargo could to pass the names of various libraries to the build script, assuming it knows them that far ahead of time.

In the meantime, I think it would be possible for skeptic to maintain a cache of knowledge about the deps folder and use heuristics to make a pretty good guess about which libs are the most fresh.

[brson]

PS: I can't locate the file /tmp/rust-skeptic.ocUVTbTQ1DCk/test.rs:1:1 from the error message above. All I can find is this

This is a tempfile that is deleted during test execution. It would be nice if there were some way to tell skeptic not to delete them for debugging purposes.

[colin-kiegel]

Ok, it's already good to know that this is a known limitation and cargo clean is the only known workaround.

Hm, this would probably benefit a lot if cargo introduced some testing plugin slot, or some other query mechanism (maybe on nightly only). Compiletests seem to have similar problems. There it already starts with finding the right target directory, which can be a problem in workspaces.

I'm not familiar with cargo plugins. But could it help to introduce a cargo skeptic or cargo query subcommand? I could imagine that cargo plugins might already have some access to these kinds of information, no?

[colin-kiegel]

PS: I'm not sure how involved that is (or if it is even comparable), but cargo clippy seems to be doing a good job here. I never experienced these problems with clippy.

[oli-obk]

Cargo clippy forwards to cargo rustc and injects a custom compiler through the RUSTC env var. This way we avert these issues entirety. But it has the downside of overwriting the RUSTC env var and thus breaking builds that set it. This is an open issue in clippy

[colin-kiegel]

that sounds like this might work for skeptic, too. And I think skeptic wouldn't need to inject a custom compiler, right?

[brson]

I'm not familiar with cargo plugins. But could it help to introduce a cargo skeptic or cargo query subcommand? I could imagine that cargo plugins might already have some access to these kinds of information, no?

Unfortunately cargo plugins have no special information. They are just executables that cargo runs - there's no info passed between them.

[brson]

Cargo clippy forwards to cargo rustc and injects a custom compiler through the RUSTC env var. This way we avert these issues entirety. But it has the downside of overwriting the RUSTC env var and thus breaking builds that set it. This is an open issue in clippy

I can't quite imagine a way to fix this issue for skeptic by intercepting rustc itself, but if there is such a solution it may be worth pursuing.

[brson]

One plausible way to fix this would be to have skeptic synthesize an entire cargo project from the original cargo project, for every test it generates, sharing target directories. Then run cargo instead of rustc, and have cargo figure out the deps.