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Fix documentation example for gcov profiling #72615

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Jun 8, 2020
Merged

Fix documentation example for gcov profiling #72615

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2b26b8b
Fix documentation example for gcov profiling
jschwe May 26, 2020
7bf026e
Doc: unstable book - profile.md: improve wording
jschwe Jun 3, 2020
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7 changes: 6 additions & 1 deletion src/doc/unstable-book/src/compiler-flags/profile.md
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Expand Up @@ -12,10 +12,15 @@ For example:
```Bash
cargo new testgcov --bin
cd testgcov
export RUSTFLAGS="-Zprofile"
export RUSTFLAGS="-Zprofile -Ccodegen-units=1 -Copt-level=0 -Clink-dead-code -Coverflow-checks=off -Zpanic_abort_tests -Cpanic=abort"
export CARGO_INCREMENTAL=0
cargo build
cargo run
```

Once you've built and run your program, files with the `gcno` (after build) and `gcda` (after execution) extensions will be created.
You can parse them with [llvm-cov gcov](https://llvm.org/docs/CommandGuide/llvm-cov.html#llvm-cov-gcov) or [grcov](https://github.com/mozilla/grcov).

Please note that `RUSTFLAGS` apply to everything that cargo builds and runs during a build, including build scripts!
To avoid this, pass a `RUSTC_WRAPPER` program to cargo that only adds the profiling flags to rustc for the specific
crates you want to profile.
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When --target flag is passed to cargo, the RUSTFLAGS no longer apply to build scripts and procedural macros. This could be more convenient to use than a wrapper.

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Thanks for the feedback. I added a sentence regarding --target and mentioned RUSTC_WRAPPER as an option if one wants more control over which crates get instrumented.