Incompatibility using both `-Zinstrument-coverage` and `-Z build-std`

[catenacyber]

I tried this code:

git clone https://github.com/OISF/suricata.git
cd suricata
export RUSTFLAGS="-Zinstrument-coverage"
sh autogen.sh
./configure --disable-shared --enable-fuzztargets --disable-gccmarch-native --enable-debug-validation
make

This ends up running cargo build --release -Z build-std

I expected to see this happen:
Compilation succeeds

Instead, this happened:
Compilation failed with

Updating crates.io index
   Compiling core v0.0.0 (/Users/catena/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/core)
error[E0152]: found duplicate lang item `f32`
   --> /Users/catena/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/core/src/num/f32.rs:319:1
    |
319 | / impl f32 {
320 | |     /// The radix or base of the internal representation of `f32`.
321 | |     #[stable(feature = "assoc_int_consts", since = "1.43.0")]
322 | |     pub const RADIX: u32 = 2;
...   |
937 | |     }
938 | | }
    | |_^
    |
    = note: the lang item is first defined in crate `core`.
    = note: first definition in `core` loaded from /Users/catena/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/x86_64-apple-darwin/lib/libcore-e3cd79f6ad1173fd.rlib
    = note: second definition in the local crate (`core`)

and many more errors about other duplicate items

Meta

This is part of #34701 cc @richkadel

rustc --version --verbose:

rustc 1.50.0-nightly (603ab5bd6 2020-11-15)
binary: rustc
commit-hash: 603ab5bd6e0ffefafa7411cd8bd23a6ca82bcff0
commit-date: 2020-11-15
host: x86_64-apple-darwin
release: 1.50.0-nightly

Backtrace

    Updating crates.io index
   Compiling core v0.0.0 (/Users/catena/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/core)
error[E0152]: found duplicate lang item `f32`
   --> /Users/catena/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/core/src/num/f32.rs:319:1
    |
319 | / impl f32 {
320 | |     /// The radix or base of the internal representation of `f32`.
321 | |     #[stable(feature = "assoc_int_consts", since = "1.43.0")]
322 | |     pub const RADIX: u32 = 2;
...   |
937 | |     }
938 | | }
    | |_^
    |
    = note: the lang item is first defined in crate `core`.
    = note: first definition in `core` loaded from /Users/catena/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/x86_64-apple-darwin/lib/libcore-e3cd79f6ad1173fd.rlib
    = note: second definition in the local crate (`core`)

[catenacyber]

By the way, there is a current PR OISF/suricata#5582 with a fix : not using -Z build-std when -Zinstrument-coverage is in RUSTFLAGS

[richkadel]

@catenacyber - Thanks for the bug report!

I did some experimenting (briefly), and I can repro this issue, for instance, with:

RUSTFLAGS="-Zinstrument-coverage" cargo build --release -Z build-std --target x86_64-unknown-linux-gnu

I don't fully understand how -Z build-std works, but from the error messages, it looks like the compiler may have built a new version of the std library (as intended), and then the compiler uses that new version but also (?) loads another version of std (or two versions of core).

One copy loads the standard LangItems, and the second copy also tries to load them but fails because they were already loaded.

(This is a minimually-educated guess, from the error messages.)

Anyhow, I don't think -Zinstrument-coverage would be doing anything specifically to cause this problem, so I thought, maybe it's not specific to -Zinstrument-coverage.

So I tried:

RUSTFLAGS="-Zprofile" cargo build --release -Z build-std --target x86_64-unknown-linux-gnu

Sure enough, I get the same errors.

Since cargo -Z build-std is also an "experimental" feature, I would redirect this issue to the build-std feature experts, to see if they can figure out why some compiler options (like -Zinstrument-coverage and -Zprofile) conflict in this way.

Note, both -Z instrument-coverage and -Z profile do have a dependency on the profiler_builtins library in rust/library, so one place they might look is where the profiler_builtins library gets loaded. Maybe it's forcing the default std library to load, as a direct dependency of profiler_builtins, rather than using the std that was just built.

[catenacyber]

Indeed, I do not know which experimental feature should fix this.
So, please redirect this report how you see fit 👍

[richkadel]

@alexcrichton - Is cargo -Z build-std something you would be familiar with? Or if not, do you know who might be? (Thanks!)

[alexcrichton]

I believe the profiler_builtins crate is the key here. Using -Zbuild-std builds a fixed set of crates and profiler_builtins is not included in that set. This would probably be an issue for https://github.com/rust-lang/wg-cargo-std-aware/issues since I don't think there's much rust-lang/rust can do here for this.

[richkadel]

@catenacyber - Would you mind copy/pasting your issue into that list (perhaps just linking back to here for context if helpful)?

Thanks!

[taiki-e]

@rustbot label +A-code-coverage