-Z profile fails to produce any .gcno/.gcda files for bench target

[mzabaluev]

I want to generate profiling coverage information for a benchmark in my project. I run this command:

RUSTFLAGS='-Z profile=yes -Z profile-emit=/tmp/myproject-profile -C codegen-units=1' cargo build --benches -Z unstable-options --profile=bench --target=x86_64-unknown-linux-gnu

Neither the build nor running the bench executable creates .gcno/.gcda files with the coverage information. Also tried without profile-emit.

Tried to instrument cargo test for comparison, that results in a build failure:

     Running `rustc --crate-name chunked_bytes --edition=2018 src/lib.rs --error-format=json --json=diagnostic-rendered-ansi --crate-type lib --emit=dep-info,metadata,link -Cembed-bitcode=no -C debuginfo=2 -C metadata=acf3ef1e1fe2447e -C extra-filename=-acf3ef1e1fe2447e --out-dir /home/mzabaluev/work/rust/chunked-bytes/target/x86_64-unknown-linux-gnu/debug/deps --target x86_64-unknown-linux-gnu -C incremental=/home/mzabaluev/work/rust/chunked-bytes/target/x86_64-unknown-linux-gnu/debug/incremental -L dependency=/home/mzabaluev/work/rust/chunked-bytes/target/x86_64-unknown-linux-gnu/debug/deps -L dependency=/home/mzabaluev/work/rust/chunked-bytes/target/debug/deps --extern bytes=/home/mzabaluev/work/rust/chunked-bytes/target/x86_64-unknown-linux-gnu/debug/deps/libbytes-fcec2a592652b77a.rmeta -Z profile=yes -Z profile-emit=/tmp/chunked-bytes-profile -C codegen-units=1`
error: can't instrument with gcov profiling when compiling incrementally

The -C incremental=... option cannot be suppressed by disabling incremental in the test profile in .cargo/config.toml.

Edit: the cargo test failure was solved, I used a custom target to which config did not apply. That build does try to produce GCDAs and fails in some build script executable, but I guess it's a subject for a different issue.

Meta

rustc --version --verbose:

rustc 1.45.0-nightly (56daaf669 2020-06-03)
binary: rustc
commit-hash: 56daaf669ebc3d5083db5cded719f780dc31104e
commit-date: 2020-06-03
host: x86_64-unknown-linux-gnu
release: 1.45.0-nightly
LLVM version: 10.0

[mzabaluev]

Tracking issue for the feature: #42524

[ehuss]

I think profiling requires debuginfo. Try setting this in Cargo.toml:

[profile.bench]
debug = true

[profile.release]
debug = true

I'm not sure how well it will work with an optimized build, though. You may want to consider using the dev profile instead (and setting incremental=0).

[mzabaluev]

The compiler could have put out a warning about the options being incompatible.

[link2xt]

An option -Cdebuginfo=2 should be documented in https://doc.rust-lang.org/unstable-book/compiler-flags/profile.html then.

Had to add it here: deltachat/deltachat-core-rust#3546