Weird behavior with target.cfg.dependencies/build-dependencies

[khyperia]

For this src/lib.rs, compiling with various configurations produces some weird results. I'm using bitflags as a random example, I don't think there's anything special about it:

use bitflags;
pub fn f() {}

This Cargo.toml fails, as expected (I am on linux, flip to cfg(unix) or whatever if you're on windows):

[package]
name = "rusttest"
version = "0.1.0"
edition = "2018"

[target.'cfg(windows)'.dependencies]
bitflags = ""

# [target.'cfg(not(windows))'.build-dependencies]
# bitflags = ""

error[E0432]: unresolved import `bitflags`
 --> src/lib.rs:1:5
  |
1 | use bitflags;
  |     ^^^^^^^^ no external crate `bitflags`

This Cargo.toml also fails, as expected, with the same error.

[package]
name = "rusttest"
version = "0.1.0"
edition = "2018"

# [target.'cfg(windows)'.dependencies]
# bitflags = ""

[target.'cfg(not(windows))'.build-dependencies]
bitflags = ""

But, this Cargo.toml succeeds compilation, very unexpectedly (with a warning: unused import: bitflags):

[package]
name = "rusttest"
version = "0.1.0"
edition = "2018"

[target.'cfg(windows)'.dependencies]
bitflags = ""

[target.'cfg(not(windows))'.build-dependencies]
bitflags = ""

Is this intended behavior? This seems really weird and not at all what I expected. The first example shows that target.'cfg(windows)'.dependencies is not included in builds on linux. The second example shows that target.'cfg(not(windows))'.build-dependencies (which is enabled on linux) does not "leak" into the main program. So why, when combined together, does the build succeed?

Output of cargo version: cargo 1.50.0 (f04e7fab7 2021-02-04)
Also reproduces on cargo +nightly version: cargo 1.52.0-nightly (572e20153 2021-02-24)

---

Discovered while trying to investigate the issues hit in EmbarkStudios/rust-gpu#423

[khyperia]

@eddyb suggested adding resolver = "2" to the Cargo.toml:

[package]
name = "rusttest"
version = "0.1.0"
edition = "2018"
resolver = "2"

[target.'cfg(windows)'.dependencies]
bitflags = ""

[target.'cfg(not(windows))'.build-dependencies]
bitflags = ""

and uh,

khyperia@cobalt ~/me/rusttest> cargo +nightly build
thread 'main' panicked at 'activated_features for invalid package: features did not find PackageId { name: "bitflags", version: "1.2.1", source: "registry `https://github.com/rust-lang/crates.io-index`" } false', src/tools/cargo/src/cargo/core/resolver/features.rs:235:14
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Backtrace, click to expand

khyperia@cobalt ~/me/rusttest> RUST_BACKTRACE=full cargo +nightly build
thread 'main' panicked at 'activated_features for invalid package: features did not find PackageId { name: "bitflags", version: "1.2.1", source: "registry `https://github.com/rust-lang/crates.io-index`" } false

Stack backtrace:
   0: cargo::core::resolver::features::ResolvedFeatures::activated_features_int
   1: cargo::core::compiler::unit_dependencies::new_unit_dep_with_profile
   2: cargo::core::compiler::unit_dependencies::compute_deps
   3: cargo::core::compiler::unit_dependencies::deps_of
   4: cargo::core::compiler::unit_dependencies::build_unit_dependencies
   5: cargo::ops::cargo_compile::create_bcx
   6: cargo::ops::cargo_compile::compile_ws
   7: cargo::ops::cargo_compile::compile
   8: cargo::commands::build::exec
   9: cargo::cli::main
  10: cargo::main
  11: std::sys_common::backtrace::__rust_begin_short_backtrace
  12: std::rt::lang_start::{{closure}}
  13: core::ops::function::impls::<impl core::ops::function::FnOnce<A> for &F>::call_once
             at /rustc/e37a13cc3594004663738bd18d8100e6db9666cf/library/core/src/ops/function.rs:259:13
      std::panicking::try::do_call
             at /rustc/e37a13cc3594004663738bd18d8100e6db9666cf/library/std/src/panicking.rs:379:40
      std::panicking::try
             at /rustc/e37a13cc3594004663738bd18d8100e6db9666cf/library/std/src/panicking.rs:343:19
      std::panic::catch_unwind
             at /rustc/e37a13cc3594004663738bd18d8100e6db9666cf/library/std/src/panic.rs:431:14
      std::rt::lang_start_internal
             at /rustc/e37a13cc3594004663738bd18d8100e6db9666cf/library/std/src/rt.rs:51:25
  14: main
  15: __libc_start_main
  16: <unknown>', src/tools/cargo/src/cargo/core/resolver/features.rs:235:14
stack backtrace:
   0:     0x55dd1e2ad400 - std::backtrace_rs::backtrace::libunwind::trace::h9d49145f95eb5894
                               at /rustc/e37a13cc3594004663738bd18d8100e6db9666cf/library/std/src/../../backtrace/src/backtrace/libunwind.rs:90:5
   1:     0x55dd1e2ad400 - std::backtrace_rs::backtrace::trace_unsynchronized::hab1d020365bb6864
                               at /rustc/e37a13cc3594004663738bd18d8100e6db9666cf/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x55dd1e2ad400 - std::sys_common::backtrace::_print_fmt::h7659588431e304bd
                               at /rustc/e37a13cc3594004663738bd18d8100e6db9666cf/library/std/src/sys_common/backtrace.rs:67:5
   3:     0x55dd1e2ad400 - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h09f4a9e3befae3c7
                               at /rustc/e37a13cc3594004663738bd18d8100e6db9666cf/library/std/src/sys_common/backtrace.rs:46:22
   4:     0x55dd1e2d459c - core::fmt::write::hf3fdfde304b9a088
                               at /rustc/e37a13cc3594004663738bd18d8100e6db9666cf/library/core/src/fmt/mod.rs:1092:17
   5:     0x55dd1e2a47d2 - std::io::Write::write_fmt::h1cb850689c7116f0
                               at /rustc/e37a13cc3594004663738bd18d8100e6db9666cf/library/std/src/io/mod.rs:1568:15
   6:     0x55dd1e2b0355 - std::sys_common::backtrace::_print::hdbccd5aa093ba544
                               at /rustc/e37a13cc3594004663738bd18d8100e6db9666cf/library/std/src/sys_common/backtrace.rs:49:5
   7:     0x55dd1e2b0355 - std::sys_common::backtrace::print::hc639c4f320222558
                               at /rustc/e37a13cc3594004663738bd18d8100e6db9666cf/library/std/src/sys_common/backtrace.rs:36:9
   8:     0x55dd1e2b0355 - std::panicking::default_hook::{{closure}}::hdb012dd7a485bb5d
                               at /rustc/e37a13cc3594004663738bd18d8100e6db9666cf/library/std/src/panicking.rs:208:50
   9:     0x55dd1e2afe03 - std::panicking::default_hook::h75facbce77b6ba91
                               at /rustc/e37a13cc3594004663738bd18d8100e6db9666cf/library/std/src/panicking.rs:225:9
  10:     0x55dd1e2b0af1 - std::panicking::rust_panic_with_hook::hbcaa5de2cb5e22d5
                               at /rustc/e37a13cc3594004663738bd18d8100e6db9666cf/library/std/src/panicking.rs:591:17
  11:     0x55dd1e2b0637 - std::panicking::begin_panic_handler::{{closure}}::h4ee6cde415c8f62d
                               at /rustc/e37a13cc3594004663738bd18d8100e6db9666cf/library/std/src/panicking.rs:497:13
  12:     0x55dd1e2ad8bc - std::sys_common::backtrace::__rust_end_short_backtrace::h895319f2d3f611c0
                               at /rustc/e37a13cc3594004663738bd18d8100e6db9666cf/library/std/src/sys_common/backtrace.rs:141:18
  13:     0x55dd1e2b0599 - rust_begin_unwind
                               at /rustc/e37a13cc3594004663738bd18d8100e6db9666cf/library/std/src/panicking.rs:493:5
  14:     0x55dd1e2d2a91 - core::panicking::panic_fmt::h0123abb763a6e96f
                               at /rustc/e37a13cc3594004663738bd18d8100e6db9666cf/library/core/src/panicking.rs:92:14
  15:     0x55dd1e2d28b3 - core::option::expect_none_failed::h9fff90ea7603aa1d
                               at /rustc/e37a13cc3594004663738bd18d8100e6db9666cf/library/core/src/option.rs:1300:5
  16:     0x55dd1da11f8f - cargo::core::compiler::unit_dependencies::new_unit_dep_with_profile::h5964cde2c2abca55
  17:     0x55dd1da1095f - cargo::core::compiler::unit_dependencies::compute_deps::h69e985c7bbf4d1bc
  18:     0x55dd1da0f303 - cargo::core::compiler::unit_dependencies::deps_of::h123cd8ebc552c83b
  19:     0x55dd1da0dbc1 - cargo::core::compiler::unit_dependencies::build_unit_dependencies::h83836656f1b0fb95
  20:     0x55dd1dc0b2ad - cargo::ops::cargo_compile::create_bcx::h78fae7bd213ba8be
  21:     0x55dd1dc06506 - cargo::ops::cargo_compile::compile_ws::h4fd239b9a12b765b
  22:     0x55dd1dc0641e - cargo::ops::cargo_compile::compile::hc792daa77ab07f92
  23:     0x55dd1d96ad82 - cargo::commands::build::exec::h1f89262783db7921
  24:     0x55dd1d98616b - cargo::cli::main::h0cbd726e1a4a310b
  25:     0x55dd1d97e7d8 - cargo::main::h3ab2228c9b96dd76
  26:     0x55dd1d9764b6 - std::sys_common::backtrace::__rust_begin_short_backtrace::h3f798627fa82d4dd
  27:     0x55dd1d96665c - std::rt::lang_start::{{closure}}::h571364af7911c191
  28:     0x55dd1e2b0ffa - core::ops::function::impls::<impl core::ops::function::FnOnce<A> for &F>::call_once::h31ee16c075ae2d62
                               at /rustc/e37a13cc3594004663738bd18d8100e6db9666cf/library/core/src/ops/function.rs:259:13
  29:     0x55dd1e2b0ffa - std::panicking::try::do_call::h3993344bf97066b6
                               at /rustc/e37a13cc3594004663738bd18d8100e6db9666cf/library/std/src/panicking.rs:379:40
  30:     0x55dd1e2b0ffa - std::panicking::try::h09e523df7c9c2d28
                               at /rustc/e37a13cc3594004663738bd18d8100e6db9666cf/library/std/src/panicking.rs:343:19
  31:     0x55dd1e2b0ffa - std::panic::catch_unwind::h9d4ddb10ebb815ff
                               at /rustc/e37a13cc3594004663738bd18d8100e6db9666cf/library/std/src/panic.rs:431:14
  32:     0x55dd1e2b0ffa - std::rt::lang_start_internal::hc92e27a69d75de2a
                               at /rustc/e37a13cc3594004663738bd18d8100e6db9666cf/library/std/src/rt.rs:51:25
  33:     0x55dd1d980b25 - main
  34:     0x7fe6bfc2ab25 - __libc_start_main
  35:     0x55dd1d916071 - <unknown>

[eddyb]

@ehuss @alexcrichton resolver = "2" is in beta, right? Seems like this might be important to prioritize for an investigation/fix before it gets into stable - but I can't any labels myself to nominate it or anything.

[ehuss]

This was an unintended regression introduced by #8777 (first released in 1.49). I can try to take a look.