build: add "coverage" profile to cargo

Cargo doesn't fully pay attention to the RUSTFLAGS variable.
As such, to pass appropriate flags to rustc, Cargo uses a profile.
We thus create a profile specifically for generating code coverage and
updating the code coverage docs accordingly.

See rust-lang/rust#78011.

Closes build-trust#3553.

Cargo.toml

@@ -32,3 +32,16 @@ members = [
 ]
 
 exclude = ["implementations/rust/ockam/ockam_examples/example_projects"]
+
+# Coverage profile for generating code coverage with grcov.
+#
+# See https://github.com/rust-lang/rust/issues/78011.
+#
+[profile.coverage]
+panic = "abort"
+opt-level = 0
+overflow-checks = false
+incremental = false
+codegen-units = 1
+inherits = "test"
+

implementations/rust/DEVELOP.md

@@ -83,7 +83,7 @@ Get a code coverage report:
 ```
 cargo +nightly install grcov
 
-env CARGO_INCREMENTAL=0 RUSTFLAGS="-Zprofile -Ccodegen-units=1 -Copt-level=0 -Clink-dead-code -Coverflow-checks=off -Zpanic_abort_tests -Cpanic=abort --cfg tokio_unstable" RUSTDOCFLAGS="-Cpanic=abort" cargo +nightly test
+env CARGO_INCREMENTAL=0 RUSTFLAGS="-Zprofile -Clink-dead-code --cfg tokio_unstable" cargo +nightly test --profile coverage
 
 grcov --llvm . -s . --binary-path ./target/debug/ -t html --branch --ignore-not-existing -o ./target/debug/coverage/