[GR-19691] Update the How-To guide

PullRequest: truffleruby/4247

doc/contributor/how-to-guide.md

@@ -31,6 +31,9 @@
   * [How to tag slow ruby/spec tests](#how-to-tag-slow-rubyspec-tests)
   * [How to introduce a constant in specs](#how-to-introduce-a-constant-in-specs)
   * [How to add a new spec](#how-to-add-a-new-spec)
+  * [How to work on a new CRuby feature](#how-to-work-on-a-new-cruby-feature)
+  * [How to prepare PR for changes in the next CRuby version](#how-to-prepare-pr-for-changes-in-the-next-cruby-version)
+  * [How to use jt CLI tool](#how-to-use-jt-cli-tool)
 
 ## How to find a Core Method implementation
 
@@ -1564,3 +1567,68 @@ There are the following main directories:
 - `spec/ruby/core` - specs for the Core Library
 - `spec/ruby/library` - specs for the Standard Library
 - `spec/ruby/language` - specs for the Ruby syntax itself
+
+## How to work on a new CRuby feature
+
+When you work on a new feature/method usually you make the following steps:
+
+- check a feature/method description in the Ruby documentation
+- check a corresponding issue (on the Ruby bug tracker <https://bugs.ruby-lang.org>):
+  - a feature description
+  - discussion
+  - "Associated revisions" tab with commits and diffs (pay attention to added MRI and ruby/spec tests)
+- check existing ruby/spec tests in the TruffleRuby repository
+- add missing test cases
+- untag ruby/spec tests
+- implement the feature
+- untag MRI tests and ensure they pass too
+- mention in a PR description an original issue in a way it's described in the Ruby
+  release notes or provide a link to an original ticket (on <https://bugs.ruby-lang.org>)
+
+## How to prepare PR for changes in the next CRuby version
+
+When you work on a new feature that is introduced in Ruby version which TruffleRuby doesn't support yet
+(e.g. TruffleRuby is compatible with Ruby 3.1 now but the feature is introduced in Ruby 3.2) there is a
+problem with running proper specs locally and on CI (both internal and GitHub Actions).
+
+So you should ensure that the specs for the new feature are run and passed successfully on CI and the
+whole suit pass successfully as well:
+
+- add specs for the new feature if they are missing (with a `ruby_version_is` guard)
+- use `PRETEND_RUBY_VERSION` environment variable to run specs for the new feature
+- add a file with specs for the new feature to the next list in `spec/truffleruby.next-specs` (to run them on CI)
+- use `jt test :next` to run locally all the specs for the next CRuby version
+
+If the new feature breaks existing behavior and some specs fail - disable them temporary (until TruffleRuby
+is switched to the next CRuby version completely) with tagging as failed (use `jt tag <path-to-spec-file>`)
+
+Look for additional details in [The "Running specs for Ruby 3.3 features" section of the Contributor Workflow document](workflow.md#running-specs-for-ruby-33-features).
+
+## How to use jt CLI tool
+
+The `jt` CLI tool has a lot of commands but there are very few basic ones that you may need to use often:
+
+- `jt ruby` - run TruffleRuby REPL (`irb`)
+    - `jt ruby <filename>` - execute a file with Ruby source code with TruffleRuby
+    - `jt ruby -e '<Ruby source code>'` - execute a string of Ruby source code with TruffleRuby REPL
+    - `jt -q ruby --jdebug -e '<Ruby source code>'` - to launch debugger
+- `jt test <path to test file>` - run ruby/spec test file
+    - `jt --use ruby test <path to test file>` - to use tests against current CRuby, not TruffleRuby
+        - `jt -u ruby test <path to test file>` - there is a short form `-u`
+    - `jt test <path to test file> -- --jdebug` - to launch debugger
+    - `jt test fast <path to test file>` - run subset of specs and tag slow test cases with tag `slow`
+    - `jt test :next` - run specs for features introduced in the next CRuby version, not supported by TruffleRuby currently
+- `jt test mri <path to test file>` - run MRI test file
+  - `jt test mri <path to test file> --jdebug` - to launch debugger for MRI tests (without `--`)
+  - `jt test mri <path to MRI test file> -- -n <test case name>` - to run only specified UnitTest case
+- `jt build`
+- `jt clean`
+- `jt rebuild`
+- `jt sync` - to synchronize changes in Standard Library Ruby source code and apply them without rebuilding TruffleRuby
+- `jt tag` - ruby/spec only
+- `jt untag <path-to-file>` - (ruby/spec only) regenerate an exclude file for particular ruby/spec file (and list all the failed test cases)
+- `jt retag <path-to-file>` - (MRI test only) regenerate an exclude file for particular MRI test file (and list all the failed test cases)
+- `jt lint fast` - to run some linter checks (that are run on CI) locally
+- `jt rubocop` - to run Ruby source code linter separately from other linter checks.
+    - `jt rubocop -a` - to correct safely Rubocop warnings automatically
+    - `jt rubocop -A` - to correct unsafely Rubocop warnings automatically