RFC: Add native code coverage support in Cargo

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+- Feature Name: `cargo_test_coverage`
+- Start Date: 2022-06-01
+- RFC PR: [rust-lang/rfcs#0000](https://github.com/rust-lang/rfcs/pull/0000)
+- Rust Issue: [rust-lang/rust#0000](https://github.com/rust-lang/rust/issues/0000)
+
+# Summary
+[summary]: #summary
+
+This RFC aims to improve the process of collecting code coverage data for
+Rust libraries. By including Cargo in the process of instrumenting Rust
+libraries and running the unit tests, the sequence of steps to get coverage
+results will be simplified. This RFC also proposes adding support for Cargo
+to selectivly choose which crates get instrumented for gathering coverage results.
+
+# Motivation
+[motivation]: #motivation
+
+Why are we doing this? What use cases does it support? What is the expected outcome?
+
+The motivation behind this feature is to allow for a simple way for a Rust developer
+to run and obtain code coverage results for a specific set of crates to ensure confidence
+in code quality and correctness. Currently, in order to get instrumentation based code coverage,
+a Rust developer would have to either update the `RUSTFLAGS` environment variable or Cargo
+manifest keys with `-C instrument-coverage`. This would automatically enable instrumentation
+of all Rust crates within the dependency graph, not just the current crate. Instrumenting
+all crates including transitive dependencies does not help the developer ensure test coverage
+for the crates they actually want to test. This also adds unnecessary work for both the Rust
+compiler and codegen backend, `LLVM` in this case, to instrument all libraries as opposed to
+the subset a Rust developer actually cares about. This support is currently limited to the
+`LLVM` codegen backend and will have no effect when another codegen backend is used.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+This section examines the features proposed by this RFC:
+
+## CLI option
+
+Three new flags will be added for the `cargo test` command, `--coverage`, `--coverage-format` and `--coverage-output-directory`
+The `--coverage` flag would instruct Cargo to add the rustc flag `-C instrument-coverage`, for the current
+crate only. Cargo also supports two types of workspaces, a workspace with a root package and one without called a
+virtual manifest. In the case where the `cargo test --coverage` command is run from a Cargo workspace which has a root package,
+the `-C instrument-coverage` rustc flag would only be enabled for the root package. In the case where the `cargo test --coverage`
+command is run from a Cargo workspace without a root package, the `-C instrument-coverage` rustc flag would be enabled for
+all default members of the workspace. This would mean that only the crates selected would be instrumented and code
+coverage results would only be collected for those crates and not all crates in the dependency graph.
+
+As an example, let's take the following crate `foo`:
+
+```text
+/Cargo.toml
+/src
+    +-- lib.rs
+```
+
+Where crate `foo` has a dependency on the `regex`:
+```toml
+[dependencies]
+regex = "*"
+```
+
+And `lib.rs` contains:
+
+```rust
+use regex::*;
+
+pub fn match_regex(re: Regex, text: &str) -> bool {
+    let Some(caps) = re.captures(text) else {
+        return false
+    };
+
+    let matches = caps.iter().filter_map(|s| s).collect::<Vec<Match>>();
+    matches.len() > 0
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn find_match() {
+        let result = match_regex(Regex::new(".*").unwrap(), "Hello World");
+        assert_eq!(result, true);
+    }
+
+    #[test]
+    fn find_no_match() {
+        let result = match_regex(Regex::new("a+").unwrap(), "Hello World");
+        assert_eq!(result, false);
+    }
+}
+```
+
+Now running `cargo test --coverage` would build the `foo` crate and instrument all libraries created by this crate.
+Each test executable run will generate a unique `*.profraw` file. At the end of all test execution, Cargo will be
+responsible for merging the generated `profraw` files, and by default would generate code coverage results in HTML
+format for simple viewing within a browser. The code coverage results produced would be only for the crate `foo`
+and ignore all code coverage for any function defined outside of this crate.
+
+LLVM supports exporting code coverage results in a number of formats. `--coverage-format` would be responsible for selecting
+the code coverage results format and `--coverage-output-directory` would allow a Rust developer to select the
+output directory for the code coverage report. The default for `--coverage-format` is `html` and the default for
+`--coverage-output-directory` would be `target/coverage/`
+
+The `cargo test` subcommand also supports the `--package` and `--workspace` flags. This instructs Cargo to run the
+tests for those specific packages. When combined with the new `--coverage` flag, a Rust developer will be able to
+selectively choose which Rust crates will be instrumented and have tests run. Since potentially unit tests from
+multiple crates will be run, the code coverage results will include coverage results for a crate that is being
+covered by a test from another crate.
+
+The `--coverage` flag will also support a comma separated list of crates so that a Rust developer can control
+the exact set of crates which will be instrumented during a build and test run.
+
+For example, let's take the following workspace with default members:
+
+```text
+/Cargo.toml
+/member1
+  +-- src
+    +-- lib.rs
+/member2
+  +-- src
+    +-- lib.rs
+/member3
+  +-- src
+    +-- lib.rs
+```
+
+Where crate `foo` has a virtual manifest:
+```toml
+[workspace]
+members = ["path/to/member1", "path/to/member2", "path/to/member3"]
+default-members = ["path/to/member2"]
+```
+
+Running the following cargo invocation would only instrument the workspace member
+`member2` and run the tests for `member2`:
+
+```
+cargo test --coverage
+```
+
+To instrument a different workspace member, the following command is also supported:
+
+```
+cargo test --coverage=member1,member3
+```
+
+This cargo invocation will instruct Cargo to run the tests for the default workspace member `member2`, but
+will only instrument the crates `member1` and `member3` to collect coverage results for. This flexibility
+allows a Rust developer to see just how much a given crate is covered by tests from another crate.
+
+With this feature, existing custom Cargo commands can leverage Cargo to do all of the heavy work to instrument specific
+crates and generate coverage results in a multitude of formats. This will allow in more flexibilty in generating
+for custom commands that generate code coverage for crates today.
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+As mentioned earlier, this RFC proposes adding multiple flags to the `cargo test` command. The flag `--coverage`
+would be responsible for setting the `-C instrument-coverage` flag that Cargo would pass on to the rustc invocation of the
+current crate. In the previous example, `foo` would be the crate being instrumented and `regex` would be an upstream dependency.
+
+Using the `--coverage` flag, Cargo would only set the `-C instrument-coverage` flag for the crate `foo`. If the `RUSTFLAGS`
+environment variable had already been set to include the `-C instrument-coverage` flag, then Cargo would still pass that
+flag to all crates within the dependency graph, including the `regex` crate and any transitive dependencies.
+
+This should not break any existing workflows and is strictly an opt-in feature.
+
+To use this new feature do the following:
+
+`cargo test --coverage`
+
+This flag would also be responsible for setting the `LLVM_PROFILE_FILE` environment variable which is read by LLVM
+to generate a `.profraw` file for each test executable that is run. Once again if the environment variable is already set,
+then Cargo would not make any changes and would leave the value as is to use the user-defined file name. If the environment
+variable is not set, Cargo would set it to ensure a unique naming scheme is used for each `.profraw` file that would be
+generated.
+
+Once the tests have finished running, Cargo would leverage LLVM tooling to analyze profraw files and generate
+coverage reports for a Rust developer. These tools would be `llvm-profdata` and `llvm-cov` and can be used by
+adding the `llvm-tools-preview` component to the current toolchain, `rustup component add llvm-tools-preview`.
+This would be a requirement to use this new feature since these are the tools necessary to analyze coverage results
+and generate a code coverage report. If the component is not installed for the current toolchain, an error will occur
+with a message stating the the `llvm-tools-preview` component is required to generate a code coverage report.
+
+For example, a Rust developer can invoke the following Cargo command to generate an HTML coverage report
+for the current crate or `default-members` in a workspace:
+
+```
+cargo test --coverage
+```
+
+This will generate a code coverage report in HTML format in the `target/coverage/` directory. If a workspace
+does not have `default-members`, then all members would be instrumented.
+
+Run the following cargo CLI to choose a specific set of crates to instrument and override the defaults:
+
+```
+cargo test --coverage:crate1,crate3,foo
+```
+
+To override the default output format and directory, the `--coverage-format` and `--coverage-output-directory`
+can be passed to the cargo CLI:
+
+```
+cargo test --coverage --coverage-format=lcov --coverage-output-directory=src/coverage
+```
+
+This will generate a code coverage report in lcov format in the `src/coverage/` directory.
+
+The supported options for the `--coverage-format` option are:
+
+1. `html`
+2. `lcov`
+3. `json`
+
+These updates to Cargo would be sufficient enough to ensure that a Rust developer would have control over what crates are
+instrumented, the format in which code coverage results are generated and where they are stored. This would also allow
+the Rust developer to no longer have to set environment variables manually to ensure crates are instrumented
+for gathering coverage data and can generate code coverage results with a single command.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+A drawback of this feature would be that Cargo would need to enable the `LLVM_PROFILE_FILE`
+environment variable in order to ensure unique profile data is generated for each test
+executable. I am not aware of any other Cargo features that set environment variables today
+so this could potentially create new issues in Cargo.
+
+# Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+## Rationale
+
+This design provides a simple mechanism to integrate collecting code coverage
+results for a given crate. Allowing Cargo to be part of the coverage process
+would reduce the need for setting environment variables manually. Simply running
+`cargo test --coverage` would automatically run a build setting the `-C instrument-coverage`
+rustc flag only for the crates selected and set the `LLVM_PROFILE_FILE` environment
+variable to ensure each test run produces a unique `profraw` file. Once the tests have
+finished running, Cargo would leverage LLVM tooling to analyze profraw files and generate
+coverage reports for a Rust developer.
+
+This design does not break any existing usage of Rust or Cargo. This new feature would
+be strictly opt-in. A Rust developer would still be able to manually set the
+`-C instrument-coverage` rustc flag and instrument all binaries within the dependency
+graph. Since this is a Cargo specific feature, the Rust compiler will not need any updates.
+
+## Alternatives
+
+### Alternative 1: leave the existing feature
+
+Supporting this alternative would mean that no changes would be necessary to either Cargo
+or Rust. Getting instrumentation-based code coverge is still supported and would continue
+to work as it does today.
+
+The drawback for this option is that it would require setting the flag for all crates in
+the dependency graph, including upstream dependencies. This would also instrument all
+binaries and report on coverage for functions and branches that are not defined by the
+current crate with the potential of skewing coverage results.
+
+### Alternative 2: use a new manifest key instead of a CLI option
+
+Supporting this alternative would mean that changes would need to be made to the existing
+manifest format to possibly include a new section and/or key. A new `coverage` key could
+be added to the target section, `coverage = true`. This still has the added benefit of not
+requiring any changes to the Rust compiler itself and the feature could be scoped to Cargo only.
+
+The drawback for this option is that it could potentially add clutter to the `Cargo.toml`
+files. Adding this new section to every crate that a Rust developer wants to have instrumented
+would only add to all the data that is already contained within a toml file.
+
+### Alternative 3: use a RUSTC_WRAPPER program to selectively choose which crates to profile
+
+Supporting this alternative would mean that there wouldn't need to be any changes to Cargo
+at all.
+
+This would require creating a RUSTC_WRAPPER program specifically for selecting which crates to profile.
+This means more boiler plate code for each Rust developer that simply wants to profile their own crate.
+I believe the feature this RFC proposes would both be a cleaner solution long term and more in line
+with the Cargo workflow of potentially reading these kinds of behaviors from the `Cargo.toml` manifest
+file.
+
+### Alternative 4: use existing custom subcommands to run code coverage analysis
+
+Supporting this alternative would mean that there wouldn't need to be any changes to Cargo
+at all.
+
+There are multiple custom subcommands that exist today to achieve instrumenting Rust libraries
+analyzing code coverage metrics. A couple of examples would be cargo-llvm-cov and cargo-tarpaulin.
+Both of these custom subcommands are available via crates.io and support running tests with code
+coverage enabled and creating a coverage report to be viewed by a Rust developer.
+
+Cargo-llvm-cov currently leverages the `cargo-config` subcommand which is still unstable. To do so,
+cargo-llvm-cov sets the `RUSTC_BOOTSTRAP` environment variable to allow its usage from a stable
+toolchain. This is not a recommended approach especially for Rust developers that want to use
+such tools in production code.
+
+Tarpaulin has a great set of features for collecting and analyzing coverage metrics but it only
+supports x86_64 processors running Linux which limits how this can be used by Rust developers
+working on other platforms such as Windows.
+
+# Prior art
+[prior-art]: #prior-art
+
+## VSInstr
+
+Visual Studio ships with a tool `vsinstr.exe` which has support for instrumenting binaries after
+they have already been built. Since LLVMs instrumentation-based code coverage hooks into each object
+file it generates this scenario is a bit different than the feature this RFC proposes. `vsinstr` does
+allow for excluding namespaces of functions to skip over so that everything within a binary does not
+get instrumented.
+
+ - [vsinstr](https://docs.microsoft.com/en-us/previous-versions/visualstudio/visual-studio-2017/profiling/vsinstr?view=vs-2017)
+
+## gcov based coverage
+
+Rust also has support for another type of code coverage, a GCC-compatible, gcov-based coverage implementation.
+This can be enabled through the `-Z profile` flag. This uses debuginfo to derive code coverage. This is different
+than the source-based code coverage which allows for a more precise instrumentation to be done.
+
+ - [Source-Based Code Coverage](https://blog.rust-lang.org/inside-rust/2020/11/12/source-based-code-coverage.html)
+
+# Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+- Are there any drawbacks from having Cargo set the `LLVM_PROFILE_FILE` environment variable that LLVM uses to name each
+of the generated `profraw` files? This is used to ensure each test run generates unique profiling data as opposed to overwriting
+the previous run.
+
+# Future possibilities
+[future-possibilities]: #future-possibilities
+
+## A single command to generate coverage results and open in a browser
+
+Running `cargo test --coverage --open` would greatly simplify the experience of viewing coverage results.
+With a simple command, a Rust developer can run all of their tests with source-based instrumentation
+enabled, and view the coverage results in a browser. This would need a new flag `--open` flag to be
+added to the `cargo test` command and would only be valid if the `--coverage` flag was enabled. This
+would produce a very similar experience to how `cargo doc --open` works today. `cargo doc` compiles all
+of the documentation for a crate and the `--open` flag automatically opens a browser showing all of the
+generated documentation. This would be a great feature to have when generating code coverage.